The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

4.17 The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity MT Auer, NA Auer, BB Barkdoll, and TJ Bornhorst, Michigan Technological University, Houghton, MI, USA C Brooks, Michigan Tech Research Institute, Ann Arbor, MI, USA D Dempsey, International Joint Commission, Washington, DC, USA PV Doskey, SA Green, MD Hyslop, WC Kerfoot, AS Mayer, and JA Perlinger, Michigan Technological University, Houghton, MI, USA R Shuchman, Michigan Tech Research Institute, Ann Arbor, MI, USA NR Urban and DW Watkins Jr., Michigan Technological University, Houghton, MI, USA r 2014 Elsevier Inc. All rights reserved.

4.17.1 4.17.2 4.17.2.1 4.17.2.2 4.17.2.3 4.17.2.4 4.17.2.5 4.17.3 4.17.3.1 4.17.3.2 4.17.3.3 4.17.3.4 4.17.3.5 4.17.4 4.17.4.1 4.17.4.2 4.17.4.3 4.17.5 4.17.5.1 4.17.5.2 4.17.5.3 4.17.6 4.17.6.1 4.17.6.2 4.17.6.3 4.17.6.4 4.17.6.5 4.17.7 References

Introduction Formation Bedrock Geology Glacial Geology Glacial Lakes Soils Billions of Years Later Hydrology Flow Patterns and the Hydrologic Budget Water Level Regulation Regulating Diversions Planning for the Future Perspective Physics Light Diffusive Mass Transport – Temperature and Thermal Stratification Advective Mass Transport – Wind and Currents Water Chemistry – Oxygen, Nutrients and Trophic State Oxygen Nutrients Trophic State Biodiversity The Interwoven Nature of All Life and Importance of Water for Survival So What is Biodiversity? Food Web Connections in the Historic versus Contemporary Great Lakes Ecosystem Changing Biodiversity in the Great Lakes Perspectives Summary and Conclusions

Glossary Adaptive management strategies A systematic approach for improving resource management by learning from the outcome of antecedent outcomes. Advection It is one of the two forms of mass transport, the other being diffusion. This process is movement with the bulk fluid flow, for example, currents. Amphipod A macroinvertebrate crustacean; the amphipod Diporeia is 1–8 mm in length and figures prominently in the diet of lake trout and lake whitefish. Anoxia It is a total depletion of oxygen. Anticyclonic In the Northern Hemisphere, circulating clockwise and contrary to the direction of Coriolis forces.

Comprehensive Water Quality and Purification, Volume 4

362 363 363 365 365 366 367 368 368 370 370 371 371 371 372 373 375 379 379 380 381 383 383 385 385 386 388 388 388

Areas of Concern Designated geographic areas within the Great Lakes basin that show severe environmental degradation. Attenuating It is a reduction in concentration or intensity. Bathymetry The measurement of the depth of water bodies. Benthic Water in a lake that is near the bottom. Biodiversity The variety of all forms of life, from genes to species, through the broad scale of ecosystems. Circulation cells A package of air or water with a distinct circulation pattern. Cladophora A native, filamentous green alga that grows to nuisance proportions in the Great Lakes and inland waters.

doi:10.1016/B978-0-12-382182-9.00092-X

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The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

Coastal jets Transient along shore circulation patterns where currents are driven by river inputs, wind, the meander of an offshore current over coastal waters, and other physical phenomena. Cold water fish Fish species with preference for cold waters, for example, lake trout and salmon. Compensating works Structures designed to regulate the release of water from lakes. Condition An index used to assess the overall health of a fish by comparing its weight with other fish of the same length and species. Conglomerates A sedimentary rock consisting of rock pieces cemented together in a matrix of finer rock material. Convective The transfer of heat from one location to another by the movement of fluids. Coriolis effect A force caused by the rotation of the Earth and the inertia of the mass experiencing that effect; in the Northern Hemisphere, river plumes are deflected to the right and currents travel in a counterclockwise direction due to this effect. Crustal Relating to the Earth’s crust, the outmost solid shell of the planet. Cyanobacteria A group of bacteria, formally referred to as the blue-green algae, that obtain their energy through photosynthesis. Cyclonic In the Northern Hemisphere, circulating counterclockwise in the direction of Coriolis forces. Density The mass of a material per unit density, for example, grams per cubic centimeter. Diffusion It is one of the two forms of mass transport. This is the mixing of molecules within a fluid to random motion; molecular diffusion is driven by the basic kinetic energy of the molecules; turbulent or eddy diffusion is driven by the action of eddy motion; and the latter is significantly more effective in transporting materials than the former. Dimictic Lakes that are thermally stratified and then mix twice annually. Dreissinids Referring, in the Great Lakes, to two species of invasive mussels: Dreissena polymorpha, the zebra mussels and Dreissena bugensis, the quagga mussel. Ecosystem services These are the benefits received by humankind from the resources and processes associated with natural ecosystems. Eddies The swirling of a fluid and the reverse current generated when a fluid (air and water) encounters an obstacle or is slowed by friction with a fluid of differencing density. Effluent Outflow, often from a municipal or industrial wastewater treatment plant. Entrain It is to draw in and transport. Environmental determinism It is originally and inappropriately used to relate environment and human behavior and culture. The term is used here to relate the ancient environment and environmental dynamics to contemporary environmental conditions. Enzymatic hydrolysis A process in digestion where macromolecules are split from food by the enzymatic addition of water.

Epilimnion The warm, well mixed upper layer in a lake experiencing summer thermal stratification. Eutrophic A body of water rich in nutrients; in the Great Lakes characterized by high levels of phosphorus and algae, low transparency and, in some locations hypolimnetic oxygen depletion. Evaporite A water-soluble mineral sediment formed by concentration, evaporation, and crystallization from an aqueous solution. Evapotranspiration The sum of evaporation and transpiration; the latter being plant uptake of liquid water and its release to the atmosphere as water vapor. Exotic species Organisms introduced to habitats where they are not native; invasive exotic species are those that cause harm to the ecosystem. Filter feeders Animals that feed by straining suspended matter and food particles from water. Food web A depiction of feeding connections in an ecosystem; the upper food web includes predator and forage fish and the lower food web includes algae, zooplankton, and macroinvertebrates. Forage fish Fish preyed upon by larger predator species. Glacial drift Material, including rocks, cobbles, gravel, sand, or clay, transported and deposited by a glacier of glacial meltwater. Glacial till An unsorted glacial sediment or drift. Global climate models A system of equations describing fluid dynamics and chemical or biological processes contributing to climate behavior; also known as a general circulation model. Harmful algal blooms Algae or cyanobacteria that can cause harm to humans through excessive growth and/or the production of toxins. Heat flux The transfer of heat at the boundary between a lake and the atmosphere. Henry’s Law coefficient A coefficient describing the ratio of the concentration of a gas in water to its partial pressure in the overlying atmosphere; see saturation. Homeostasis The property of a system to regulate its internal environment (or composition) to maintain an essentially constant, stable condition. Hydrologic cycle The continuous movement of water on, above, or below the Earth through evaporation, condensation, precipitation, infiltration, runoff, and subsurface flow. Hypolimnion The cold, well mixed lower layer in a lake experiencing summer thermal stratification. Hypoxia Oxygen concentrations below those required to support most animal life. Igneous It is one of the three main rock types. These are formed from magma, liquid phase of solid rock. Internal wave Wave motion below the lake surface; often the ’rocking’ of the thermocline. Limiting nutrient The nutrient, phosphorus in freshwaters, that limits plant growth. Limnology The study of lakes. Littoral The shallow, nearshore region of a lake, sometimes defined as extending to the depth where attached plant growth is no longer supported.

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

Load The amount of material delivered to a lake from pipes, rivers, direct runoff, and the atmosphere, expressed in units of mass/time. Macroinvertebrates Animals that have no backbone and are visible to the naked eye. Magma Molten or semi-molten rock found beneath the surface of the Earth. Mesotrophic A body of water with levels of nutrients intermediate to those of oligotrophic and eutrophic lakes; often with both high production and adequate oxygen reserves. Metalimnion The region below the epilimnion and above the hypolimnion in a lake experiencing summer thermal stratification where temperature changes rapidly with depth. Metamorphic It is one of the three main rock types. These are formed from existing igneous or sedimentary rocks. Mixing depth The depth of the bottom of the well mixed upper layer, the epilimnion, in lakes exhibiting thermal stratification. Natal An adjective that refers to birth. Nested models In simulating mass transport, a smaller, high density network of model cells is nested within a larger system with a lower density of cells. Nitrogen fixation The process by which nitrogen gas is converted into ammonia; in lakes this is carried out by some species of cyanobacteria. Oligotrophic A body of water poor in nutrients; in the Great Lakes characterized by low levels of phosphorus and algae, high transparency, and abundant oxygen resources. Optical depth A measure of transparency, quantified as the negative logarithm of the fraction of light not absorbed while penetrating the water column. Organic matter The vast array of carbon compounds, formed originally through photosynthesis; including living and dead photosynthetic plants and bacteria and the other organisms which draw upon them as energy sources. Orogeny Processes leading to the structural deformation of the Earth’s lithosphere, including mountain building by folding and faulting. Outwash Sand and gravel, transported and sorted by glacial meltwater and laid down in stratified deposits. Partial pressure The hypothetical pressure of a gas if it is alone occupied the volume of the gas mixture that it is contained within; the partial pressure of oxygen is 0.21 atm as it occupies 21% of the atmosphere. Pelagic Any water in a lake which is not near the bottom or shore; see benthic, littoral. Photic zone The depth of water in a lake where sufficient sunlight is received to support photosynthesis. Photon The basic unit of electromagnetic energy, and thus light; the energy input to the Earth from the sun may be described as a photon flux, the number of packets of light energy impinging on a unit area of the Earth’s surface per unit time. Photosynthesis The process used by plants and some bacteria to capture and convert light energy from the sun to chemical energy which may be stored.

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Phytoplankton The microscopic, free-floating plants in lakes; cyanobacteria are often casually considered as part of this group. Plate tectonics Large scale motion of the plates that make up the Earth’s lithosphere (the crust plus the uppermost mantle, a stiff, but viscous rock layer). Precursor A substance from which another substance is formed. Primary production The production of organic compounds from carbon dioxide; in lakes, largely through the process of photosynthesis, using light as a source of energy. Proglacial Forming immediately in front of or just beyond a glacier; in North America, glacial lakes were formed to the south of the retreating ice sheet. Reaeration The transfer of oxygen from the atmosphere to water. Respiration The metabolic process through which an organism obtains energy by reacting oxygen with organic matter yielding carbon dioxide, water, and energy. Salmonids Fish belonging to the family Salmonidae, including trout, whitefish, and salmon. Sandstones A sedimentary rock consisting of sand and a matrix of mineral matter that cements it together. Saturation The concentration of a gas in water when in equilibrium with the partial pressure of that gas in the overlying atmosphere; see Henry’s Law coefficient, partial pressure. Secchi disk A circular disk, white or with black and white quadrants, that is lowered into the water to measure transparency in lakes. Sediment oxygen demand The sum of all biological and chemical processes in sediment that utilize (take up) oxygen. Sedimentary One of the three main rock types, these are formed from pieces of preexisting rock or the remains of once-living animals that settle (sediment) at the Earth’s surface, often in water. Seiche A standing wave in an enclosed bofy of water, best visualized as a liquid oscillating in a cup. Shales A fine-grained sedimentary rock composed of a mud that is a mix of flakes of clay minerals and tiny fragments of other minerals. Shortwave radiation High energy, short wavelength (visible and ultraviolet) radiation; compare to low energy, long wavelength (infrared) radiation. Shunt A pathway that moves something to an alternate course; in the Great Lakes, dreissinids have altered the pathways of phosphorus cycling. Sluice gates A channel (sluice) with a valve (gate) used to regulate flow. Solar spectrum The continuum of the sun’s electromagnetic radiation; in lakes, effectively including ultraviolet, visible, and infrared radiation. Spectral range A particular segment of the solar spectrum. Stability In relation to thermal stratification, resistant to mixing.

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Stable isotope Isotopes of chemical species that are not radioactive, i.e., decay spontaneously; used in aquatic ecology to trace food web operation. Stakeholders Those affected by and/or involved in the decisions regarding environmental policy, management, and regulation. Stamp mills A machine that was used to separate copper from the rock matrix by pounding or stamping. Static Showing little or no change; the opposite of dynamic. Stratification Include summer and winter. Superfund site Locations where toxic chemical dumps have been identified and selected for cleanup under the Comprehensive Environmental Response, Compensation, and Liability Act (Superfund). Sustainable Short for sustainable development, sustainable is defined by the Brundtland Commission as ‘‘development that meets the needs of the present without compromising the ability of future generations to meet their own needs.‘‘ Syncline A fold in rock strata where the layers dip inward from both sides. Taconite A low grade iron ore. Terminal moraines Ridges of unconsolidated glacial drift deposited at the end of a glacier during a standstill in its retreat.

