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Physiography of Lakes and Reservoirs
FIGURE 6.1
Satellite images of the Great Lakes (left) and Smithville Reservoir (Missouri) (right). The reservoir is 16 km long. Note the dendritic pattern of the reservoir and the relatively smooth shorelines of the glacially formed Great Lakes. The numerous black dots around the reservoir are farm ponds (data from the U.S. Geological Survey).
6
Physiography of Lakes and Reservoirs Formation: Geological Processes Lake Habitats and Morphometry Stratification Water Movement and Currents in Lakes Summary Questions for Thought
Lakes of all sizes provide us with fisheries, recreation, drinking water, and scenic splendor. Having a clean lake nearby increases property values. Large lakes (Table 6.1) have played a part in the history, economy, and culture of many nations. Lakes also provide an excellent system for ecological study. The boundaries of the lake community and ecosystem often appear distinct, the water well mixed, and the bottom relatively homogeneous, making lakes a tractable system for ecologists. Much effort has been made to study the physical and biological aspects of lakes (e.g., some of those in Table 6.2 have been studied intensively for approximately a century) and to manage pollution. The base of these studies is an understanding of the geomorphology of the lakes. Different lake morphologies give rise to different levels of productivity and physical effects of water retention, circulation, currents, and waves. For example, the fates of toxins and nutrients in lakes depend partly on lake circulation, which is a function of lake physiography. In this chapter, I describe formation, lake morphometry, the process of stratification, and water movement in lakes.
FORMATION: GEOLOGICAL PROCESSES What is a lake? I define a lake as a very slowly flowing open body of water in a depression of ground not in contact with the ocean. This definition
91
TABLE 6.1 Properties of the 10 Largest Lakes by Depth, Area, or Volume, Globally Arranged by Maximum Deptha
Lake
Continent
Formation
Mixis
Baikal Tanganyika Caspian Nyasa Issyk Kul Great Slave Crater Matano Toba Hornindalsvatn Great Bear Superior Michigan Huron Victoria Aral
Asia Africa Asia/Europe Africa Asia North America North America Asia Asia Europe North America North America North America North America Africa Asia
Tectonic Tectonic Tectonic Tectonic Tectonic Glacial Volcanic Tectonic Volcanic–tectonic Glacial Glacial Glacial Glacial Glacial Tectonic Tectonic
Meromictic Meromictic Meromictic Meromictic Meromictic Dimictic Monomictic
a
Monomictic Dimictic Dimictic Monomictic Monomictic Monomictic Polymictic Meromictic
Area (km2)
Maximum depth (m)
Mean depth (m)
Volume (km3)
Length (km)
31,500 34,000 436,400 30,800 6,200 30,000 55 164 1,150 508 29,500 83,300 57,850 59,510 68,800 62,000
1741 1470 946 706 702 614 608 590 529 514 452 307 265 223 79 68
730 572 182 273 320 70 364 240 216 237 81 145 99 76 40 16
23,000 18,940 79,319 8,400 1,732 2,088 20 39 249 12 2,381 12,000 5,760 4,600 2,700 970
2200 1900 6000 1500 760 2200 35 80 100 65 2100 3000 2210 2700 3440 2300
Data are from several sources, including Herdnedorf (1990), Hutchinson (1957), Horne and Goldman (1994), and Gasith and Gafny (1990).
DL 3.4 3.1 2.6 2.7 2.8 3.6 1.3 1.8 2.6 3.3 2.9 2.6 3.1 3.7 2.6
Retention (years) 323 5500 Sink 305 4.9
124 184 104 21 23 Sink
TABLE 6.2 Selected Lakes Not Listed in Table 6.1 with Historical or Research Interesta Area (km2)
Maximum depth (m)
Mean depth (m)
Volume (km3)
Length (km)
46.2 64 4 310 43 230 25.3 225 501 280 67
45 21 2.9 153 26 133 12.8 91 249 106 33.8
28 540 23 89 4.3 7.5 0.47 1720 124 820 0.17
46 1200
Monomictic Amictic
618 25,820 0–8,583 580 1.7 56.4 39.8 18,760 499 7,700 5.2
Monomictic
14.3
64
23
Lake
Continent
Formation
Mixis
Biwa Erie Eyre Geneva Kinneret Loch Ness Mendota Ontario Tahoe Titicaca Vanda
Asia North America Australia Europe Asia Europe North America North America North America South America Antarctica
Tectonic Glacial Tectonic Glacial Tectonic Glacial Glacial Glacial Tectonic Tectonic Glacial
Monomictic Monomictic Polymictic Monomictic Monomictic Monomictic Dimictic Monomictic
Windermere (both basins)
Europe
Glacial
a
0.39
Retention (years)
2.6 2.1 4.9
5.4 3 Sink
70 2.2 39 9.1 1380 125 176 5.6
1.16 3.2 1.6 2.8 1.6 3.46 2.28
17
1.2
Data are from several sources, including Herdnedorf (1990), Hutchinson (1957), Horne and Goldman (1994), and Gasith and Gafny (1990). Ablation is evaporation directly from the ice that covers the lake.
b
DL
7.3 2.8 4.6 8 700 1343 75 (ablationb)
94
6. Physiography of Lakes and Reservoirs
includes saline lakes but excludes estuaries and other mainly marine embayments. The distinction between a small shallow lake or pond and a wetland is not clear, and neither is that between a very slow, wide spot in a river and a lake or reservoir with high water throughput. Remember, all aquatic habitats occur across a continuum of physical attributes, such as depth and water velocity. Permanent lakes are common where more precipitation occurs and where geology allows for formation of water-retaining basins (Fig. 6.2A). Some areas have geological histories that result in more lakes. For example, if we compare the distribution of wetlands (Fig. 4.11) to the distribution of freshwater lakes, relatively more lakes than wetlands occur in northern North America, and wetlands are relatively important in northern Asia and northeast Europe. Intermittent lakes (those that dry sometimes) are distributed sparsely throughout the world, with greater numbers in drier areas (Fig. 6.2B). The western United States, south Australia, India, central Asia, and central Africa all have high numbers of intermittent lakes. Humans have made many lakes and ponds. Most regions inhabited by humans with few natural lakes and even moderate precipitation have significant numbers of ponds and reservoirs. The large number of rivers in the Northern Hemisphere that have been altered by the construction of dams was discussed in Chapter 5. More small than large lakes exist in the world. However, the sum of the area of lakes globally of each size is fairly constant, with the few very
FIGURE 6.2 Cogley, 1994).