4.17.1

Introduction

It is almost de rigueur that a chapter devoted to the Laurentian Great Lakes begin by pointing out that these systems hold onefifth of the Earth’s freshwater supply; that their basins are home to one-tenth of the population of the US and onequarter of the population of Canada; that one of their tributaries caught fire, many of their fish are inedible and their use as a source of potable water has been called into question. Such an approach might, however, be considered to be ‘so 1980s’ given the advances in public consciousness emerging, after passage of the U.S. National Environmental Policy Act, the Clean Water Act and the Great Lakes Water Quality Agreement more than 40 years ago. Instead, let us first seek to place environmental quality and the concept of sustainable futures in a historical context, the better to see the challenges and opportunities that lie ahead. Consider, for example, the establishment of Yellowstone National Park in 1865, only 7 years after the close of the Civil War. This landmark event, carrying with it the belief that some natural places should be preserved just as God created them, suggests an implicit awareness of unsustainable practices was already in play at that time. This awareness became explicit in 1907 when President Theodore Roosevelt charged that, ‘‘The conservation of our natural resources and their proper use constitute the fundamental problem which underlies almost every other problem of our National life.’’ He encouraged the people of the US to ‘‘look ahead,’’ recognizing that ‘‘to waste, to destroy, our natural resources, to skin and exhaust the land y will result in undermining in the days of our children the very prosperity which we ought by right to hand down to

Thermal bar A band of water at 4 1C that separates warm nearshore waters from cold offshore waters as lakes move toward thermal stratification. Thermal niche A preferred temperature, usually expressed as a range, for example, 1072 1C. Thermal regime The seasonal cycle of stratification and turnover; as used here, especially the timing of initiation and destruction of stratified conditions. Thermocline The plane, within the metalimnion, where the highest rate of change in temperature with depth is observed. Trophic state A reference to the level of plant and animal production in a lake; characterized by nutrient and chlorophyll levels and transparency. Turnover The complete mixing of the water column, top to bottom, following periods of thermal stratification. Upwelling The introduction of cold, bottom waters to the surface of a lake; occurs when winds push surface waters away from the shore and bottom waters rise up to replace them. Warm water fish Fish species with preference for warm waters, for example, yellow perch and smallmouth bass. Wind stress Application of the force of wind, for example, in moving water masses. Zooplankton The microscopic, free-floating animals in lakes.

them.’’ He called on Congress to ‘‘get our people to look ahead and to substitute a planned and orderly development of our resources in place of a haphazard striving for immediate profit.’’ A foreshadowing of twenty first century sustainability, writ large with a big stick. Science at that time was already exploring lake ecosystem structure and function. Noted Harvard professor of natural history, Louis Agassiz, led the first scientific expedition on Lake Superior in 1848. By 1917, a decade after Roosevelt called for a ‘planned and orderly development,’ Birge and Juday were pioneering the field of limnology on the lakes of North America. What would become the field of environmental engineering can be recognized in 1925 when Streeter and Phelps published their studies of the oxygen resources of the Ohio river. The fundamental equation described in this work addressed ‘pollution and the natural purification’ of rivers and eventually became the basis for the wasteload allocation models that sought to engineer discharges in a manner that would not compromise the sustainability of aquatic life. One could go on, chronicling the grass roots environmental movement of the 1960s and the policy and regulatory framework of the 1970s developed in response to public outcry. Yet the point is made. An interested public has long been aware of the need to foster a perspective which would guide sustainable living, an existence in harmony with natural features of the landscape such as the Great Lakes. That the responsible parties failed to do so is evidenced in the selection of Sleeping Bear Dunes on lake Michigan as the Most Beautiful Place in America. The selection was certainly merited but do not look too closely. A visit to the water’s edge reveals rotting emerald piles of the

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

nuisance alga Cladophora strewn with freshwater jewels – the bodies of an invasive fish, the goby. What lies at the root of this failure? Certainly some of the blame can be assigned to what might be called wanton destruction. Although the taconite processing facilities on Lake Superior’s north shore and the copper stamp mills on the south shore selected their locations to insure access to an abundant supply of water, it would be naı¨ve to conclude that there was no eye toward these same waters as a convenient means of waste disposal. The same might be said of the chemical manufacturers located along the Oswego and Niagara rivers in New York and elsewhere on the Great Lakes; the contemporary Areas of Concern and Superfund sites offer condemning testimony in this regard. Yet much of the damage done, much of the environmental legacy, many of the sins of the mothers and fathers, occurred as a result of a lack of knowledge bordering on ignorance. The insidious nature of such a lack of understanding is perfectly described this a paraphrase of a quote attributed to former Secretary of State Donald Rumsfeld ‘‘there are things we know, there are things we don’t know and there are things we don’t know we don’t know.’’ It is the last of these which is most dangerous. What brings us back to this chapter. Some fear, and rightly so, that precious little is known about creating a sustainable future – in the Great Lakes or elsewhere. One may find it challenging to identify an example of a sustainable course of action that did not evolve as a reaction to one which was unsustainable. It is possible to imagine the last of the buffalo hunters riding off into the sunset commenting, ‘‘Well, THAT certainly wasn’t sustainable!’’ Thus, the first challenge may be to develop a sensitivity to the prospect and advent of the unsustainable. The emerging field of biomimicry encourages us to emulate nature; to ask – what would nature do? Would nature have approved the delivery of tons and tons of Florida phosphorus to a phosphorus-limited Great Lakes? Have opened the gates to the sea lamprey and zebra mussel? Have condoned the discharge of a broad spectrum of endocrine disruptors to this ecosystem? Unlikely. Armed with a sensitivity to the need for sustainability, it is then necessary to acquire an in-depth understanding of ecosystem function. This is the domain of science. Decision making, based on carefully vetted scientific principles, provides a regulatory framework for guiding management actions. This is the domain of policy. Finally, technological solutions are designed and implemented, consistent with concepts of sustainability and regulatory frameworks. This is the domain of engineering. In this chapter, scientists, policy experts, and engineers offer perspectives on a sustainable future for the Great Lakes. The presentation focuses on science because, once an ethic of sustainability has been adopted, it is science that sheds light on the things that we do not know. And with a sustainable Great Lakes ecosystem as our vision, the illumination of its behavior and communication of that behavior to future generations of stakeholders becomes our mission (see Chapters 4.7, 4.8, and 4.18).

4.17.2

Formation

‘Bedrock,’ the opening chapter in J. M. Roberts’ A History of Europe begins – ‘‘Our maps lead us to take certain geographical facts for

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granted, even if some have not been there very long. Among them is a set of shapes. We can begin to look for Europe when the subcontinent at the western end of the Eurasian landmass had acquired more or less the physical form it has today, about 10 000 years ago. Then, much was already in place which was going to settle a lot of later history and shape millions of lives for thousands of years.’’ These words could well have been written about the Great Lakes basin, where bedrock was reworked by glaciers leaving the waters, soils, and mineral matrices that supported the region’s development. Here, the authors consider the hypothesis that the cultural and economic development of the Great Lakes basin, even contemporary water quality, reflects a type of environmental determinism: that what we have today is a natural and inevitable consequence of antecedent conditions.

Formation, as used here, has multiple meanings. In geology, the word refers to mappable units of the lithosphere; in limnology to the creation of the depressions which hold lake water, giving those systems their particular physical characteristics. In this section, the word formation also refers to the manner in which geology and geography have mediated the economic and cultural development of the Great Lakes basin. One need only examine an image of the Great Lakes from space at night (Figure 1) to appreciate the manner in which light and thus population are diminished as one moves north. Lights are diminished, water quality improves: why? It begins with the bedrock.

4.17.2.1

Bedrock Geology

Geologists point to the far distant past (4.5 billion years ago) for the origins of the Earth and divide the time that follows into eons, eras, periods and epochs. This first division, containing three Eons, is the Precambrian Supereon. The first Eon within the Precambrian is the Hadean, a reference to the Greek god of the underworld and reflective of the hot and barren nature of the Earth at that time (Bornhorst and Brandt, 2009). The first rocks on Earth (4.0 billion years) and oldest rocks in the Great Lakes region (3.6 billion years) were formed next during the Archean Eon. In the Great Lakes region, rocks of the Precambrian (greater than 500 million years old) include igneous forms such as granite (molten rock, cooled slowly

Figure 1 The Nighttime Lights of North America, a map layer produced by the National Atlas of the USs. The readily apparent diminution of light as one moves north across the Great lakes basin reflects the geological history of the region and the manner in which that geology influenced development. http://nationalatlas.gov

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The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

below ground) and basalt (molten rock, originating from lava flows and cooled quickly above ground), sedimentary forms such as conglomerates, sandstones and shales (laid down in ancient seas), and metamorphic rocks such as gneiss (transformed from igneous and sedimentary materials deep in the Earth; Bornhorst and Brandt, 2009). The years following and leading to the end of the Precambrian were highly dynamic, including 500 million years of erosion (no new rock formation) and periods of orogeny (crustal shift and mountain building) driven by plate tectonics. Here, the bedrock underwent doming (uplift from rising magma), block faulting (sliding along crustal cracks), and folding (syncline formation), which brought Precambrian rock above sea level in parts of the Great Lakes region (Bornhorst and Brandt, 2009). These particularly hard and dense igneous and metamorphic rocks, together with some softer sedimentary forms, are exposed at the surface in northern Minnesota, Michigan’s Upper Peninsula and parts of northern Ontario. Termed the Canadian Shield, this large area of Precambrian rock (formed in the Archean Eon) is the ancient basement of the Great Lakes basin and host to deposits of iron, copper, gold, and silver. Above the Precambrian, at an age of approximately 500 million years, the current Phanerozoic Eon and rock formed in its first Era, the Paleozoic. During this time, the Earth experienced what is termed the Cambrian explosion, an interval in which the diversity of life increased rapidly over a relatively short span of time and for which fossil remains are abundant. One might envision the bedrock geology of the Paleozoic as a series of layers, placed one on the other, with the oldest at the bottom and newest at the surface (Figure 2(a) and (b)), illustrating the principles of superposition and original horizontality (Brandt, 2009). The six periods of the Paleozoic Era (a seventh, the Permian, is missing in the Great Lakes region) have distinctive geologies, reflecting the evolution of life on Earth and, as elsewhere, impacting the formation (considered broadly) of the Great Lakes region. The sedimentary rocks included in the Paleozoic strata were deposited in marine waters at times when the sea flooded the continent. Sandstones and shales were formed through weathering and deposition of Precambrian rocks, limestones (calcium carbonate) and dolomites (calcium–magnesium carbonate), from chemical precipitates and shells deposited on the sea floor, and evaporite deposits (salt and gypsum) resulting from intense evaporation of saline waters. The Pennsylvania Period of the Paleozoic Era saw cycles of marine submergence and emergence supporting the development of extensive wetlands where peat was formed and converted into coal. Thus, the ancient seas played a critical role in the formation of geological resources of economic and industrial significance to the contemporary Great Lakes basin, for example, deposits of:

•

• •

dolomites and limestones mined today on Drummond Island and at Rogers City on Lake Huron and on islands and inland sites adjacent to western Lake Erie and Lake Ontario; salt mined today at Detroit and gypsum extracted at Alabaster, Michigan on Lake Huron’s Saginaw Bay and at Port Clinton, Ohio on Lake Erie; and coal, particularly in the Appalachian region to the south and east of the Great Lakes, ultimately providing the energy

Paleozoic periods

Age (Ma)

Pennsylvanian

Sandstone

Mississippian

Shale

299 318 359

Devonian Limestone 416 Salt and gypsum Silurian 444 Ordovician 488 542

Cambrian Precambrian Supereon

Various igneous and metamorphic rocks 1000

(a) Basin

(b)

Figure 2 (a) Precambrian and Paleozoic rock strata in the Great lakes region. This particular organization likely never existed in its entirety as structural activity reoriented the strata. The relative thickness of the stratum representing each Period is pictured here together with the approximate contributions of various rock types to the formation. This presentation does not represent stratigraphy within a Period, for example, the bedding of sandstones and shales within the Mississippian. Age is in millions of years before the present, Ma. (b) Subsidence and syncline formation tilted the strata, serving to expose bedrock deeper in the sequence at the surface. Reproduced from Michigan Tech.

needed to refine iron ore from the north and give birth to the steel and automotive industries. The extraction of gas from deposits in Marcellus shales of the Devonian Period is a topic of contemporary interest. In the Silurian Period of the Paleozoic, roughly 400 million years ago, that portion of the Earth’s crust centered in Michigan’s Lower Peninsula began to sink or subside (Figure 2(b)), perhaps a response to earlier structural activity, which weakened the crust (LoDuca, 2009). The result was formation of the Michigan basin. The central portion of the basin provided room for sediments to accumulate in Paleozoic seas and, subsequently, erosive forces flattened the system exposing alternating rock strata of differing resistance to scour. The Paleozoic Era came to a close with the building of the Appalachian Mountains, and what is today the Great Lakes basin became an upland region. No rock record remains of the more than 200 million years from the end of the Paleozoic Era (Permian Period) to the Pliocene Epoch (beginning 5.3 million years ago). Asthe Great Lakes region

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

awaited the advance of glaciers in the Pleistocene Epoch (beginning 2 million years ago), geological processes in the Great Lakes region were limited to erosion.

4.17.2.2

Glacial Geology

Compared to the vast period of time embodied in preceding Eras, the million or so years of the Pleistocene Epoch are but a tick of the clock. Beginning approximately 780 000 years ago (Larson and Schaetzl, 2001), the Great Lakes region was host to six periods of glaciation, each lasting for B50 000 years and separated by interglacial periods of several hundred thousand years. Periods of glaciation included one or more advances and retreats, each leaving deposits of glacial drift which, on weathering, left a marker of the timing of that particular glacier’s presence. The last glacial period, the Wisconsin Age, began approximately 79 000 and ended approximately 10 000 years before the present (Larson and Kincare, 2009). The record of the ice movement is well preserved as scratches on the surface of the bedrock and in the drift deposited with the glacier’s retreat. The ice margin moved north of Lake Superior, leaving the Great Lakes basin entirely, approximately 9000 years ago. Both the Precambrian rock of the north and the Paleozoic rock present at the surface in the central and south portions of the Great Lakes basin experienced sedimentation and structural activity, which led to the exposure of bedrock units of differing resistance to scour: extremely hard Precambrian granites and basalts; hard limestones and dolomites; semihard sandstones; and soft shales. Before the Pleistocene ice ages, the Great Lakes region was occupied by the Laurentian river system (Figure 3; Kincare and Larson, 2009) with tributaries flowing (although perhaps not at the same time, Larson and Schaetzl, 2001) through what would become the modern day lake basins; The tributaries eroded the softer strata, creating deep valleys, which would guide the advance of the ice sheets that followed. Subsequently, where the glaciers encountered soft substrate, often within the valleys of preglacial rivers, deep basins were scoured. When the glaciers passed over hard

Figure 3 Before glaciation, a system of rivers traversed the Great Lakes region eroding the surface and cutting valleys which served as a route of advancement for the scouring action of glaciers in the Pleistocene Epoch. Reproduced from Michigan Tech.

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substrate, less erosion occurred and surface features remain today. Although similar in some respects, the formative and glacial geology involving the Precambrian rock of the north differs from that in the south where Paleozoic rock dominates at the surface. The process in the north, leading to the formation of Lake Superior, is summarized in Figure 4. Because Precambrian rock is present at the surface in the north, softer formations vulnerable to glacial scour had to first be revealed by structural activity. In his Geology of the Great Lakes, Hough (1958) describes the relationship between bedrock geology and glaciation in the Michigan basin to the south where the glaciers encountered various Paleozoic rocks. Deposits of resistant dolomite and limestone lie near the surface in the western basin of Lake Erie. Thus, this basin has not been deeply excavated (water depth of 8–12 m) and hard rock surface features remain as Pelee, Kelleys, and the Bass Islands. In the deeper central basin of Lake Erie, the glaciers found deposits of soft shales and excavated the region until meeting deposits of resistant limestone. Finally, in the eastern basin, deposits of shale were thicker and could be eroded more, leaving this the deepest portion of the lake. These patterns of formation, structural activity, and erosion are repeated across the Great Lakes basin, with the glaciers encountering strata of differing resistance to erosion at various locations. Glacial scour was greatest in the north where the ice mass was thickest and lake basins are deeper (a maximum of 406 m in Lake Superior) than in the south, where the glaciers were thinner (a maximum of 64 m in Lake Erie). This differential resistance to scour left behind prominent surface features evident in the islands and shorelines of the Lakes, for example, Precambrian bedrock on Isle Royale and along Lake Superior’s north shore and Paleozoic limestone where the Niagara Escarpment forms the spectacular cliffs of the Bruce Peninsula and the hard strata at Niagara Falls separating Lakes Erie and Ontario.

4.17.2.3

Glacial Lakes

The Wisconsin ice sheet did not disappear all at once. The slow retreat to the north, which eventually revealed today’s lake basins, released torrents of water and foot on foot of glacial drift over a period of thousands of years. The modern basins were revealed incrementally: first as ice-dammed lakes taking on transient forms with names unfamiliar and boundaries only vaguely recognizable to us. The southernmost extent of the glacier immediately before the appearance of the firstknown lakes is described by an ice margin running from Wisconsin to New York and south of all of today’s lake basins. The retreat of the Wisconsin ice sheet left deposits of glacial till called terminal moraines marking different substages in its movement. During each substage, proglacial lakes formed at the ice margin and the direction of their outflow was determined by the height of surrounding land or the glacial ice mass itself (Figure 5). Each retreat northward altered the juxtaposition of the ice margin and ice/land masses, changing the shapes of the transient basins and the direction of outflow to the sea. At various times, the Great Lakes drained to the Gulf of Mexico through the Mississippi river and to the

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The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

(a)

(b)

(c)

(d)

(e)

Figure 5 As the ice margin (green line) receded northward, transient lakes (light green) were formed at the edge of the glacier. Drainage patterns (arrows), here approximately 13 000 years before the present, sent water to the Gulf of Mexico via the Mississippi river and to the Atlantic ocean via the Hudson river. Sedimentation in these proglacial lakes provided the material from which some of the best agricultural soils in the Great Lakes basin developed. Redrawn from Hough, J. L. (1958). Geology of the Great Lakes, p. 313. Urbana, IL: University of Illinois Press.

Atlantic ocean via central New York and the Hudson river. Although these lakes typically covered portions of what would become the modern Great Lakes, they also extended beyond those boundaries. For example, Oneida Lake near Syracuse, New York was once part of glacial Lake Iroquois when that ancestor of Lake Ontario drained to the east via the Mohawk and Hudson rivers, eventually discharging to the Atlantic Ocean (Figure 5). Approximately 10 000 years ago, the last ice sheet retreated to the far north leaving behind the Great Lakes largely as we know them today.

4.17.2.4 (f)

Figure 4 The geological underlayment of the Great Lakes has been influenced by orogenic, erosional, and depositional phenomena over a period of billions of years. This series of panels illustrates features of lake basin formation: (a) The base of the Earth’s crust is pulled apart (extension) as a result of a rising Keweenawan-aged plume of viscous mantle rock material. A more liquid material (magma) generated by mantle melting is erupted through extensional fractures (rifts) as basalt lava flows. (b) During this rift-filling magmatism, periodic erosion results in layers of gravel being emplaced between the lava flows, the gravels become conglomerate upon burial. (c) Hot water containing copper moves upward from within the rift and is precipitated in pore spaces in the tops of lava flows and conglomerates forming tabular (veins and lodes) native copper ore bodies. (d) The region is covered by an ancient sea and sedimentary rocks of the Paleozoic era bury the Precambrian rift rocks. (e) Glaciation scours the surface, removing the softer Paleozoic sedimentary rocks and leaving behind the harder rift and older rocks as ridges around the Lake Superior basin. (f) Glaciers recede and, over time, lake levels recede to the current level of Lake Superior today. Lake Superior lies within a portion of the valley of the 2000 km midcontinent rift, an ancient geological feature which gives the lake its shape. Reproduced from Michigan Tech.

Soils

Several factors interact to define the nature of a regional soil system. One of the most important of these is parent material, here, intact bedrock and great sheets of crushed rock scoured during the NE-SW advance and retreat of the glaciers. This glacial drift includes sorted materials termed till (sand, gravel, cobbles, and huge boulders called erratics) and outwash, homogenous deposits of sand and gravel left behind as glacial melt water velocities declined with distance from the ice margin. The till and outwash layer is 50–350-m thick in the lowland country around Lakes Erie and Michigan and eastern portion of Michigan’s Upper Peninsula (Blewett et al., 2009) and much thinner and even discontinuous in the uplands around Lake Superior (Larson and Schaetzl, 2001). Pre-Pleistocene soils formed over carbonate bedrock would have been pushed south during the early Wisconsin glaciation, leaving mineral-rich parent material in Michigan’s Lower Peninsula, Indiana, and Ohio. The later stages of ice advance did not extend as far south, so the source of parent material in Minnesota, northern Wisconsin and Michigan’s Upper and northern Lower Peninsula would have been the less mineralrich Canadian Shield rocks. Where lakes formed at the front of the ice margin, later disappearing as the glacier receded, clay and marl (calcium carbonate) were deposited on lake plains.

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

Soils then evolve from these parent materials with their character mediated by climate (leaching and weathering), relief (wetland vs. upland sites), and time. Because glaciers departed the Great Lakes basin fairly recently, the soils are young and resemble the parent material. The soil development process got a head start of several thousand years in the south where deglaciation first occurred, benefitting there as well from more accumulation of organic matter due to a longer growing season. Four soil orders (classifications) are of particular importance in the Great Lakes region (Schaetzl, 2009). The first of these is spodosols, a soil order found in northern Wisconsin, the Upper and northern Lower Peninsulas of Michigan, and the Tug Hill and Adirondack regions of New York (Figure 6). These are sandy (excessively drained and low nutrient) and loamy (more water holding capacity, organic matter, and nutrients) soils occurring on outwash plains and sandier tills in upland topographies. Spodosols have been leached of organic matter and most nutrients, leaving little but quartz grains. These soils are unsuitable for agriculture without significant amendment and are today typically covered by mixed conifer–deciduous forests. A second group is the histosols, characterized by thick accumulations of organic matter and largely limited to poorly drained wetland areas in Michigan’s Upper Peninsula. These soils are fertile and, if drained, could support productive agriculture. However, most histosols in the Great Lakes have not been drained and today primarily support forestry and recreation (Schaetzl, 2009). The third order, inceptisols are particularly young and thus lack development. In the Arrowhead region of Minnesota, dry inceptisols (suborder Udepts) are derived from glacial till low in nutrients and rich in cobbles and boulders. This, together with the region’s short growing season, makes them poorly suited for agriculture. A group of wet inceptisols (suborder Aquepts) developed on the clay-rich parent material of the Saginaw Bay

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Lake Plain. When drained, these are among the most productive agricultural soils in the Great Lakes (Schaetzl, 2009). The fourth group is the alfisols, a soil order common to much of Wisconsin, Minnesota, the southern half of Michigan’s lower peninsula, northern Indiana and Ohio, and the Lake Ontario shoreline of New York (Figure 6). Alfisols are very good agricultural soils, developing from loamy till and containing the organic matter and clay that serve to retain moisture and enhance fertility. Stepping back a bit, one might envision the Great Lakes basin being divided into its ‘north woods’ and agricultural regions along a line running between Duluth, Minnesota and Saginaw, Michigan, with a southern lowland region and a northern upland region and with spodosols and histosols to the north and alfisols and inceptisols to the south. At this scale, it is possible to see how the distribution of agricultural activity (Figure 7(a)) and centers of population (Figure 7(b)) in the Great Lakes basin have been influenced by the distribution of soil types and the bedrock geology, glacial activity, and environmental conditions that led to their formation.

4.17.2.5

Billions of Years Later

The Great Lakes of today rest upon a foundation of Precambrian and Paleozoic bedrock that is both ancient and deep. The sedimentary strata of the Paleozoic Era extend almost 5000 m below the surface (Schaetzl et al., 2009) and Precambrian lava flows add an additional thickness of 10 000–20 000 m (Hough, 1958). Since the retreat of the ice, the lake basins have been filling in with materials washed lakeward from the drainage basin. At the deepest point in Lake Michigan, ‘modern’ sediments are as much as 12-m thick, grounded in the pebbly–sandy–clay of glacial till and extending toward the water column through layer upon layer of clay (Hough, 1958). Above the thousands of meters of

Dry inceptisols Spodosols Histosols

Wet inceptisols

Alfisols

Figure 6 Locations of the four major soil orders found in the Great Lakes basin. A line drawn from Duluth, Minnesota through Saginaw, Michigan to Oswego, New York effectively represents the boundary between the less fertile spodosols and histosols of the north woods uplands and the more fertile alfisols and wet inceptisols of the agricultural southern lowlands. Image: University of Idaho, College of Agricultural and Life Sciences, Soil & Land Resources Division (http://soils.cals.uidaho.edu/soilorders/maps.htm).

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Ogoki/Long Lac Diversions

Superior

Mic higa

n

St. Mary’s River

Chicago Diversion

(a)

Huron

St. Clair River Lake St. Clair Detroit River

St. Lawrence River Welland Canal

Ontario Niagara River

Erie

Figure 8 The Great Lakes and their connecting channels. Blue arrows identify major in- and out-basin diversions and white arrows indicate the direction of water flow through the Great Lakes system. Reproduced from Michigan Tech.

section considers the rewards, consequences, and challenges associated with water level management and the interbasin transfer of Great Lakes water.

(b)

Figure 7 Agriculture and population: (a) percentage of land in crops, low (yellow)-high (dark green). Intensity of cropping is used here as a surrogate for soil depth, friability and fertility in the Great Lakes region. Red and black lines indicate southward limits of various glacial stages and (b) population density in the Great Lakes region (2000 census). The association of population with areas supporting agriculture is readily apparent. http://nationalatlas.gov

bedrock, lie comparatively thin layers of water (averaging 19 m in Lake Erie, 147 m in Lake Superior) and soil (a few meters at most). It is these thin surface layers that play host to all life in the Great Lakes and their basins. Although we shall see that anthropogenic perturbation has had a profound impact on Great Lakes’ water quality, even calling into question the ability to insure a sustainable future, much that we have today has evolved from and been a natural and inevitable consequence of ancient antecedent conditions. It begins with the bedrock.