Global distribution of permanent (A) and impermanent (B) lakes (data from
Formation: Geological Processes
large lakes having a major impact on total area (Fig. 6.3). A variety of geological processes lead to the formation of these lakes (Table 6.3). Hutchinson (1957) described these processes in detail; I give a brief version here. Tectonic movements of the earth’s crust (Fig. 6.4) form some of the largest and oldest lakes. For example, warping of the earth’s crust formed the Great Rift Valley in Africa and has given rise to Lakes Edward, Albert, Tanganyika, Victoria, Nyasa, and Rudolf. This group contains some of the oldest, deepest, and most ecologically and evolutionarily interesting lakes on Earth. Although small tectonic lakes are more numerous than large lakes, the large tectonic lakes cover an area that is greater than that covered by the small ones on a global scale (Fig. 6.5). Graben lakes are tectonic lakes formed where multiple faults allow a block to slip down and form a depression. Lake Baikal of Siberia, the deepest and oldest lake on Earth, is a graben lake. About 7 km of sediment has accumulated on the bottom of Lake Baikal over 16 million years (Fig. 6.6). Tectonic movements also form horst lakes. In this case, the blocks tilt and leave a depression that can be filled by water (Fig. 6.4). Damming by natural processes can form lakes. Examples of these processes include landslides, lava flows, drifting sand dunes, and glacial moraines. In addition, beaver ponds, damming by excessive plant growth, flows of rivers at deltas, glacial ice dams, and pools formed at the edges of large lakes by shore movement are classified into this general category. These lakes usually are not large, but some exceptions exist (e.g., Lake Sarez, a large landslide lake in Russia, is 500 m deep).
Total area (km 2 x 105)
7 6 5 4 3 2 1
Total number
0
106 105 104 103 102 101 5
10
4
10
3
2
10
-1
-1
-1
6
5
0
0
4
3
0
0
.1
00
-1
-1
10
1
-0
10
10
1-
01
1-
0.
0.
Lake size range (km 2) FIGURE 6.3 Global numbers and total areas of lakes by surface area size class (data from Meybeck, 1995).
95
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6. Physiography of Lakes and Reservoirs
TABLE 6.3 Ways That Lakes Form and Essential Characteristics of Each Typea Lake type
Formation process
Essential characteristics and examples
Tectonic
Basin formed by movement of Earth’s crust: graben, a block slips down between two others; horst, diagonal slippage Formed when ice left from retreating glacier is buried in till (solid material deposited by glacier) and then melts Glacial activity deposits a dam of rock and debris Movement of earth dams a stream or river
Can be very old and very deep; Lake Baikal, Asia, and Lake Tanganyika, Africa
Pothole or kettle
Moraine Earthslide Volcanic—caldera Dissolution lake Oxbow
Volcanic explosion causes hole that is filled with water Limestone dissolves and lake forms River bend pinches off, leaves lake behind
Small lakes/wetlands; prairie pothole region in Alberta and North and South Dakota Narrow, fill valley Similar to reservoirs; Quake Lake, Montana Often round and deep; Crater Lake, Oregon Small, steep sides Shallow, narrow, may be seasonally flooded
a
Many more types are possible (Hutchinson, 1957).
Glacial activity is responsible for the formation of many lakes in the temperate regions and for the formation of more lakes than any other process (Fig. 6.5). Several processes associated with glacial activity lead to lake formation. Glaciers scour as they move down valleys. The ice flow of these glaciers creates basins. Lakes occur where glaciers have scoured more deeply, leading to the formation of cirque (also called tarn lakes) lakes in the “amphitheaters” at the heads of the valleys. The glacier forms chains of paternoster lakes as it flows further down the valleys (Figs. 6.7 and 6.8). Glacial scour can lead to formation of extremely large lakes. The Great Slave Lake in Canada was carved to a depth of 464 m below sea level by the massive weight of the continental ice sheet and is the deepest lake in North America. The Laurentian Great Lakes of North America (e.g., Superior, Huron, and Erie) were partially formed by glacial action. Fjord lakes such as Loch Ness (the home of a legendary creature) are long glacial lakes formed in steep val-
FIGURE 6.4 Diagrams of two modes of lake formation associated with tectonic processes: (A) graben, a block drops below two others; and (B) horst, blocks tip and a lake forms along a single fault line.
40 30 20 10 0
106
Tectonic
Tectonic
104 102
Total Number in Class
Total surface area (10,000 km2)
Formation: Geological Processes
40 Glacial
30 20 10 0 6
Fluvial
100
0.01-0.1
0.1-1
1-10
10 2 -10 3
10-100
106
10 3 -10 4
10 4 -10 5
10 5 -10 6
Glacial
104 102 100 0.01-0.1 10
4
104
2
102
0
100
104-105
10-100
6
Fluvial
5
4
10
10
3
10
-1
-1
-1
6
5
0
4
0
3
0
0
00
-1
-1
2
10
.1
6
5
0
4
0
3
0
0
-0
1
10
10
1-
-1
-1
-1
.1
00
-1
01
1-
0.
0.
5
4
10
10
3
10
2
-1
10
-0
1
10
10
1-
01
1-
0.
0.
Surface area range (km2)
FIGURE 6.5
Global numbers and total areas of lakes of different geological origins by surface area size class (data from Meybeck, 1995).
leys. One of the strangest lakes associated with glaciers is the gigantic lake that has recently been described below the ice in Antarctica (Sidebar 6.1). As glaciers move, they entrain rocks and sediments into the ice. Where glaciers melt at the edges and front, they deposit these materials. As the glaciers retreat they leave this material, called glacial till, behind. If large blocks of ice remain in this till, they melt and eventually leave lakes, ponds, or wetlands called kettles or potholes (Fig. 6.7). This process formed the many lakes and ponds that provide vital habitat to waterfowl in the northern
Olkhon Island
Fault
Depth from lake surface(km)
Lake surface
0 1 0 - 1.7 million year old sediment
2 3 4
3.5 - 35 million year old sediment
5 6
FIGURE 6.6
Bedrock
0
1
2
3
4 5 Width (km)
6
7
8
9
Cross section of Lake Baikal, the south basin, in the region of maximum depth (1620 m) [redrawn from Belt (1992) and Mats (1993)].
97
98
6. Physiography of Lakes and Reservoirs
FIGURE 6.7
Formation of some types of glacial lakes. (A) A cross section of a glacier moving down a valley. (B) After the glacier has retreated, it leaves cirque, moraine dammed, and pothole lakes.
FIGURE 6.8
Paternoster lakes (a string of glacial lakes) in a snowy mountain valley in the Jewel Basin of Montana.
Formation: Geological Processes
99
prairies of North America, although many have been filled for agricultural purposes. If the forward flow of a glacier is approximately equal to its backward melting rate, a wall of material is formed called a terminal moraine, which can impound water flow and lead to formation of lakes. Materials deposited along the sides of glaciers form lateral moraines. Glacial lakes tend to be smaller than tectonic lakes, but a few very large glacial lakes (e.g., the North American Great Lakes) make up a considerable area when considered on a global basis (Fig. 6.5). A catastrophic mode of lake formation is the release of large volumes of water from behind glacial ice dams. Some of these outbursts happen on a moderate scale now; larger ones occurred during the last ice age. These outbursts occurred as a result of pooling of glacial water as large ice sheets receded, followed by collapse of the ice dam. Such outbursts created massive floods that scoured out existing lakes and created new lakes below any spillways that existed in the channels. Kehew and Lord (1987) suggest that such outbursts established the courses of most major rivers in the midcontinental United States and Canada. Lake Missoula was formed in western Montana behind the retreating ice sheet and was responsible for several massive floods Sidebar 6.1. downstream in the Columbia River basin. A Large Lake beneath the Ice The lakes in the Grand Coulee in eastern in Antarctica Washington state are below the spillways In 1974 and 1975, an airborne radio-echo surwhere the floods dug out massive quantities vey of Antarctic ice depths led to the discovof sediment and deeply incised the channels. ery of a lake under the ice. The ice sitting on Volcanic activities can lead to formation the lake’s surface is flat relative to the surof lakes. Explosions of volcanoes or pockets rounding ice sitting on land, and remote satelof steam can leave behind depressions in the lite measurements of ice elevation have alcraters that fill with water. These lakes are lowed determination of the size of the lake called caldera or maar lakes. An example of (Kapitsa et al., 1996). The lake is estimated to a volcanic lake is the exceptionally clear and cover about 15,000 km2, is 125 m deep, and deep Crater Lake in Oregon (Fig. 1.1). The rests below about 4 km of ice. Preliminary callake was formed when the volcano Mount culations suggest that the residence time of Mazama exploded about 6000 years ago. This the water in the lake is tens of thousands of eruption must have been a catastrophic event years, and that the lake basin is about 1 million for Native Americans living in the region duryears old. However, there is significant water ing the time since several meters of ash were exchange between the lake and the ice sheet deposited throughout western North Amer(Siegert et al., 2000). Scientists have taken ica. The resulting crater filled with water and cores through the ice sheet to 3950-m depth a subsequent eruption formed a volcanic cone (about 120 m above the lake). The ice at 3310 in the lake known as Wizard Island. m is about 420,000 years old and was formed Water can dissolve sedimentary rocks by refrozen lake water (Jouzel et al., 1999). and lead to depressions that form lakes. In Analyses of the refrozen lake water from the karst regions, sinkholes form where limeice cores indicate the presence of a microbial stone is dissolved and the cavity collapses to community (Priscu et al. 1999, Karl et al., 1999) form a lake. Similar processes can occur and some of these bacteria may still be viable where old subterranean salt deposits are dis(Karl et al., 1999). Sampling the lake without solved or where sandstone is washed away. contaminating it will be technically difficult but These dissolution lakes are generally small. is certain to yield interesting results. Activities of rivers can form fluvial lakes, including oxbow lakes where meanders
100
6. Physiography of Lakes and Reservoirs
pinch off (see Chapter 5). A levee lake is another fluvial type that is formed next to rivers where periodic floods scour and fill depressions parallel to river channels. The lowland areas surrounding the Amazon River contain many lakes of this sort. They are connected to the Amazon during times of high flow. Fluvial lakes provide an important habitat for many organisms and are involved intimately with the ecology of the river. Fluvial lakes tend to be smaller than either glacial or tectonic lakes and are less important globally than the other two lake types (Fig. 6.5). Additional processes that can form lakes include erosion by wind (aeolian lakes), crater formation by meteoric impacts, formation of depressions by alligators (Alligator mississippiensis) or bison (Bos bison), accretion of corals leading to lakes in the centers of coral atolls, and dam building by beavers (Castor).
LAKE HABITATS AND MORPHOMETRY A lake can be divided into several subhabitats. Lake habitats in general are referred to as lentic or lacustrine (i.e., habitats with deep, nonflowing waters). The open water of a lake, particularly that above sediments that do not receive enough light to maintain photosynthetic organisms, is the pelagic habitat. The profundal zone is the benthic habitat below the pelagic waters. The profundal zone is influenced by materials that settle from the pelagic waters and usually has sediment composed of fine silt or mud. The shallow zone of a lake, where enough light reaches the bottom to allow the growth of photosynthetic organisms, is referred to as the littoral zone. The relative occurrence of these different subhabitats is determined by the size and shape of the lake. Morphometry, or the shape and size of lakes and their watersheds, is one of the first ways that people classify lakes. The bathymetric map (a depth-contour map of a lake bottom) of a lake provides important information on geomorphologic properties (Fig. 6.9). Generally the first measurement made is of the area of the lake (A) and the second is of depth (z). The maximum depth (zmax), mean depth (z苶), and volume (v) are also of interest. The volume is the product of area and mean depth: v A z苶 In general, lakes with a low mean depth are more productive than deeper lakes. Greater productivity of shallow lakes is a consequence of wind mixing the nutrients up from the bottom more readily, more extensive shallow habitat for primary producers that use the lake bottom, and other morphometric considerations. If the volume of a lake and the amount of water entering and leaving the lake are known, then the retention time or water residence time of the water in the lake can be determined. The average retention time can be calculated as follows: Retention time volume/discharge into lake The retention time can vary widely from several hours for a small pond with a large inflow to thousands of years for very large lakes. The water residence time is important in determining the residence time of pollutants in a lake,
Lake Habitats and Morphometry
FIGURE 6.9
Bathymetric maps of Belton Reservoir, Texas (left), and Flathead Lake, Montana (right). The reservoir is dendritic and shallow, with the deepest portion near the dam (lower right). Flathead Lake was formed by a combination of tectonic and glacial processes and is deep with a regular shoreline and shallow outlet (lower left).