4.17.3

Hydrology

‘‘As the rain and the snow come down from heaven, and do not return to it without watering the earth y,’’ found in Chapter 55 of the Book of Isaiah, reflects an understanding of the hydrologic cycle that is more than 1500 years old. Yet, water resource management remains one of the most difficult tasks facing scientists and engineers in the US and worldwide. In providing support for the management process, these experts seek to apply advances in technology to manipulate the hydrologic cycle for the apparent benefit of society. This

The Great Lakes are an abundant natural resource, representing approximately 20% of the world’s freshwater supply. Nearly 1 billion people worldwide lack access to safe drinking water (United Nations, 2011), and 80% of the world’s population is threatened by water stress (Vo¨ro¨smarty et al., 2010). If it could somehow be distributed to the far ends of the planet, a diversion of only 10% of the average outflow from Lake Superior would supply those 1 billion people with ‘basic water access,’ that is, 20 l of freshwater per day for drinking, cooking, and personal hygiene (Howard and Bartram, 2003). Given the enormity of the resource and critical nature of the demand, one would expect the hydrology of the Great Lakes basin to be well understood and its management under careful control. It is perhaps surprising then to learn that the hydrologic cycle of the Great Lakes is still the subject of intensive inquiry. Even with control structures at the outlets of Lakes Superior and Ontario, we presently have less ability to regulate water levels, manage discharges, and evaluate the impacts of future water diversions here than in many other water resource systems.

4.17.3.1

Flow Patterns and the Hydrologic Budget

Water flows from one of the Great Lakes to another through connecting channels (Figure 8): Superior-Huron and Michigan through the St. Marys river, Huron-Erie through the St. Clair and Detroit rivers, Erie-Ontario through the Niagara river, and Ontario-the Atlantic Ocean through the St. Lawrence river. In addition, in-, out- and within-basin diversions have been constructed to support water resource management objectives. A hydrologic budget, tabulating inputs and outputs of water, provides a means of examining the factors controlling lake levels and significance of interbasin transfers. Such a budget for the Great Lakes (Table 1) would include inflow from connecting channels, precipitation on the lake surface and

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Table 1 Great Lakes water budgets: average annual values expressed as cm distributed over lake area. The ranges of allowable lake outflows under current regulation plans are also shown for Lakes Superior and Ontario

Precipitation Evaporation Runoff Diversions Channel flow in Channel flow out Lake area (km2)

Superior

Michigan–Huron

Erie

Ontario

77 54 54 6 0 83 (41–128) 82 100

81 59 64 3 58 142 117 400

86 86 86  25 650 710 25 700

83 67 150 33 960 1180 (910–1460) 18 960

Source: Reproduced from Lee, D. H., Quinn, F. H., Sparks, D. and Rassam, J. C. (1994). Modification of Great Lakes regulation plans for simulation of maximum Lake Ontario outflows. Journal of Great Lakes Research 20(3): 569–592.

177.4 177.2 177.0 Lake elevation (m)

176.8 176.6 176.4 176.2 176.0 175.8 175.6 175.4 1900

1920

1940

1960

1980

2000

Figure 9 Monthly Lakes Michigan–Huron water surface elevations, 1900–2000. Data accessed from the NOAA Great Lakes Environmental Research Laboratory (ftp://ftp.glerl.noaa.gov/publications/tech_reports/glerl-083/UpdatedFiles). Data from Croley II, T. E., Hunter, T. S. and Martin, S. K. (2001). Great Lakes Monthly Hydrologic Data, NOAA Technical Report #TM-083. Ann Arbor, MI: NOAA Great Lakes Environmental Research Laboratory.

runoff from the watershed as inputs, and evaporation and outflow through connecting channels as outputs. Diversions may represent inputs or outputs depending on the direction of flow. Contributions to the hydrologic budget are presented here as centimeters of water distributed over the lake surface annually, a convenient means of visualizing water quantity in these immense systems. These can be converted to flows by multiplying by the lake area (Table 1). For the upper Great Lakes (Superior and Michigan– Huron), precipitation on the lake surface exceeds evaporation by an average of approximately 40 cm yr1, with runoff from the contributing drainage areas providing a little more than 100 cm yr1 of additional water. For Lakes Erie and Ontario, precipitation and evaporation are roughly balanced, and runoff from their watersheds provides approximately 70 and 90 cm yr1 to the lakes, respectively. Although diversions or interbasin transfers of Great Lakes water may have a relatively small effect on the overall hydrologic budget in a given year (averaging 11 cm year1 Table 1), they may have significant impacts on lake levels over a multiple year period (see Section 4.17.3.4).

Average annual values do not tell the entire story, however. There is a regular seasonal pattern (but also considerable seasonal variability) in the hydrologic cycle and thus in lake levels. Runoff from precipitation and snowmelt tend to increase lake levels in the spring and early summer, whereas higher rates of evaporation lead to falling lake levels in the fall. Evaporation from the upper Great Lakes is typically very low in the summer. A stable boundary layer of cool air sets up over these relatively cold waters, becoming nearly saturated with water vapor and thus limiting evaporation. Winter ice cover on the upper Great Lakes reduces evaporation by offering less area where water is in contact with the atmosphere. However, sublimation of ice and evaporation from ice-free regions continue to represent significant losses in the overall water budget. Interannual variability in the hydrologic budgets of the lakes can also be substantial, leading to sustained periods of high or low lake levels. For example, the 1920s were a period of record low levels on the Upper Great Lakes, whereas the 1980s saw record high levels (Figure 9). The primary impacts of high lake levels are shoreline erosion and property damage, whereas low lake levels can restrict navigation, lake access and

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The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

recreational boating, and hydropower production. Reduced levels of discharge from connecting channels resulting from low lake levels may also have significant environmental impacts; for instance, sturgeon spawning may be adversely affected by low flow in the St. Marys river in the early summer. However, a certain degree of variability in lake levels can have environmental benefits. As an example, the health of coastal wetlands depends on both intra- and interannual cycles of drying and inundation mediated by exchange with the main lake (Wilcox et al., 2005). Consumptive use, defined as ‘‘that portion of water withdrawn or withheld from the Great Lakes basin and assumed to be lost or otherwise not returned to the Great Lakes basin due to evapotranspiration, incorporation into products, or other processes’’ (Great Lakes Commission, 1989), is typically not included in hydrologic budgets because of its relatively small magnitude. Estimates of consumptive use in the Great Lakes basin have been approximately 5% of total withdrawals (International Joint Commission, 1999). It should also be noted that while there is considerable uncertainty in consumptive use estimates, total consumptive use in the Great Lakes basin is considered to be lower today than 20 years ago due to reduced withdrawals by all water use sectors, with the largest reductions in the industrial and power generation sectors (Kenny et al., 2009).

4.17.3.2

Water Level Regulation

Great Lakes water levels have a large impact on the region’s economy, indeed the national economies of Canada and the US. Accordingly, legal doctrines directing the management of the Great Lakes by the two countries date back more than 100 years to the Boundary Waters Treaty of 1909, establishing the International Joint Commission (IJC) to monitor transboundary environmental agreements. The Orders of Approval of 1914 established the IJC International Lake Superior Board of Control to facilitate increased hydropower development on the St. Marys river and regulate the lake’s outflow. At that time, the specified purposes of regulation were commercial navigation, hydroelectric power generation, domestic and sanitary uses, and irrigation; environmental, recreational, and shoreline property impacts (flooding or low levels) were not addressed by these early agreements. The regulation of Lake Superior is generally considered to have begun in 1888, when a railroad trestle was built across the St. Marys river, restricting the river’s discharge capacity (Coordinating Committee, 1994). In the 1890s, the US and Canada constructed diversion canals for hydroelectric plants, which increased the total flow capacity of the river. In 1901, construction of ‘compensating works’ – sluice gates to regulate the river’s flow – began, and by 1914, navigation and power canals were added. By 1921, the compensating works had been expanded to a 16-gate structure approximately 300 m in length, providing modern-day control of the outlet of Lake Superior. Since 1921, several Lake Superior regulation plans have been in place, with plans typically being modified, or new plans adopted, following periods of extremely high or low levels (e.g., 1920s, 1960s, and 1980s). The current regulation plan, which primarily attempts to balance the levels of Lake Superior and those of Lakes Michigan and Huron within their historical ranges, has been in effect since 1990 (Clites

and Quinn, 2003). In 2007, IJC appointed the International Upper Great Lakes Study Board to update the regulation plan for Lake Superior. Major topics for investigation include improving understanding of the factors that affect water levels and flows (including potential impacts of climate change), developing and testing alternative new regulation plans, and assessing the impacts of these alternative plans on the ecosystem and human interests using updated environmental and socioeconomic data (International Joint Commission, 2011b). The only other Great Lake that is regulated is Lake Ontario, hydraulically separated from the other lakes by Niagara Falls. Regulation of Lake Ontario outflows began in 1960 with the completion of Canadian and US hydroelectric power facilities in the St. Lawrence river, just downstream of the lake’s outlet. Navigation locks are also operated in the Canadian and US portions of the river. From 1960 to 2000, three plans were used to regulate Lake Ontario outflows, all designed to reduce the range of fluctuations in Lake Ontario levels while maintaining flows in the St. Lawrence to facilitate navigation, ensure adequate hydropower production, and protect downstream riparian interests. Although Lake Superior’s outflow is adjusted monthly, Lake Ontario’s outflow is adjusted weekly because it is a smaller lake yet receives higher inflows, and thus its water levels change much more quickly in response to variable hydrologic conditions than do the levels of Lake Superior (Yee et al., 1993). In 2000, the International Lake Ontario–St. Lawrence River Study was initiated to update regulation plans for that system. After 5 years of data collection, plan development and evaluation using simulation modeling, and public meetings at which a wide range of preliminary options were presented, three candidate regulation plans were selected for evaluation by the Study Board. However, none of the plans was able to satisfy the range of demands and competing interests in the Lake Ontario–St. Lawrence system, under all hydrologic conditions, without harm to some interests. Work toward an IJC Order of Approval for a revised regulation plan continues to move forward in consultation with the federal governments of Canada and the US (International Joint Commission, 2011a).

4.17.3.3

Regulating Diversions

Approximately 15 in-, out- and inter-basin diversions have been constructed on the Great Lakes. Chief among these are the Ogoki and Long Lac diversions to Lake Superior (c. WWII, for hydroelectric power and log transport), the Chicago diversion (c. 1900, for shipping and wastewater diversion to the Mississippi river), and the Welland canal from Lakes Erie to Ontario (c. 1829, as a shipping bypass of Niagara Falls) (Figure 8). Today, management of water resources in the Great Lakes is guided by the Great Lakes–St. Lawrence River Basin Water Resources Compact of 2008, a legally binding agreement among the eight US states in the Great Lakes basin. In general, the compact seeks to ban diversions of Great Lakes water, with some limited exceptions, and set water use and conservation standards within the basin. Although widely lauded as a tool for consensus-based water management in the basin, all of the eight states missed the deadline in 2010 to submit water conservation programs, raising concerns about compact implementation.

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

Concern about the potential effects of large withdrawals and diversions from the Great Lakes basin was a major driving factor behind the Great Lakes Compact, and political debate over regulation of water withdrawals will likely continue well into the twenty-first century. In 2011, the state legislature of Ohio approved a bill allowing withdrawals from Lake Erie of up to 5 million gallons per day without a state permit, a volume 50 times greater than allowed by most of the other Great Lakes states. Proponents of the Ohio bill argued that a withdrawal of this magnitude would have an insignificant impact on the lake while potentially promoting economic growth in the region, and thus it is interesting to estimate exactly what the hydrologic impact would be. Assuming none of the water withdrawn is returned to the lake (which is highly unlikely if use is within the basin), and assuming inflows and outflows remain constant, the net effect of diverting 5 million gallons per day from Lake Erie would be a 0.027-cm drop in the lake level by the end of the year. Alternatively, the potential impact may be estimated roughly by considering that 5 million gallons per day is approximately 0.004% of the average discharge from the lake. If use is within the basin, the impact would almost certainly be even smaller, considering average consumptive use in the Great Lakes basin is less than 5% of water withdrawals, and thus more than 95% of the withdrawal would likely recycle back to the lake. This is not to suggest that regulation of withdrawals from the lakes and their contributing watershed areas is unnecessary, but rather the intent is to emphasize the need for an appreciation of the vast water resources of the basin and understanding of the relative scale of human activities. Indeed, large diversions from tributary rivers and streams can have significant local (watershed scale) impacts, even if they do not have significant impacts on lake levels themselves. For example, the same Ohio water resources bill sought to allow diversions of up to 2 million gallons per day from rivers and groundwater in the Lake Erie basin, which could have significant watershed-scale impacts, especially during periods of low flow. In addition, there is concern that approval of a single large diversion would be precedent setting and ‘open the faucet’ for other large diversions that have significant cumulative impacts (Peterka, 2011).

4.17.3.4

Planning for the Future

Proper evaluation of the hydrologic impacts of water withdrawals from the Great Lakes basin – indeed from any watershed – requires the use of models that account for the interactions of the various components of the hydrologic cycle and for the effects of regulation. The global climate models (GCMs; also general circulation models) used to predict future climatic conditions do not provide a consensus forecast for the Great Lakes region. Average forecasts from 16 GCMs have indicated that Great Lakes basin average annual temperatures will increase by 4–6 1F and precipitation changes will range from  5% to þ 15% by 2050 (Karl et al., 2009). Output from two GCMs were input to a watershed model by Lofgren et al. (2011) to predict hydrologic impacts. The results indicated that, depending on the lake, average annual net basin supply (NBS; the net inflow of water from natural sources over a given period, typically 1 month) could fall by approximately 51% or

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increase by 11% in 2081–2100 compared to 1981–2000. For Lakes Michigan and Huron, predicted changes in lake level range from a drop of 0.99 m to a rise of 0.41 m, depending on the GCM applied (Lofgren et al., 2011). These changes would occur due to changes in precipitation, evaporation, and runoff but may be mitigated to some extent by adjustments to the Lake Superior regulation plan. Such changes in NBS would potentially dwarf any plausible impact of diversions. This conclusion is well supported by three scenarios simulated using a Lake Superior regulation model developed at Michigan Technological University (Watkins, 2011): (1) A base case scenario with 1900–2008 hydrology; (2) a diversion scenario, with the same hydrology and a 1 billion gallons per day diversion from Lake Superior; and (3) a climate change scenario, with no diversion but a 10% decrease in NBS for the upper Great Lakes (Superior, Michigan, and Huron) compared to the historical hydrology. The model primarily attempts to balance lake levels on Superior and Michigan–Huron while providing adequate flow in the St. Marys river for navigation, hydropower, and fisheries. The results, shown in Figure 10, indicate significantly greater impacts under the climate change scenario than under the diversion scenario. Further, large-scale diversions from the lakes could be partially offset by water level regulation, a less viable option for dealing with climate change impacts.

4.17.3.5

Perspective

The Great Lakes represent a vast amount of freshwater, perhaps so vast that volumes in the Lakes and flows in the connecting channels are difficult for many people to fathom. In fact, the volumes and flows are so great that even the uncertainties in their estimation represent vast quantities of water. Although interbasin water transfers in the twenty-first century may certainly affect water resources on local or watershed scales in the Great Lakes basin (as they already have in some locales in the twentieth century), it is questionable that they would have significant effects on lake levels. Great amounts of resources would have to be spent to divert water at such a large scale. Evidence suggests, however, that climate change is more likely to have significant impacts on the Great Lakes on all scales. This will call for refocusing regulatory efforts on the development and adoption of adaptive management strategies for a variable and changing Great Lakes climate. Ongoing lake regulation studies seek both to develop adaptive management strategies to monitor and assess climatic conditions and to refine the regulation plans as needed; similar strategies are required at local and watershed scales throughout the Great Lakes basin.

4.17.4

Physics

A lake is not a static pool of water. Photons leaving the sun arrive in a mere 8 min, penetrate the lake surface and then move through the water column fueling photosynthesis and undergoing conversion to heat energy. Warming, cooling, and the action of the wind place water molecules in constant motion as lapping waves, sweeping currents, and spinning eddies y cycling eternally among the vapor, liquid, and ice phases. Physics est rex rgis y physics rules. The case may be made, in Latin or in the vernacular, that physical phenomena

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184.0 183.8

Lake level (m)

183.6 183.4 183.2 183.0 182.8 182.6 182.4 182.2

Superior

Historical

Diversion

Climate change

182.0 0

200

400

600 800 1000 Month since initiation of simulation

1200

1400

178.0 177.5

Lake level (m)

177.0 176.5 176.0 175.5 175.0 Michigan−Huron

Historical

Diversion

Climate change

174.5 0

200

400

600 800 1000 Month since initiation of simulation

1200

1400

Figure 10 Monthly lake elevations simulated with historical net basin supplies under three scenarios: (1) Baseline case, (2) Diversion from lake Superior (1 billion gallons per day), and (3) 10% reduction in net basin supplies for Lakes Superior and Michigan–Huron. (ftp://ftp.glerl.noaa.gov/ publications/tech_reports/glerl-083/UpdatedFiles). Data from Croley II, T. E., Hunter, T. S. and Martin, S. K. (2001). Great Lakes Monthly Hydrologic Data, NOAA Technical Report #TM-083. Ann Arbor, MI: NOAA Great Lakes Environmental Research Laboratory.

are of singular importance in governing lake behavior. The exchange of energy across the lakes’ surfaces regulates temperature, dictating the timing and extent of vertical and horizontal mixing. Working in concert with temperature gradients, winds generate the currents and eddies, which mediate the transport of suspended sediments, organisms, nutrients, and pollutants.

4.17.4.1

Light

As discussed subsequently in relation to Energy Cycling, sunlight is the ultimate source of energy to the lakes (see Chapter 4.18). Of the visible (shortwave, 350–700 nm) light falling on a lake, approximately 1% is reflected at the surface and a few percent is scattered back to the surface by suspended material and water itself. The majority of the light penetrates the water column and is absorbed by dissolved organic matter (DOM), suspended mineral particles, and pigmented organisms and is ultimately converted to heat. The concentrations of these materials determine the rate at which light is absorbed with depth and thus the locations where heat is generated and transferred. A small but critically important fraction of the

light fuels photosynthesis, the process that forms the base of the aquatic food web. The spectral range absorbed by plants is termed photosynthetically active radiation (PAR) and consists of wavelengths 400–700 nm. Light attenuating materials absorb photons in a characteristic range of the solar spectrum, and the light that is eventually scattered back to the surface gives lakes their characteristic color. Backscattered light can be detected by satellites permitting remote sensing of chlorophyll, DOM, and suspended sediment. The ratio of light remaining at any depth to that penetrating the surface is used to calculate the optical depth, a measure of water transparency. For example, although 99% of green light (wavelength 530 nm) is absorbed by the time it reaches a depth of 3 m in Lake Erie, it is not until a depth of 50 m that this wavelength is virtually extinguished in Lake Superior (Barbiero and Tuchman, 2004). Thus, differences in transparency (optical depth) provide an indication of trophic state (algal biomass) as well as discharges and resuspension of suspended solids. The simple Secchi disk (Figure 11(a)) provides a valuable, if less technically refined, measure of transparency. In 1850, an expedition on Lake Superior led by Louis Agassiz used a tin cup as a surrogate for the Secchi disk and estimated transparency to be

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373

(a)

Surface 30 m

Surface waters are well lit year around and warm to approximately 18 °C in summer. Light extends to a depth of about 30 m. Below that depth, it is always dark and the water temperature remains at 4 °C. (b)

300 m

Figure 11 (a) Secchi disk transparency varies widely across the Great Lakes, from the waters of western lake Erie laden with sediment and algae to lake Superior where, in spring, the absence of particles that absorb and reflect light leave the water looking so clear it is almost black. Images, left to right – http://courses.washington.edu/uwtoce06/webg3/methods.html; http://www.serc.si.edu/labs/phytoplankton/primer/ hydrops.aspx, and http://www.benmeadows.com/LaMotte-Secchi-Disk-Weighted-Rope_31226299/. (b) Our interactions with lakes occur mostly in well-lit waters. In lake Superior, this photic zone extends more deeply than in the other Great Lakes, yet the majority of the water column is both dark and cold all through the year. Reproduced from Michigan Tech.

13 m. Today, Superior’s transparency remains the best of the Great Lakes, with depths up to 20 m reported. Transparency regulates the depth of the photic zone, that is, the stratum of surface water in a lake that can support photosynthesis. Despite its great clarity, only a small portion of the Lake Superior water column is within the photic zone; a majority of the lake remains forever dark (Figure 11(b)). The activities of aquatic organisms can influence light attenuation and thus the depth of penetration; for example, algal growth in the western basin of Lake Erie reduced Secchi disk depths to as little as 2 m, inducing light limitation in algae below that depth. Exploding populations of dreissenid mussels, voracious filter feeders, have clarified the waters of the Great Lakes (Kerfoot et al., 2010; Vanderploeg et al., 2010) increasing light penetration and expanding the bottom area available for colonization by Cladophora, an attached green alga that grows to nuisance proportions in Lakes Erie, Michigan, and Ontario (see Section 4.17.5; Auer et al., 2010).

4.17.4.2

Diffusive Mass Transport – Temperature and Thermal Stratification

Heat transfer in lakes may occur directly by exchange between the water and overlying air, through evaporation (removes

heat), condensation (adds heat) and melting of ice (absorbs heat from surrounding water), and (locally) through tributary or point source discharges. Direct heating from the sun is, by far, the largest input to the system. A critical aspect of freshwater is its temperature of maximum density, 4 1C. In the vertical, this means that a water mass that is either warmer or colder will ‘float upon’ a water mass at 4 1C. The same situation evolves in the horizontal, with dense 4 1C water acting as a barrier separating warmer and colder waters. Diffusion refers to the movement or transport of mass and temperature due to random motion or mixing (Chapra, 1997). Although molecular diffusion occurs slowly and at a microscopic scale, turbulent diffusion occurs more rapidly and over a much larger scale due to the formation of eddies of various sizes (Figure 12(a) and (b)) that serve to reduce concentration gradients. Although turbulent diffusion occurs at scales much larger than those of molecular diffusion, there remains a great range of mixing intensities within the turbulent category. For example, the degree of vertical mixing observed in the surface layers of lakes (exposed to wind energy; Figure 12(b)) can be as much as a million times greater than that experienced in deeper layers (Chapra, 1997). The vertical and horizontal layering resulting from thermal stratification plays an important role in establishing the magnitude of

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Figure 12 (a) The intensity of turbulent mixing increases as the size of the water body increases. In the Great Lakes, winds blow across great distances, generating eddies and mixing surface waters at rates hundreds of times greater than in small inland lakes. Image: http:// americanhistory.si.edu/onthewater/exhibition/4_2.html. (b) The gyres or turbulent eddies, which promote mixing in lakes, vary in size from a few millimeters to tens of kilometers. Eddies are visible through tracking of their transport of heat or materials. The image below of sea ice transport in the Bering Sea provides a most dramatic visualization of eddies. Image http://eol.jsc.nasa.gov/scripts/sseop/ photo.plframe=N&mission=STS045&roll=79

turbulent diffusion, profoundly influencing the transport of temperature, nutrients, and dissolved gases and thus ecosystem function and water quality (see Section 4.17.5). Vertical layering is observed in the form of summer and winter stratification. Summer stratification is characterized by warm surface waters floating on top of colder, denser underlying water. The onset of stratification occurs when the surface water warms to above 4 1C and density gradients offer resistance to mixing of the water column. Stable stratification occurs as early as May for Lake Erie and as late as August for Lake Superior with considerable interannual variability in all the lakes. Once stratification is established, the lake becomes

divided into a surface layer that is well mixed by the wind and in contact with the atmosphere and a relatively isolated deep layer. The surface layer is termed the epilimnion (epi ¼ above, limnion ¼ lake) and the bottom layer, hypolimnion (hypo¼ under). An intermediate zone exhibiting a thermal gradient is called the metalimnion (meta ¼ changing) with the thermocline defined as the plane of maximum temperature/density gradient (Figure 13(a) and (b)). Throughout the summer the mixed layer warms and deepens as it absorbs solar energy and is mixed by winds (Figure 13(b)). High winds deepen the position of the thermocline, entraining, and subsequently warming colder hypolimnetic waters. During periods of weak mixing, several ‘epilimnetic’ layers may form with the metalimnion appearing as a series of steps rather than as a single region of transition. In the shallow western basin of Lake Erie, the mixed layer can extend all the way to the bottom, and in its deeper but still shallow central basin (24 m), the hypolimnion is compressed to only a few meters thickness by the end of summer. A key consequence of summer stratification is that the epilimnion, the stratum receiving tributary inputs and the only point of contact with the atmosphere, becomes chemically and biologically isolated from the hypolimnion (see Section 4.17.5). As solar inputs decrease and winds increase in the fall, the net heat flux becomes negative, that is, heat leaves the lake at the surface. The mixed layer cools and ultimately approaches 4 1C where the absence of vertical density gradients leads to complete mixing of the water column (fall turnover). At this point, and on into winter, the water column is homogenous with respect to temperature (4 1C) and density and chemical and biological species are thoroughly mixed from top to bottom. Less well known is the process of inverse or winter stratification, which occurs because water becomes less dense as it is cooled from the 4 1C temperatures of fall toward the freezing point. Winter temperatures range from zero or near zero at the surface to 4 1C at depth (Figure 13(a)). In Lakes Michigan and Superior, this state can persist from January or February to April (Figure 13(b)). Spring warming of surface waters returns the water to a homogenous condition at 4 1C leading to spring turnover. The water is again thoroughly mixed for 1–3 months until the summer regime is established. The Great Lakes are thus classified as dimictic (miktos ¼ mixed) systems, turning over twice a year. All of the Great Lakes exhibit a coastal phenomenon in spring known as the thermal bar. Before the onset of summer stratification, solar input rapidly warms the shallow waters of the nearshore. A density gradient develops extending from the shallow, warm nearshore water to cold offshore water, with an interface at the 4 1C density maximum (the thermal bar) that inhibits mixing. Water approaching the density barrier from either side sinks slowly (B1–20 cm min1; Holland and Kay, 2003) forming a vertical convective mixing cell on each side of the thermal bar (Figure 14(a) and (b)). The thermal bar usually forms in the month of April on Lake Erie; May on Lakes Huron, Michigan, and Ontario; and June on Lake Superior (Ullman et al., 1998). Tributary discharges of suspended sediment and nutrients may be trapped shoreward of the thermal bar, potentially influencing phytoplankton dynamics in spring. The position of the thermal bar (4 1C density maximum) propagates offshore over a period of weeks

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to months (Figure 14(b)); the rate of propagation influenced by bathymetry and wind stress (Rao et al., 2004). Floating debris collects at the bar, as the convective cells move water toward the interface (Figure 14(a)). Here, the bar becomes visible at the surface and sportfishing boats can often be found tracking this ‘scum line’ and the fish that feed on the insects and other debris that accumulate at the surface. Climate change is recognized as being capable of influencing the thermal properties of lakes with attendant impacts on organism physiology, population abundance, and community and food web structure (Shimoda et al., 2011). Changes in the thermal regime would be expected to appear first on the scene, and this expectation has been validated by recent research reporting that Great Lakes ice cover declined by an average of 71% over the interval 1973–2010, with reductions of 79% and 88% reported for Lakes Superior and Ontario, respectively (Wang et al., 2012). Predictions based on general circulation models indicate that Lake Erie will be 96% ice free by 2090 (Lofgren et al., 2002). The reduced albedo (reflection) associated with ice free-conditions leads to more absorption of shortwave radiation, an earlier onset of thermal stratification (Austin and Colman, 2007; Wang et al., 2012) and warmer summer average temperatures (Austin and Colman, 2007; Figure 15). Nested physical and biological models, driven by general circulation models, have been applied to examine the impact of climate change phenomena on

ecosystem structure and function (Lehman, 2002). Simulation results point to increases in surface and bottom water temperatures of as much as 5 1C during this century, a longer duration and stability of thermal stratification and deeper daily mixing depths during peak thermal stratification. In all but Lake Erie, the duration of nutrient limitation of algal growth is projected to increase and light limitation associated with the deeper mixing may limit algal growth (Lehman, 2002). In the upper food web, habitat warming would lead to a shift in fish populations from warm water to cold water species as competition for space within thermal niches increases. Further, attendant rapid changes in water level would reduce the efficacy of wetlands and littoral regions as spawning nursery areas (Meisner et al., 1987).