how quickly the biota can be washed out, and the general influence of tributaries entering a lake. For example, Lake Tahoe has a 700-year residence time (Table 6.2) so it is very sensitive to nutrient pollution (see Chapter 17). Another important aspect of morphometry is the irregularity or degree of convolution of the shore. An index used to quantify this is called the shoreline development (DL). This index compares the minimum possible circumference of the lake, given a specific surface area (i.e., a perfect circle), to the actual circumference of the lake and its surface area. A value of 1 for shoreline development is a perfect circle and a larger value means that the shoreline is highly dissected. High shoreline development is generally related to small values of mean depth and mode of formation, and it is indicative of a high degree of watershed influence. Shoreline development is calculated as follows: L DL 2兹 A 苶0 Where L is the length of the shore, and A0 is the surface area of the lake. Lakes with high shoreline development are often naturally productive relative to those with low shoreline development (Example 6.1). The idea of a dissected shoreline resulting in increased productivity leads to consideration of the watershed of a lake as a determinant of productivity. A lake with a relatively large watershed will have much land from which nutrients can be washed. Such a lake is likely to be more productive than a lake with a small watershed. Land-use practices also play a major role in determining nutrient inputs. Reservoirs are common features of today’s landscape, so understanding how they vary from natural lakes is important. Damming can form
101
102
6. Physiography of Lakes and Reservoirs
EXAMPLE 6.1. Compare Morphology of Two Lakes—Crater Lake and Milford Reservoir Crater Lake, Oregon, and Milford Reservoir, Kansas, are lakes of contrasting properties, despite the fact that they are similar in surface area. Crater Lake (Fig. 1.1) is a deep oligotrophic lake in the crater of a large volcano; its scenic grandeur has earned it national park status. Milford Reservoir is a typical Midwest U.S. lake; it is eutrophic but widely used for recreation, including a vibrant fishery. Much of the difference in levels of productivity in these two systems can be related to their contrasting morphometric properties, but the level of agricultural activity in the watershed of Milford Reservoir is also much greater. The following are vital characteristics of the two lakes:
Area (km2) Mean depth (m) Watershed area (km2) Shoreline length (km) Inflow (m3/s)
Crater Lake
Milford Reservoir
55 364 81 42 4.3
65 7.4 64,465 122 27.2
Calculate volume, water replacement time, and shoreline development. Also calculate the ratio of the area of the watershed to the lake volume and speculate how these features may alter trophic state. Volume for Crater Lake area mean depth 55 0.364 20.3 km3 Volume for Milford Reservoir 65 0.0074 0.48 km3 Water replacement for Crater Lake volume/discharge 203/ 0.136 1500 years Water replacement for Milford Reservoir 0.48 km3/(0.858 km3/y1) 0.56 years Shoreline development for Crater Lake DL 42/(2兹 55 苶) 1.6 Shoreline development for Milford Reservoir DL 122/(2兹 65 苶) 4.3 Watershed area/ lake volume for Crater Lake 4.0 Watershed area/ lake volume for Milford Reservoir 134,000
All these parameters but one suggest that the watershed will have a much greater influence on the water in Milford Reservoir and that nutrients and light should be greater in Milford Reservoir. The only caveat is that the water replacement is so slow in Crater Lake that once a nutrient enters the system, it could be recycled for some time. The high shoreline development index, watershed area to volume, and low mean depth suggest that Milford Reservoir should indeed be more eutrophic than Crater Lake.
Stratification
natural lakes, and presumably reservoirs are not much different, except that natural lakes usually do not release deep waters downstream. Occasionally, the capacity of natural lakes is increased and outflow is regulated by adding a dam. Unlike natural lakes, reservoirs are deep near the dam and generally become shallower near the deltas of the rivers that feed them. Reservoirs are often limited by the surrounding topography, so they have a lower mean depth than many natural lakes. Low mean depth can lead to increased mixing and associated suspended solids. Reservoirs fill the drainage basins of rivers and streams, and each arm of a reservoir moves up into a former stream channel. Thus, a typical reservoir has a dendritic or tree-like shape (Figs. 6.1 and 6.9). A dendritic shape results in a high value for the shoreline development index. The shallow mean depth and high shoreline development index indicate that many reservoirs are very productive unless turbidity limits light for photosynthetic production.