4.17.4.3

Advective Mass Transport – Wind and Currents

Advection is movement with the bulk flow, that is, transport is unidirectional (diffusion moves mass away from higher concentrations in both directions) and does not change the identity of the substance being transported (reduce concentration gradients) (Chapra, 1997). In lakes, advection occurs not only through the action of currents, formed primarily through wind action, but is also induced by temperature gradients. Although diffusive eddies act to reduce

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0 Figure 14 (a) Water driven toward the thermal bar from either direction sinks on reaching the density maximum present at 4 1C setting up a convection cell and causing floating materials to accumulate where they meet – the ‘scum line’ well known to those fishing for trout and salmon on the Great Lakes. (b) Continued warming of nearshore waters and erosion of the cold mass by convection at the interface causes the thermal bar to move lakeward in the period before thermal stratification.

concentration gradients locally within the water column, currents serve to transport nutrients, plankton, ice, and even boats in distress long distances within the lakes. Currents are formed and driven by a combination of wind stress and thermal gradients, modulated by bathymetry (Bennington et al., 2010). Winds have the greatest impact in offshore waters, whereas thermal gradients act mainly in the coastal zones. Wind stress is spatially nonhomogeneous, that is, the intensity of a west wind varies along a north–south transect. This leads to vorticity, a curl in the wind velocity field, which imparts a rotation to current structure. In the Great Lakes region the wind stress curl is typically counterclockwise (cyclonic) and is a major driver of the cyclonic circulation patterns observed in Lakes Superior and Michigan. Wind stress curl is probably significant in the other lakes as well (Schwab, 2003; Bennington et al., 2010). Mean circulation patterns reflect average annual water flow, the travel of a water mass and its constituents over many months of transport. A cyclonic mean circulation is characteristic of all the Great Lakes (Beletsky and Saylor, 1999) except Lake Erie where flows are west-to-east along the north and south shores with a return flow to the west in the center. The first comprehensive map of Great Lakes currents was produced by Harrington (1895), who released and tracked the recovery of floating bottles. Harrington’s maps of surface currents (Figure 16(a)) hold up well today for the open waters of the

largest lakes (Superior, Huron, and Michigan), reflecting the typical cyclonic mean circulation. Computer models, supported by modern instrumentation have dramatically improved the accuracy and resolution of current studies (Figure 16(b)) and provide a capacity for tracking chemical spills and disabled vessels as well as the transport of water quality constituents. Current patterns and magnitude can vary significantly from the mean circulation over periods of hours to months. Winds speeds are greatest in winter, generating higher current speeds and shifting the position of circulation cells from that of summer (Figures 16(c)). Although conceptually appealing, the concept of mean circulation fails to reflect the dynamic nature of current structure imparted by coastlines, seasonally variable wind fields, and short-term forcing by storms. These short-lived or intermittent currents are superimposed on mean circulation patterns and are often much stronger and sometimes in opposite directions to the average flow. For example, an anticyclonic current pattern observed in summer in the southern basin of Lake Michigan contrasts with the mean cyclonic pattern. This feature of summer conditions varies dramatically in size and intensity; in some years expanding north to Milwaukee and in other (warm) years being entirely absent (Beletsky et al., 2006). Transient currents are more relevant to some processes than mean circulation, for example, in dispersing fish larvae, which are subject to conditions of water

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Figure 15 Reductions in the extent and duration of ice cover have been observed in the Great lakes over the past three decades. The satellite image in the top panel (NOAA NWS) shows nearly complete ice cover on lake Superior in early March of 2008–09. On the same date in the following year, there was almost no ice present on the lake. Panels at the bottom left illustrate the trend of increasing summer average temperature paralleling reductions in ice cover. In a similar fashion, the lake becomes thermally stratified at an earlier date in years when ice cover is low. Redrawn from Austin, J. A. and Colman, S. M. (2007). Lake Superior summer water temperatures are increasing more rapidly than regional air temperatures: A positive ice-albedo feedback. Geophysical Research Letters 34, L06604. doi:10.1029/ 2006GL029021.

movement when and where they hatch, not to long-term mean circulation patterns (Beletsky et al., 2007). Currents in deep water do not necessarily mirror those at the surface, especially when the lakes undergo thermal stratification. In winter, depth-averaged water movement typically tracks surface flows, and the resulting currents are relatively slow but can transport large volumes of water. During summer stratification, surface currents differ significantly from water movement below, with high velocity coastal jets typically confined to the surface. The strongest surface currents in Lakes Michigan and Superior are seen during October and November when strong autumn winds act on water that has retained its summer stratification and horizontal thermal gradients (Bennington et al., 2010). Absent wind forcing, water movement can be induced by horizontal thermal (density) gradients. In summer, as warm coastal water begins to flow over colder offshore water the Coriolis force drives it to the right, which generates counterclockwise longshore currents. Steeper temperature gradients yield faster flows and thus these density currents are most important in the warmest months. In Lake Superior, the strong east-flowing coastal jet, known as the Keweenaw, Current reacts rapidly to changes in wind forcing, adjusting (and even reversing) direction in response to shifts in wind velocity or

temperature gradients in as little as 1 day (Bennington et al., 2010). Longshore, density-driven flow may be reversed in fall when differential cooling in the coastal zone can lead to colder water in the nearshore than exists offshore. Another form of water movement is that associated with a seiche (pronounced saysh), a periodic ‘sloshing’ of water in a basin much as coffee in a cup (Figure 17(a)). A seiche is initiated with a wind ‘set up’ or pressure front that pushes water toward one shore, whence it returns across the lake in the form of a wave. Because there is always some wind, small seiches of a few centimeters are continuous throughout the lakes. These oscillations act to flush coastal wetlands, much as tides do in the oceans, and can range in magnitude from the daily fluctuations of 5 cm observed in the main basin of Lake Huron to 20 cm at the east and west ends of Lake Erie (Trebitz, 2006). The ‘sloshing’ associated with seiches occurs at regular, lake-specific intervals with periods (high water-low water-high water) of 2–14 h. Seiches have their greatest impact at the ends of the long axes of the lakes (e.g., western Lake Erie and Duluth harbor in Lake Superior) or at the ends of inlets such as Green Bay and Saginaw Bay. A related phenomenon, the storm surge, results from an unusually large wind set up and can lead to extreme fluctuations on water level at the shoreline poles of the oscillation. A low-pressure

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Figure 16 Lake Superior currents: (a) cyclonic circulation patterns from 1892–94 (reproduced from Harrington, M. W. (1895). Surface currents of the Great Lakes. Weather Bureau Bulletin. Washington, DC: U.S. Department of Agriculture), (b) 2011 current forecasts developed by NOAA’s Great Lakes Environmental Research Laboratory and the National Weather Service, and (c) computer simulations of summer and winter current patterns (reproduced from Bennington, V., McKinley, G. A., Kimura, N. and Wu, C. H. (2010). General circulation of Lake Superior: Mean, variability, and trends from 1979 to 2006. Journal of Geophysical Research-Oceans 115, C12015. doi:10.1029/ 2010JC006261).

system that swept through Toledo, Ohio between October 14th and 20th in 2011 triggered lakewide oscillations that increased and decreased water levels by almost 1.5 m (Figure 17(b)). Oscillations appeared as mirror images at opposite ends of the lake (Toledo and Buffalo, New York) and gradually dampen down to the background seiche amplitude. Events such as this are readily evident along the shoreline,

where receding waters associated with the down oscillation can leave vessels resting on the bottom (Figure 17(c)). A seiche can also take the form of an internal wave where wind stress on a stratified lake exerts downward pressure on the thermocline at the upwind side. The thermocline may then oscillate, driving, mixing, and deepening the surface mixed layer. In an extreme case, the thermocline can breach the surface, producing an upwelling of hypolimnetic water. Upwellings can also be driven by shore parallel winds and when offshore winds drive warm epilimnetic waters away from shore with replacement by cold, hypolimnetic water from below. Swimmers who report that yesterday’s warm beach is frigid today are usually experiencing upwelling conditions. Transport associated with the entry of river plumes to the lakes is mediated by density differences (temperature and solids concentration) and by the Coriolis force, causing moving water to be deflected to the right (in the northern hemisphere). For example, the Niagara river enters Lake Ontario from Lake Erie as a warm, positively buoyant plume and spreads over the lake surface for approximately 10 km at a depth of 8–10 m before eventually dissipating as it mixes with

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lake water (Figure 18(a); Rao and Schwab, 2007). Coriolis forces steer the plume to the right, along the south shore, carrying constituents originating in Lake Erie and the Niagara river with it. The Ontonagon river, which enters Lake Superior from the western portion of Michigan’s Upper Peninsula (Figure 18(b)), is an example of a negatively buoyant plume. Here, even when the river and lake have similar temperatures, the high river water density due to extreme clay sediment content can cause the river to plunge below the lake surface (Churchill et al., 2003).

4.17.5

Water Chemistry – Oxygen, Nutrients and Trophic State

Comedian Johnny Carson described Lake Erie as ‘‘the place where fish go to die.’’ In 1969, an NBC documentary declared the entire lake dead. The scientific community indicted phosphorus discharges, aided and abetted by the thermal stratification process, as the perpetrators of the crime. In this section, the interaction between nutrient loads, mixing regime, and the needs of aquatic organisms that forms the basis for this indictment is examined.

A comprehensive examination of topics relating to Great Lakes water chemistry could easily occupy hundreds of pages.

These systems exhibit seasonally and spatially diverse chemistries, which have attracted the attention of environmental scientists for decades and yielded a body of knowledge published in a host of scientific publications including a dedicated outlet, the Journal of Great Lakes Research. Management of the lakes with respect to water chemistry, however, focuses on two topic areas: the persistent bioaccumulative toxins treated in Chapter 4.18 and the phosphorus–algae–oxygen dynamic examined here.

4.17.5.1

Oxygen

Among the general rules established under the Clean Water Act (formally, the 1972 amendments to the Federal Water Pollution Control Act), ‘fishable/swimmable’ conditions are to be achieved for all of the Nation’s waters. Fundamental to such conditions is the maintenance of adequate concentrations of dissolved oxygen. Absent consumptive chemical or biological processes, levels of oxygen in lakes remain in equilibrium with the essentially constant partial pressure of the gas in the atmosphere and concentrations are said to be at saturation. This relationship is described by Henry’s Law where dissolved oxygen concentrations increase as water temperature decreases due to the impact of temperature on the Henry’s Law coefficient

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(equilibrium constant). For the range of temperatures typically observed in Great Lakes waters (0–25 1C), saturation oxygen concentrations range from approximately 8 to 14 mg l1. Biological and chemical consumption of oxygen in lakes is balanced by reaeration, that is, transfer of the gas across the air–water interface. The rate of reaeration increases with temperature, turbulence at the air–water interface, and the degree of departure from saturation. In the spring and fall, when the water column in the Great Lakes is well mixed, oxygen concentrations are near saturation (B13 mg l1 at 4 1C) from surface to bottom. During summer stratification, mixing between well-aerated surface waters (the epilimnion) and bottom waters (the hypolimnion) becomes limited (see Section 4.17.4). Microbial consumption and respiration of organic matter (largely algae) settling to the lake bottom exerts a sediment oxygen demand (SOD), the magnitude of which is proportional to the degree to which the lake receives fertilizing nutrients. In Lake Superior, where nutrient levels are low and consumptive processes are negligible, oxygen levels remain near saturation and the summer depth profile tracks that of temperature (Figure 19(a)). More highly enriched systems such as Lake Erie and Green Bay in Lake Michigan harbor organic-rich sediments, which exert a significant demand on oxygen resources. In this case, bottom water oxygen resources are consumed over the summer with little replenishment from surface waters (limited vertical mixing) and hypoxia (oxygen o2 ppm) or anoxia (absence of oxygen) may result (Figure 19(b)). Hypoxia in Lake Erie covers much of the system’s central basin (Figure 20), an area roughly equivalent to the combined size of Delaware and Rhode Island. Resupply of oxygen to bottom waters occurs at turnover. Oxygen depletion under the ice in winter, a common feature of ponds and shallow, inland lakes is not an important phenomenon in the Great Lakes.

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4.17.5.2

Nutrients

The Great Lakes Water Quality Agreement states that oxygen in hypolimnetic waters should not be less than that necessary for the support of fish life and specifically calls for restoration of the required levels of oxygen to the bottom waters of the central basin of Lake Erie. However, the engineered approaches used to address this problem in ponds and small inland lakes, for example, increasing oxygen input through mechanical mixing or hypolimnetic aeration, are not feasible for systems to the size of the Great Lakes. Thus, attention must turn to reducing the degree of anthropogenic impact on those ecosystem processes resulting in oxygen consumption. In his classic text on Surface Water Quality Modeling, Dr. Steven C. Chapra introduces the concept of slow and fast response times (eigenvalues) for the water and sediment components of lakes. Supported by rates of flushing and reaction typically much faster than those of the sediments, the water column will respond to changes in the input of fertilizing materials more rapidly. Sediments containing a substantial fraction of the materials discharged to and produced within lakes over

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

timeframes of decades will require an extended period to reach equilibrium with the ‘remediated’ water column lying above them. The first step in initiating that response is to implement controls on sources of the fertilizing nutrients stimulating algal growth and attendant accumulation of organic matter in the sediment. Like all plants, algae require light, a temperature consistent with their growth optimum, and a suite of nutrients to support growth. The requirement for some nutrients is a significant fraction of the total plant biomass (macronutrients, e.g., N, P, and K) and for others much less so (micronutrients, e.g., Fe, Mn, and Zn). Leibig’s Law of the Minimum (Figure 21), formulated and popularized in the nineteenth

century, states that plant growth will be limited by the nutrients present in the smallest amount relative to its requirement. In the Great Lakes, this limiting nutrient is phosphorus. As phosphorus is added to the lakes, algal growth (represented by the water level in Figure 21) increases until the next nutrients, present in the least amount relative to the needs of the algae, becomes limiting. Phosphorus-rich waters will exhibit a drawdown in nutrients such as silicon and nitrate over the growing season (Figure 22); however, this does not result in a limitation on production of total algal biomass. Rather, shifts in the algal species contributing to the phytoplankton community occur, with diatom biomass declining as silica is depleted and cyanobacteria capable of nitrogen fixation increasing in abundance as concentrations of fixed inorganic nitrogen (NH3, NO 3 ) levels fall.

4.17.5.3

Trophic State

The degree to which a lake is supplied with nutrients is described by its trophic state (from the Greek trophe or nourishment). Trophic state may be categorized based on the total phosphorus (TP) concentration and lakes of a particular trophic state share certain characteristics: oligotrophic – poorly nourished, TP o10 ppb, low levels of algae, high water clarity and abundant hypolimnetic oxygen reserves; mesotrophic – moderately nourished, TP 10–20 ppb; and eutrophic – well nourished, high levels of algae, poor water clarity, and oxygendepleted hypolimnia, TP 420. It is generally assumed that all of the Great Lakes were oligotrophic following the retreat of the glaciers, that formed them and that the lakes are moving along a natural continuum of nutrient enrichment (eutrophication) that will transform them into dry land over a period of thousands of years. The rate of enrichment has, however, been vastly accelerated through anthropogenic activity (cultural eutrophication), leading to the contemporary interest in reducing nutrient levels and returning the lakes to a trophic state condition consistent with the geology of their watersheds.

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Figure 21 Liebig’s Law of the Minimum. Algal growth (level of water in the barrel) rises only as high as the shortest stave (limiting nutrient). Increasing the concentration of that nutrient (raising the stave height) allows algal growth to increase to the level where it is limited by the next nutrient least in supply with respect to its growth requirement. http://en.wikipedia.org/wiki/ Liebig’s_law_of_the_minimum

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Figure 23 Lake 226, the divided lake, in Canada’s Experimental Lake Area was separated into two parts with a curtain. Carbon and nitrogen were added to one side (top) and carbon, nitrogen and phosphorus to the other (bottom). The algal bloom that developed in the side receiving phosphorus helped confirm that this nutrient was limiting in freshwaters. Photo by D.W. Schindler.

An extensive body of scientific study, including the dramatic ‘divided lake’ experiment of Dr. David W. Schindler (Figure 23), has clearly identified phosphorus as the driving force behind excessive algal growth leading to oxygen depletion, nuisance growth of the attached alga Cladophora, and proliferation of harmful algal blooms (HABs). Phosphorus is thus the appropriate focus for management. Work pursuant to the Great Lakes Water Quality Agreement established guidelines for in-lake total phosphorus concentrations and trophic state and called for the allocation of total phosphorus loads to meet these guidelines. A suite of mathematical models (Figure 24) were used to calculate a target load, which would yield the lake response called for in the guidelines. The major point and nonpoint sources of phosphorus to the Great Lakes were identified (Table 2) and loading reductions were sought by: (1) requiring all municipal waste treatment facilities discharging more than 1 million gallons per day to the Great Lakes or waters tributary to the Great Lakes to achieve an effluent TP concentration of 1 ppm, (2) placing restrictions on the phosphorus content of detergents, and (3) implementing phosphorus management plans for urban and agricultural nonpoint sources. The response to implementation of load reductions has, in some cases, been dramatic (Table 3). For example, TP levels

in Lake Ontario declined to one-third of their 1979 concentrations, moving the trophic state from eutrophy to oligotrophy. The oligotrophication of the offshore waters of lake Ontario has been decried by the sport fishing industry due to suspicion that attendant changes in food web dynamics do not favor target recreational species, for example, introduced salmonids (see Section 4.17.6). Total phosphorus in Lake Erie has declined by about one-half, a monumental achievement in this heavily loaded system, with trophic state conditions in the eastern and central basins approaching mesotrophy. It is only in the shallow water of Lake Erie’s western basin where eutrophic conditions persist. Total phosphorus concentrations in Lakes Superior, Michigan, and Huron have declined slightly, maintaining the target trophic state of oligotrophy. The Great Lakes phosphorus story is not yet, however, fully told. Rather than declining in response to changes in phosphorus loading, HABs, dominated by potentially toxic cyanobacteria, are more prevalent and impact a greater portion of Lake Erie than ever before. Further, the extent of hypoxia in Lake Erie’s central basin is both significant and expanding. Growth of the attached, filamentous, green alga Cladophora, thought to have been under control, has once again reached nuisance proportions (Figure 25) in all of the lakes except Superior. Although some of this may be attributed to changes in loads, particularly the fraction of algal available P reaching the lake, it is the changes in transparency and P cycling (Auer et al., 2010; Tomlinson et al., 2010) driven by invasive zebra and quagga mussels that has recently drawn the attention of the Great Lakes scientific community (see Section 4.17.6). The role of light is undeniable. Filtering activity by mussels has increased Secchi disk transparency (see Section 4.17.4) and made available additional solid substrate to be colonized by Cladophora. The situation with respect to P nutrition is more complicated. The total phosphorus analyte consists of three fractions: soluble reactive P, which is fully and immediately available to algae, and dissolved organic and particulate phosphorus, portions of which may eventually be made available to algae through enzymatic hydrolysis. The particulate fraction yields phosphorus slowly and thus has historically been transported out of the nearshore and deposited in deep waters before it can be made available to algae (Figure 26(a)). Attached algae (Cladophora) and phytoplankton (including cyanobacteria, HABs) then competed for the remainder, that is, the dissolved fraction. Filterfeeding mussels impact this cycle in two ways: first by capturing particulate P and releasing it to nearshore waters as dissolved P and second by consuming phytoplankton and recycling the P contained therein (Figure 26(b)). The effect of this phosphorus shunt (as conceptualized by Hecky et al., 2004) is to restructure phosphorus cycling in the Great Lakes nearshore in a manner that favors organisms that are not grazed by zooplankton, for example, HABs species and Cladophora. Although an appropriate management response to this functional ecosystem change has not yet been formulated, scientists and engineers laying the groundwork for a proposed revision of the Great Lakes Water Quality Agreement have recognized that nuisance conditions may be experienced in nearshore areas even as water-quality

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

383

W = 4000

Superior (4.6) Green Bay W = 1400

T = 315 W = 5500 S = 3685

T = 370

S = 1030

Georgian Bay W = 875

W = 2275

Main Michigan

T = 582

T=2

Main Huron

W = 4650

W = 1425 S = 5288

T = 1027 S = 877

T = 850 S = 2993

W = 18150 T = 3794

S = 575 Saginaw Bay

Erie (19.8)

T = 5515 Ontario (21.0)

S = 4929

S = 15383 Figure 24 Mathematical models for total phosphorus (TP) were used to set target loads for the Great Lakes. The mass balance or budget model illustrated here tallies inputs from loads (W) and losses to sedimentation (S) and interbasin transfer (T) and calculates TP concentrations (mgP per cubicmeter) for each of the Great Lakes (values in white). Concentrations for Green Bay are for (left to right) the inner and outer bay, respectively, and for Lake Erie (left to right) for the west, central, and east basins. Redrawn from JWPCF.

Table 2 Top-ranked point and nonpoint source total phosphorus loads to the Great Lakes (metric tons per year) a. Nonpoint sources River

Load

Discharging to

1. 2. 3. 4. 5.

2947 741 630 586 533

Lake Lake Lake Lake Lake

Load

Discharging to

Maumee river Sandusky river Fox river Cuyahoga river Thames river

Erie Erie Michigan Erie Erie

b. Point sources Discharger Detroit WWTP Toronto Metro WPCP Birds Island (Buffalo) WWTP Cleveland Metro WWTP Rochester WWTP

662 269 161 157 145

Lake Lake Lake Lake Lake

Erie Ontario Ontario Erie Ontario

Abbreviations: WPCP, water pollution control Plant; WWTP, waste water treatment plant Source: Courtesy of D. M. Dolan, University of Wisconsin – Green Bay.

objectives are met in offshore waters. Finally, Higgins et al. (2012) have made it clear that mussels are not a source of P, but rather a means of reengineering its delivery, and will not stimulate nuisance algal growth absent a source of P enrichment. External loads thus remain the appropriate focus for management activity.

4.17.6

Biodiversity

All animal, plant, and human life depends on water and all life on earth is interconnected. In the Great Lakes ecosystem native species biodiversity is being lost, whereas artificial biodiversity is increasing at unprecedented rates through establishment of invasive species. Here, native biodiversity is examined, changes leading to an artificial biodiversity are reviewed and the need to focus on this critical ecosystem feature is addressed.

4.17.6.1

The Interwoven Nature of All Life and Importance of Water for Survival

Scientists believe life on earth began when heat, electrical energy from storms or ultraviolet rays from the sun interacted with on a primitive atmosphere composed of nitrogen, ammonia, methane, hydrogen gas, carbon dioxide, and water forming chemical precursors, eventually yielding simple organic compounds. Over time, these combined to create amino acids and other organic compounds found in life forms today. It is important to note the presence of water and the role that it played in the beginning of life, with waterlain fossil evidence of early life forms dating from 3.8 to 3.5 billion years ago (see Section 4.17.2). Every-living plant or animal needs water to thrive and survive from those that live in water, such as fish, to plants and animals which live in even the driest conditions on earth. Humans are composed of 60–65% water and the foods that we consume from animal (an adult cow is 50–65% water) or plant (90–95% water) sources, require much water for growth and production.

384

Table 3

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

Great Lakes phosphorus and trophic state management: targets and status

Lake

a

1976 Superior Michigan Huron Erie western basin Erie central basin Erie eastern basin Ontario

TP concentration (mg P l  1)

TP load (metric tons per year) a,b

Target

c

1970s

Target

2000s

1970se

Targeta

2000se

3.8 5.9 4.6 52.9 20.2 19.8 21.5

5 7 5 15 10 10 10

2.8 3.9 4.1 28.3 12.6 12.3 7.3

O O O E E E E

O O O M O-M O-M O-M

O O O E M M O

3400 5600 4360

6512 3037 3018

20 000

11 000

8924

11 000

7000

5287

a

Trophic state (TP-based)

2006

4212 6700 5050

d

d

a

Phosphorus Management Strategies Task Force (1980). Annex 3 of the Great Lakes Water Quality Agreement. c Courtesy of D. M. Dolan, University of Wisconsin – Green Bay. d Calculated from data presented in SOLEC (2009). e Based on the trophic state classification of Chapra, S. C. (1997). Surface water quality modeling, p. 844. New York, NY: McGraw-Hill. Trophic state abbreviations: O, oligotrophic; O-M, oligomesotrophic, M, mesotrophic; E, eutrophic. b

Figure 25 The filamentous, green alga Cladophora grows to nuisance proportions in the Great Lakes where substrate for attachment and phosphorus are available, colonizing that substrate to depths dictated by light penetration. The long, hair-like algal filaments slough, becoming detached from the substrate and significant quantities of algal biomass are transported to the nearshore where they clog water intakes and decay, fouling beaches. Image at upper right by H. A. Bootsma.

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

Premussels

Phytoplankton

Postmussels

Transport and deepwater deposition

Phytoplankton

Cladophora

Watershed dissolved P

385

Mussels

Cladophora

Watershed particulate P

(a)

Watershed dissolved P

Watershed particulate P

(b)

Figure 26 The filtering activity of mussels served to modify phosphorus cycling in the nearshore waters of the Great Lakes. Particulate phosphorus originating from the watershed and delivered largely unutilized to offshore waters and dissolved phosphorus sequestered by phytoplankton were captured by mussels and released in soluble form through this nearshore phosphorus shunt. Reproduced from Michigan Tech.