STRATIFICATION The factors influencing density of water that were discussed in Chapter 2 and the heating effects of light discussed in Chapter 3 have profound effects on mixing in lakes. These effects influence the biogeochemistry, biology, and physical geology of lakes. A primary factor creating stratification of lakes is the difference in density resulting from temperature or salinity variation. The classical understanding of lake stratification is based on consideration of cold-temperate lakes, so this seasonal sequence of stratification is considered first. During the early spring in a cold-temperate lake, the water is isothermal, or approximately the same temperature from top to bottom (Fig. 6.10). An isothermal lake can be completely mixed by wind, leading to spring mixing. The entire lake will continue to mix as long as the wind continues to blow. As the spring season progresses, the surface of the water is warmed by solar energy. The surface waters of the lake heat the most because the infrared radiation (heat) is absorbed quickly with depth. If you have ever swum in cold water on a calm, sunny spring day, you are familiar with the phenomenon of the top several centimeters of the water being much warmer than the deeper water. Such stratification is only temporary because the wind can mix a shallow layer of warm water into the lake. When a series of calm, warm days occurs, the lake stratifies. The surface waters of the lake heat enough so that the wind cannot completely mix the warmer, less dense water into the cooler water below. The top of the stratified lake is called the epilimnion. The zone of rapid temperature transition is the metalimnion or thermocline. The bottom of the lake at fairly constant temperature is called the hypolimnion (Fig. 6.11). The stratified layers will stay distinct until a prolonged period of cool weather occurs. The period with distinct layers is called summer stratification. Prolonged summer stratification is a combined function of the very slow rate of diffusion of heat across the metalimnion and the continued heating of the epilimnion. Because there is minimal mixing across the metalimnion, no eddy diffusion of heat occurs; only molecular diffusion occurs. The slow rates of molecular diffusion were discussed in Chapter 3.
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6. Physiography of Lakes and Reservoirs
5
14
2
6 7 8
11 10
ice 18 17
10 16
9
4
7
11
3
15
8
0 2 4 6 8 10 12 14
Depth (m)
13
4
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Temperature 0
0
5 10 15 20
5 10 15 20
0
Dec
Jan
Feb
Mar
(0C) 5 10 15 20
0
5 10 15 20
0 2
Depth (m)
4 6 8 10 12 14
Spring mixing Summer stratification Winter stratification Fall mixing A depth contour plot of lake temperature over the course of a year in a dimictic cold-temperate lake (Esthwaite Water, an English lake). The thick black line at the top right corner of contour plot indicates ice cover. (Bottom) Two-dimensional representations of the temperature versus depth at each phase of stratification (data from Mortimer, 1941).
FIGURE 6.10
The epilimnion is very stable relative to the ability of the wind to mix a lake. There can be some mixing of the top of the hypolimnion (entrainment) with extreme winds, but even hurricane-force winds will not fully mix a well-stratified lake (Fig. 6.12). The stratification will break down only when the autumn weather can cool the epilimnion to approximately the same temperature as the hypolimnion. Cool air coupled with continued heat losses from surface evaporation decrease the temperature of the
Temperature (0C) 0
0
2
4
6
8
10
12
14
16
18
Epilimnion
5 Depth (m)
104
Metalimnion (thermocline)
10
15 Hypolimnion
20
25 Temperature as a function of depth for Triangle Lake, Oregon, on October 1, 1983, and positions of epilimnion, metalimnion, and hypolimnion (data from R. W. Castenholz).
FIGURE 6.11
Stratification
Temperature (oC) 0
6
8
10
12
14
16
18
20
3
Depth (m)
5 8 10
After hurricane Before hurricane
13 15
FIGURE 6.12
Stability of the thermocline in Linsley Pond, Connecticut, before and after a hurricane on September 21, 1938, with wind speeds up to 100 km h1 (from G. E. Hutchinson, A Treatise on Limnology, Vol. 1, Geography, Physics and Chemistry. Copyright © 1957 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.).
surface. The cooled surface water is denser than the water immediately below, so it sinks. The wind can mix the lake once the entire lake is isothermal, and the fall mixing period begins. The lake will continue to cool and mix until formation of an ice cover on the surface of the lake. The surface of the lake can freeze when the temperature of the entire lake is below 3.9°C. If the lake is warmer, cool water from the surface will continue to sink and mix with the less dense water below it. If the lake temperature is 3.9°C, the water is at its densest, so cooler water will sit on the surface of the lake as long as the wind does not mix it. The surface of the lake can freeze if there is a cold, calm night. The low-density ice and cold, low-density water will sit on the surface of the lake once the surface has frozen. No more wind reaches the surface of the water so no more mixing occurs. Winter stratification is the second period during the year when a cold-temperate lake does not mix (Fig. 6.10), and it lasts as long as cold weather maintains the ice cover. Duration of ice cover provides one of the best long-term records of the effects of global warming on freshwater systems. There are good data on dates of formation and breakup of ice cover for many lakes and rivers for the past 100 years, and records exist for Lake Suwa in Japan almost continuously since 1450. These data suggest that freeze dates have become later by 5.8 days over the past 100 years and breakup dates 6.5 days earlier (Magnuson et al., 2000). Much terminology is used to describe the mixing regimes of lakes (Table 6.1). Lakes that mix twice a year are called dimictic. Lakes that only mix once during the year are called monomictic and are common in temperate and subtropical regions where winters are not cold enough to freeze lake surfaces but are cool enough to allow the lakes to become isothermal. Lakes that never mix are called amictic. Polymictic lakes stratify or mix several times a year and are found mainly in tropical regions. Meromictic
105
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6. Physiography of Lakes and Reservoirs
METHOD 6.1. How Do Limnogists Sample Water from Lakes? The main consideration in determining how to sample water from a lake is what information is desired. If the information can be obtained with a submersible sensor (such as for temperature, dissolved oxygen, pH, and conductivity) results are obtained most easily without removing samples. However, for many chemical and biological parameters, water must be removed from known depths with a minimum of contamination or turbulence. Several devices are available that allow water to be sampled from depth. Many different types of limnological equipment are lowered into the water on ropes or cables. The devices are generally triggered with a weight that is dropped down the line to the sampling equipment. This weight is called a messenger. Van Dorn bottles and Kemmerer bottles are devices used to remove