The body of an organism works to maintain homeostasis, a biological process that balances energy intake and loss, oxygen and CO2 regulation, and many other bodily functions. Unless unnaturally perturbed by humankind, aquatic systems such as lakes also work to maintain a homeostasis or equilibrium. In the Great Lakes, freshwater is the medium, which holds components critical for organisms: oxygen for breathing, nutrients and trace metals for growth, and a means of cycling waste products. An example of the connectedness of water and living things is evident in the life of a lake sturgeon. This fish lays eggs in the clear, cool waters of tributaries in the spring, where turbulent freshwater provides plenty of oxygen and where predators are few. The eggs hatch and the young drift downstream over several months feeding along the bottom as they go. Once flushed out of the river, they grow as juveniles in rich wetland river mouth areas, which provide refuge and food organisms. As they gain in size, the sturgeon move deeper into lakes and they feed on macroinvertebrates including crayfish and small clams and any organic material, that may have settled to the lake bottom (e.g., dead fish). Lake sturgeons can cover hundreds of kilometers in their lifetimes of 100 years or more. The entire area from the farthest upstream reaches of rivers natal to sturgeon to the deepest portions of the Great Lakes constitutes the ecosystem, that is home to these fish that cannot live an optimal existence without the habitat and coexisting organisms found across the entirety of its natural range.

4.17.6.2

So What is Biodiversity?

Biodiversity is ‘short’ for biological diversity; much as many different races constitute human diversity, many different types of organisms create the biological diversity observed within the Great Lakes ecosystem. Before the arrival of the industrial age, the lakes maintained a natural biodiversity, absorbing the relatively small perturbations threatening their homeostasis. But as humans developed the land surrounding the lakes and extracted natural resources such as minerals and lumber, and as population centers grew near shipping ports, the lakes could not absorb and recover from perturbations such as the discharge of wastes, over harvest of fisheries, and onslaught of exotic species. The stable, natural biodiversity of the Great Lakes ecosystem has changed to one of unstable, artificial biodiversity within the past 100 years. It is well accepted that stability is necessary in order to provide the ecosystem services (e.g., clean water and natural resources), which humans need to survive. Resource managers, cooperating across state, federal and international agency boundaries, regularly adjust management strategies in an effort to return the ecosystem to some semblance of stability.

4.17.6.3

Food Web Connections in the Historic versus Contemporary Great Lakes Ecosystem

Although biological diversity is measured by the number of species within a particular ecosystem, the stability of

386

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

populations was often measured by the ability of each lake ecosystem to maintain top predatory fishes, usually those that humans have found fit for consumption and which have supported commercial fisheries. Native peoples were the first to appreciate these fisheries and they shared this valuable knowledge with early fur traders and explorers. Historically, the biodiversity of the larger Great Lakes supported commercially viable fisheries for lake trout (Salvelinus namaycush) and lake whitefish (Coregonus clupeaformis). Each of these fishes had its foundation, a unique, natural, and stable food web; the lake trout’s driven by pelagic (open lake) interactions and the lake whitefish’s by benthic (lake bottom) interactions (Figure 27). Food webs, or the progression of who-eats-who from the phytoplankton to top predators, were historically traced by analysis of stomach contents for each organism and now by the developing field of stable isotope analysis of component organisms. A typical natural bottom-up (here referring to the progression from smallest to largest organism eaten) food web for lake trout consists of the sun’s energy supporting primary production by phytoplankton (free-floating algae) in the open lake water with these algae eaten by zooplankton (small crustaceans, much like tiny shrimp). Zooplankton are found from the surface of the water all the way to the lake bottom, and different types consume the various species of phytoplankton found at different depths. In the Great Lakes ecosystem, phytoplankton and detritus sinking toward the lake bottom are consumed largely by two species of macroinvertebrates: the Opossum shrimp, Mysis relicta, and the amphipod, Diporeia spp. Mysis are truly shrimp like, with large eyes, which allow them to feed on smaller organisms, especially at night, and Diporeia, which also have a shrimp-like appearance and feed on and in the sediment. These two organisms make up large portions of the diets of a group of benthic forage fish known as sculpins (four species inhabit the Great Lakes) which, in turn, contribute a significant portion of the diet of the top predator, the lake trout. Pelagic forage fish consumed by lake trout include the cisco (Coregonus artedi; once known as lake herring) and bloater (Coregonus hoyi). Pelagic forage fish preyed on by lake trout, filter zooplankton from the water column. This food web produces the pink, fatty flesh found in lake trout, that makes it a good candidate for eating fresh or smoked. A typical natural bottom-up benthic food web for the Great Lakes system shares a similar beginning, with the sun’s energy driving algal production, which supplies food to pelagic (zooplankton) and benthic (Mysis and Diporeia) primary consumers. The top of the food web here is the lake whitefish, a large silver-colored fish with a small, downturned mouth. In the Great Lakes, there are several whitefish species but the lake whitefish, known for its fine flesh, as well as its liver and roe (eggs), which can be made into caviar, is the only commercially valued species. The fine, white muscle and fatty connective tissue of this fish makes it a great species for serving in a variety for ways, including smoked.

4.17.6.4

Changing Biodiversity in the Great Lakes

Since the time of first European explorations seeking timber and furs in the mid-1600s, the Great Lakes have experienced

numerous assaults on their ecosystem stability. Those lakes nearer to trade routes and closer to ocean transportation like Lake Ontario, experienced change earlier than Lakes Michigan and Superior. Even so, the Lake Michigan ecosystem has seen dramatic changes in its biodiversity compared to that of Lake Superior, partly as a result of differences in size (volume), but importantly due to the extent of land development in the surrounding watershed. One of the first great disturbances to biodiversity in the Great Lakes was the loss of lake trout populations in all, but Lake Superior as the invasive sea lamprey passed through the Welland canal, built between Lakes Ontario and Erie as shipping passage around Niagara Falls. The sea lamprey literally dissolves and consumes the muscle and organs of fishes. It was not until sea lamprey control was initiated in the 1950s that some stability returned but at a tremendous cost for control, which is still employed today. The sea lamprey was only one of numerous invasive species, which have changed the face of the Great Lakes. A second species, the alewife (Alosa pseudoharengus), invaded and, by the 1950s exploded in population as the top predator, the lake trout, declined in abundance. Alewives began dying off in huge numbers, washing ashore and accumulating in rotting piles on beaches. Fisheries managers, unable to reestablish lake trout populations, decided to introduce several species of Pacific salmon to take the top predator position and reduce alewife numbers. The Pacific salmon population grew and became a tremendous economic boon to the Great Lake states as interest in sport fishing for record size salmon grew. The stability built on this new strategy was not long lived, however. Alewife numbers began to decline and Pacific salmon did not return to record sizes. Although still a popular fishery, it is not as successful as it was in the late 1960s. During this time some unexpected visitors began to appear in the Great Lakes ecosystem, changing native biodiversity forever. Ballast water, released into the lakes as international ships filled their holds with cargo at Great Lakes ports, unleashed new fish, invertebrates (including mussels) and even algae. Examples include several fish, the ruffe (Gymnocephalus cernuus), and round and tubenose gobies (Apollonia melanostomus and Proterorhinus marmoratus), which have established themselves in most Great Lake harbors and nearshore waters, outcompeting sunfishes and other small, native species that provide forage for predators such as walleye. Many exotic species are voracious consumers of food important for the young of many native species. The invasive zooplankter, the spiny water flea (Bythotrephes cederstroemi), has displaced native zooplankton, and its long spiny tail is believed to reduce the feeding efficiency of many fishes as they struggle to swallow the organism. Within the timeframe of only 10 years, 1995–2005, populations of Diporeia, a major diet item for important commercial fish species, have crashed in the lower four Great Lakes for reasons yet to be completely explained. The loss of this nutrient-rich food source has reduced the condition and health of lake whitefish in Lakes Huron and Michigan, impacting their survival and commercial value. Research on the Diporeia decline and on the potential survival of the species in Lake Superior is important to fishery managers.

The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

Top predator

Historical food web

lake trout

(similar to that of lake superior today)

Benthic predator lake whitefish

Pelagic forage fish cisco, kiyi, smelt, bloater

Benthic forage fish sculpin

Benthic macroinvertebrates mysis, diporeia

Zooplankton copepods cladocerans rotifers

Phytoplankton diatoms, green algae, cyanobacteria, flagellates

Contemporary food web (similar to that of Lake Ontario today)

Pelagic predators coho salmon, lake trout chinook salmon

Invasive predator sea lamprey

Pelagic forage fish alewife

Zooplankton copepods, cladocerans rotifers, spiny water flea

Benthic forage fish round goby tubenose goby

Benthic macroinvertebrates Mysis, zebra and quagga mussels

Phytoplankton diatoms, green algae, cyanobacteria, flagellates

Figure 27 A comparison of a historical Great Lakes food web, represented today in lake Superior and a contemporary food web as might be found today in lake Ontario. Here, the terms historical and contemporary refer to the degree to which the food webs have been impacted by introduced or invasive species. Reproduced from Michigan Tech.

387

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The Great Lakes: Foundations of Physics, Hydrology, Water Chemistry, and Biodiversity

Other invasive species, suspected of arriving in the Great Lakes in ballast water, include two species of mussels: the zebra (Dreissena polymorpha) and quagga mussels (D. rostriformis bugensis) (see Section 4.17.5). Mussels now colonize many natural substrates and exposed structures such as docks and power plant water intake and discharge pipelines. Removal efforts required to deal with mussel accumulation on pipes at power plants and other industries have cost millions of dollars. Mussels also wash up on beaches and care must be taken when walking near piles of sharp shells. Species invasions and changes in nutrient loading are not the only stressors which have disturbed ecosystem balance and food web dynamics. Manipulations of water flow and diversions have opened new pathways for exotic species to move into the Great Lakes. This is evident in the current threat of four species of Asian carp moving into the basin should the Chicago ship canal electronic barrier fail. Increases in temperature and evaporation associated with global climate change will affect lake levels and agency management of this issue will become more controversial as the fishery and shipping industries vie for attention to their interests.

4.17.6.5

Perspectives

The native biodiversity of the Great Lakes ecosystem has been forever changed. This is clearly marked by the loss of several species, for example, lake trout from the lower Lakes and by population imbalance as the ecological niche of a stressed or extirpated species becomes filled by another. Even if improvements in water quality or fisheries are realized, species whose niche has been usurped by another may not rebound. The dynamics of a Great Lakes ecosystem populated by native communities have evolved over the thousands of years since the retreat of the glaciers (see Section 4.17.2). Disruption of ecological connections and/or the loss of species results in diet shifts away from organisms providing optimal nutrition and energy relationships. This reduces the fitness of the organism and weakens their ability to contribute to the homeostasis, which insures food web and ecosystem stability. Without continued management and attention to stopping the introduction and spread of invasive species, the Great Lakes will eventually lose their unique, system-specific food webs, yielding to a structure that is homogenous across the lakes and largely composed of a mixture of species from around the globe (Figure 27). This threat merits the attention not only of scientists, regulators, and management officials but also the various stakeholders who derive pleasure and food resources from these remarkable and beautiful lakes.

4.17.7

Summary and Conclusions

This article begins by placing the water quality of the Great Lakes and the concept of sustainable futures in a historical context, recognizing that a desire to pass on natural resources to future generations undiminished has been a longstanding goal. The introduction concludes by emphasizing the need for sensitivity to unsustainable practices and by describing the roles of science, policy, and engineering in achieving a

sustainable future for the Great Lakes. The section on formation suggests that contemporary water quality and attendant environmental stressors have roots in the bedrock, orogenic and glacial activity, and soil development that has occurred in the Great Lakes basin over millions of years. Next, the attention turns toward hydrology; specifically to water levels and water diversion. The section traces the development of policies regulating Great Lakes water levels and explores current interests in the impacts of small- and large-scale water diversions. It is concluded that the potential effects of a changing climate are of much greater concern than would most interbasin transfers. Physical processes set the stage on which the biological and chemical dynamics determining water quality are played out. This section described the fate of incident solar radiation, the ultimate source of energy and thus life, as it is penetrates the water column. Thermal stratification and the temperature regime are then characterized, and the role of diffusive mass transport in mediating these processes is introduced. Finally, the concept of advective mass transport is considered, describing the movement of temperature and biogeochemical constituents by wind-driven currents. Eutrophication is among the longstanding water quality concerns in the Great Lakes, recognized as meriting regulatory attention for over a half century. The section on water chemistry takes this phenomenon as its topic, introducing the concept of nutrient limitation, identifying phosphorus as the limiting nutrient in the Great Lakes and exploring the role of excessive phosphorus loading in stimulating excessive algal growth and hypolimnetic oxygen depletion. The section concludes by illustrating the approach to phosphorus management in the Great Lakes and describing the challenge to that management associated with invasive mussels. The final section of this chapter treats the topic of biodiversity, the feature of ecosystem structure, and function most deserving of attention in efforts to ensure sustainability. Historical and contemporary Great Lakes food webs are contrasted, including changes related to introduced and invasive species and a call is issued to protect unique, system-specific natural food webs. These sections lay the foundation for considering nutrient and energy cycling, persistent bioaccumulative toxins, air-water and sedimentwater interactions, and the application of mathematical models in managing for a sustainable Great Lakes as treated in the following chapter.

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