lakes rarely if ever mix because they contain dissolved compounds in the hypolimnion that stabilize density layers. Several conditions cause meromictic lakes. Seasonal temperature regimes can be constant enough (mainly in tropical areas) that lakes rarely mix. The temperature difference in tropical lakes does not need to be as great to form a stable stratification as in the temperate zone because the water temperatures are higher and a greater relative difference in density occurs for each degree difference in water temperature (Fig. 2.3). For example, the density difference is greater between 20 and 25°C than between 10 and 15°C water. Salinity differences can also cause stable stratification. In this case, more saline water can sit below cooler surface waters when the salinitycaused density difference is greater than the temperature-related differences. Several conditions can cause such salinity differences. In tropical lakes that are stratified for long periods of time, the nutrients enter the surface waters from rivers. These nutrients enter the biomass of the planktonic food web, and when organisms die they sink. The sinking organisms slowly release nutrients and a portion is transported to the hypolimnion. Slowly, the salinity of the hypolimnion increases and the stratification is stabilized. In arid regions, evaporation can lead to increases in dissolved salt concentrations. Fresh river water flowing into the lake will remain on top of the denser saline water. A fresh surface lake is a common occurrence in closed basins where saline lakes form. Such was the case in the Dead Sea, where a stable stratification was maintained for nearly 300 years until water diversions for human uses reduced inflow to the lake and the dilute surface layer disappeared in the 1970s (Gavreili, 1997). Fjord lakes can also have saline waters below freshwater. In this case, a glacial valley is formed below sea level. As the glacier recedes, saline marine water floods the valley. The floor of the valley rebounds from the weight of the glacier and if a raised portion exists at the end of the valley (e.g., a terminal moraine), the saline ocean water can be isolated. Freshwater flows on top of the saline water and a lake forms with saline water
Water Movement and Currents in Lakes
water from lakes (Lind, 1974). The Van Dorn bottle consists of a tube with covers over each end. The messenger releases a catch so the stretched rubber connectors can pull the covers onto the tube, sealing the water into it. Kemmerer bottles operate similarly, but rather than using rubber connectors to pull ends onto a tube, gravity is used (Fig. 6.13). Additional devices include pumps to remove water from depth, bottles with strings attached to stoppers that can be unplugged at depth, and pipes that allow water to flow up to containers that displace surface waters (Fig. 6.13). The choice of device depends on the type of sample that is required. Toxic materials generally are to be avoided, and if chemical analysis on metals is to be done metal samplers should not be used. The violent closure of some samplers can harm some organisms that are susceptible to pressure shock. Zooplankton may avoid an opaque sampler more than a clear one because of their predation avoidance behaviors.
on the bottom and freshwater on the top. Saline springs on the bottom of lakes have also formed stable layers. Some of the dry-valley lakes in Antarctica have such stable layers (see Chapter 15). The biological and biogeochemical effects of stratification on lake organisms are strong. Molecular diffusion rates that dominate movement of dissolved materials across the metalimnion are slow enough that a significant depletion of O2 in the hypolimnion will lead to anoxia during the summer. In turn, O2 loss from the hypolimnion means that biogeochemical cycling and lake productivity are altered. Anoxia and the biogeochemistry are discussed in detail in Chapters 11–13. Given the very complex chemical and physical characteristics in many stratified lakes, several methods for sampling lake waters from different depths have been developed (Method 6.1).
WATER MOVEMENT AND CURRENTS IN LAKES The movement of wind is generally the main cause of waves across lakes, although motorboat activity can cause significant wave action. Wave action is important partially because it is associated with surface mixing and erosion of the shoreline. Lakeshore erosion can lead to habitat destruction and large financial losses associated with property damage; many environmental engineering firms specialize in controlling erosion. Wave action can influence which species can successfully inhabit the different depths of the shallow benthos (littoral zone). The two main determinants of wave height are the strength (speed and duration) of the wind and the length of lake on which the wind acts. The influence of the wave also is dependent on the geometry and materials that make up the shoreline. The length of lake on which the wind acts is called the fetch (Fig. 6.14). The longer the fetch, the higher the waves (Fig. 6.14). A perfectly round lake would be affected similarly by wind from any direction. On an irregularly shaped lake, certain wind directions lead to the largest waves. In smaller
107
108
6. Physiography of Lakes and Reservoirs
FIGURE 6.13 (A) A simple water sampler made from a weighted bottle and stopper, (B) a sampler that collects water from depth by displacing water at the surface, and (C) a Kemmerer sampler (photograph courtesy of Wildlife Supply Company). Only samplers (such as type C) that close at depth are suitable for collecting dissolved gas samples.
FIGURE 6.14 How fetch of an irregularly shaped lake varies with wind direction (A, B) and relationship between maximum wave height and fetch (C) (equation for C from Wetzel, 1983).
Summary
ponds and lakes, features such as hills or trees can prevent the wind from making very large waves. For example, deforestation can lead to a deeper epilimnion (by about 2 m in some small Canadian lakes) because of increased mixing during spring warming (France, 1997a). Even though the processes of surface wave formation are the most apparent to human observers, the wind causes other water movements in lakes that can also be important. As wind moves water at the surface of a lake forward, it must be replaced by water from below. This process leads to spiral circulation patterns called Langmuir circulation cells (Fig. 6.15). The spiraling water moves in alternating directions, leading to lines of downwelling water alternating with lines of upwelling water. These lines form along the direction of wind. Floating materials aggregate at the water surface along the downwelling lines and form streaks in the same direction as the wind is blowing. These circulation cells are several meters wide. In addition to the smaller scale waves and Langmuir cells, movement of water can also occur within the whole lake’s volume. When a sustained wind occurs, it causes water to pile up on the downwind side of the lake, and when the wind suddenly ceases the surface of the lake can rock. This rocking of a lake’s entire surface is called a seiche. An interesting phenomenon occurs in stratified lakes that are subjected to a sustained unidirectional wind. The force of the wind causes the water in the epilimnion to move across the lake to the downwind side (Fig. 6.16), and the depth of the epilimnion is greater downwind than upwind. Under extreme winds, the hypolimnion can come to the surface on the upwind side. When the wind ceases, the less dense water of the epilimnion moves back across the
Convergence
Spiral water currents
Divergence
Wind direction
Top view
Floating streaks
Water surface
Divergence
Convergence
Divergence
Convergence
Divergence
Convergence
Side view FIGURE 6.15
Langmuir circulation cells on a lake (top view and side view).
109
110
6. Physiography of Lakes and Reservoirs
Wind stops A
Epilimnon
D
Hypolimnion Hypolimnion begins to rock
Wind E
B
Hypolimnion rocks to opposite extreme, then returns to original position
Wind C
F
Mixing zones
Seiche continues but slowly dissipates
FIGURE 6.16
Formation of an internal seiche and entrainment associated with wind. Dashed arrows show water flow. (A) The lake under calm conditions; (B) the wind deepens the epilimnion on the right; (C) a strong wind mixes some of the epilimnion with the hypolimnion; (D) the wind stops and the hypolimnion begins to oscillate; and (E and F) the amplitude of the seiche diminishes over time.
surface of the lake, and the hypolimnion moves back toward its original position. Like a pendulum, the surface of the hypolimnion rocks back farther than its original position. Thus, an internal seiche is created where the surface of the lake appears still, but the plane that forms the top of the hypolimnion continues to oscillate for hours or days after the wind ceases. Aside from the intrinsic elegance of the seiche as a physical phenomenon, this type of water movement has an important biological implication. Even though the hypolimnion is very stable, seiches can lead to a moderate amount of mixing of hypolimnetic and epilimnetic water. The movement of this water up to the epilimnion is called entrainment. Entrainment causes nutrient-rich water from the hypolimnion to reach the epilimnion (Fig. 16.16C), causing stimulation of primary production. Nutrient mixing can be significant biologically because the mixing rate far exceeds the rate of molecular diffusion that usually predominates between the hypolimnion and the epilimnion. Seiches can also influence rooted plants and benthic invertebrates by altering temperature and nutrient regimes. The lake model exercises discussed by Wetzel and Likens (1991) are highly recommended for students who want a clearer understanding of the processes of stratification and seiches.
SUMMARY 1. A variety of processes form lake basins, including tectonic, glacial, fluvial, volcanic, and damming processes. Glacial lakes are the most numerous
Questions for Thought
2.
3. 4. 5.
6.
7.
8.
worldwide, but some of the largest, deepest, and oldest lakes are formed tectonically. Fluvial lakes can be very important to riverine ecology. Lake basin morphology is described with various parameters, including mean depth, area, maximum depth, volume, shoreline development (DL), and watershed area relative to lake surface area. Shallow lakes with large watersheds and highly dissected shorelines are generally the most productive. Waves are greatest where the wind has the longest length of lake (fetch) to act on. Wind causes Langmuir circulation patterns, which lead to streaks of floating material on the water surface but also mix the lake to depth. Stratification can alter the water circulation in lakes and thus alter biogeochemical, ecosystem, and community properties. Mixing can occur often (polymictic), once a year (monomictic), twice a year (dimictic), or rarely (amictic or meromictic), depending on climate and type of stratification. Thermal stratification occurs when warm surface water sits above denser, cooler waters. The warm surface layer of a thermally stratified lake is the epilimnion, the zone of steep temperature transition is the metalimnion, and the deepest stable zone is the hypolimnion. High concentrations of dissolved substances can also lead to stratified layers in lakes. Such chemically driven stability can exceed temperature-driven stability because density differences can be greater than are possible with natural temperature differences. A sustained wind that suddenly stops can cause oscillation of the lake surface (an external seiche) or the hypolimnion (internal seiche). This rocking can lead to breakdown of stratification. Mixing of deeper waters into the surface is called entrainment.
QUESTIONS FOR THOUGHT 1. Why is it sometimes difficult to assign a single geological explanation for a lake’s origin? 2. Why is a lake with a high value for DL likely to have smaller waves than a lake of comparable surface area with a DL close to 1? 3. Why do more lakes occur farther from the equator? 4. In which order (from greatest to least) should lakes be ranked with respect to the ratio of maximum depth divided by the mean depth: tectonic, glacial, and fluvial? 5. Langmuir circulation cells concentrate particles slightly more dense than water below the surface of the water: Where will these particles be concentrated and why? 6. Under what conditions would thermal stratification lead to anoxia in the hypolimnion? 7. Explain why some rivers flowing into lakes flow down into the hypolimnion, some flow across the surface, and others flow into the metalimnion.
111
Satellite images of the Great Lakes (left) and Smithville Reservoir (Missouri) (right). The reservoir is 16 km long. Note the dendritic pattern of the reservoir and the relatively smooth shorelines of the glacially formed Great Lakes. The numerous black dots around the reservoir are farm ponds (data from the U.S. Geological Survey).
6
Physiography of Lakes and Reservoirs Formation: Geological Processes Lake Habitats and Morphometry Stratification Water Movement and Currents in Lakes Summary Questions for Thought
Lakes of all sizes provide us with fisheries, recreation, drinking water, and scenic splendor. Having a clean lake nearby increases property values. Large lakes (Table 6.1) have played a part in the history, economy, and culture of many nations. Lakes also provide an excellent system for ecological study. The boundaries of the lake community and ecosystem often appear distinct, the water well mixed, and the bottom relatively homogeneous, making lakes a tractable system for ecologists. Much effort has been made to study the physical and biological aspects of lakes (e.g., some of those in Table 6.2 have been studied intensively for approximately a century) and to manage pollution. The base of these studies is an understanding of the geomorphology of the lakes. Different lake morphologies give rise to different levels of productivity and physical effects of water retention, circulation, currents, and waves. For example, the fates of toxins and nutrients in lakes depend partly on lake circulation, which is a function of lake physiography. In this chapter, I describe formation, lake morphometry, the process of stratification, and water movement in lakes.
FORMATION: GEOLOGICAL PROCESSES What is a lake? I define a lake as a very slowly flowing open body of water in a depression of ground not in contact with the ocean. This definition
91
TABLE 6.1 Properties of the 10 Largest Lakes by Depth, Area, or Volume, Globally Arranged by Maximum Deptha
Lake
Continent
Formation
Mixis
Baikal Tanganyika Caspian Nyasa Issyk Kul Great Slave Crater Matano Toba Hornindalsvatn Great Bear Superior Michigan Huron Victoria Aral
Asia Africa Asia/Europe Africa Asia North America North America Asia Asia Europe North America North America North America North America Africa Asia
Tectonic Tectonic Tectonic Tectonic Tectonic Glacial Volcanic Tectonic Volcanic–tectonic Glacial Glacial Glacial Glacial Glacial Tectonic Tectonic
Meromictic Meromictic Meromictic Meromictic Meromictic Dimictic Monomictic
a
Monomictic Dimictic Dimictic Monomictic Monomictic Monomictic Polymictic Meromictic
Area (km2)
Maximum depth (m)
Mean depth (m)
Volume (km3)
Length (km)
31,500 34,000 436,400 30,800 6,200 30,000 55 164 1,150 508 29,500 83,300 57,850 59,510 68,800 62,000
1741 1470 946 706 702 614 608 590 529 514 452 307 265 223 79 68
730 572 182 273 320 70 364 240 216 237 81 145 99 76 40 16
23,000 18,940 79,319 8,400 1,732 2,088 20 39 249 12 2,381 12,000 5,760 4,600 2,700 970
2200 1900 6000 1500 760 2200 35 80 100 65 2100 3000 2210 2700 3440 2300
Data are from several sources, including Herdnedorf (1990), Hutchinson (1957), Horne and Goldman (1994), and Gasith and Gafny (1990).
DL 3.4 3.1 2.6 2.7 2.8 3.6 1.3 1.8 2.6 3.3 2.9 2.6 3.1 3.7 2.6
Retention (years) 323 5500 Sink 305 4.9
124 184 104 21 23 Sink
TABLE 6.2 Selected Lakes Not Listed in Table 6.1 with Historical or Research Interesta Area (km2)
Maximum depth (m)
Mean depth (m)
Volume (km3)
Length (km)
46.2 64 4 310 43 230 25.3 225 501 280 67
45 21 2.9 153 26 133 12.8 91 249 106 33.8
28 540 23 89 4.3 7.5 0.47 1720 124 820 0.17
46 1200
Monomictic Amictic
618 25,820 0–8,583 580 1.7 56.4 39.8 18,760 499 7,700 5.2
Monomictic
14.3
64
23
Lake
Continent
Formation
Mixis
Biwa Erie Eyre Geneva Kinneret Loch Ness Mendota Ontario Tahoe Titicaca Vanda
Asia North America Australia Europe Asia Europe North America North America North America South America Antarctica
Tectonic Glacial Tectonic Glacial Tectonic Glacial Glacial Glacial Tectonic Tectonic Glacial
Monomictic Monomictic Polymictic Monomictic Monomictic Monomictic Dimictic Monomictic
Windermere (both basins)
Europe
Glacial
a
0.39
Retention (years)
2.6 2.1 4.9
5.4 3 Sink
70 2.2 39 9.1 1380 125 176 5.6
1.16 3.2 1.6 2.8 1.6 3.46 2.28
17
1.2
Data are from several sources, including Herdnedorf (1990), Hutchinson (1957), Horne and Goldman (1994), and Gasith and Gafny (1990). Ablation is evaporation directly from the ice that covers the lake.
b
DL
7.3 2.8 4.6 8 700 1343 75 (ablationb)
94
6. Physiography of Lakes and Reservoirs
includes saline lakes but excludes estuaries and other mainly marine embayments. The distinction between a small shallow lake or pond and a wetland is not clear, and neither is that between a very slow, wide spot in a river and a lake or reservoir with high water throughput. Remember, all aquatic habitats occur across a continuum of physical attributes, such as depth and water velocity. Permanent lakes are common where more precipitation occurs and where geology allows for formation of water-retaining basins (Fig. 6.2A). Some areas have geological histories that result in more lakes. For example, if we compare the distribution of wetlands (Fig. 4.11) to the distribution of freshwater lakes, relatively more lakes than wetlands occur in northern North America, and wetlands are relatively important in northern Asia and northeast Europe. Intermittent lakes (those that dry sometimes) are distributed sparsely throughout the world, with greater numbers in drier areas (Fig. 6.2B). The western United States, south Australia, India, central Asia, and central Africa all have high numbers of intermittent lakes. Humans have made many lakes and ponds. Most regions inhabited by humans with few natural lakes and even moderate precipitation have significant numbers of ponds and reservoirs. The large number of rivers in the Northern Hemisphere that have been altered by the construction of dams was discussed in Chapter 5. More small than large lakes exist in the world. However, the sum of the area of lakes globally of each size is fairly constant, with the few very
FIGURE 6.2 Cogley, 1994).
Global distribution of permanent (A) and impermanent (B) lakes (data from
Formation: Geological Processes
large lakes having a major impact on total area (Fig. 6.3). A variety of geological processes lead to the formation of these lakes (Table 6.3). Hutchinson (1957) described these processes in detail; I give a brief version here. Tectonic movements of the earth’s crust (Fig. 6.4) form some of the largest and oldest lakes. For example, warping of the earth’s crust formed the Great Rift Valley in Africa and has given rise to Lakes Edward, Albert, Tanganyika, Victoria, Nyasa, and Rudolf. This group contains some of the oldest, deepest, and most ecologically and evolutionarily interesting lakes on Earth. Although small tectonic lakes are more numerous than large lakes, the large tectonic lakes cover an area that is greater than that covered by the small ones on a global scale (Fig. 6.5). Graben lakes are tectonic lakes formed where multiple faults allow a block to slip down and form a depression. Lake Baikal of Siberia, the deepest and oldest lake on Earth, is a graben lake. About 7 km of sediment has accumulated on the bottom of Lake Baikal over 16 million years (Fig. 6.6). Tectonic movements also form horst lakes. In this case, the blocks tilt and leave a depression that can be filled by water (Fig. 6.4). Damming by natural processes can form lakes. Examples of these processes include landslides, lava flows, drifting sand dunes, and glacial moraines. In addition, beaver ponds, damming by excessive plant growth, flows of rivers at deltas, glacial ice dams, and pools formed at the edges of large lakes by shore movement are classified into this general category. These lakes usually are not large, but some exceptions exist (e.g., Lake Sarez, a large landslide lake in Russia, is 500 m deep).
Total area (km 2 x 105)
7 6 5 4 3 2 1
Total number
0
106 105 104 103 102 101 5
10
4
10
3
2
10
-1
-1
-1
6
5
0
0
4
3
0
0
.1
00
-1
-1
10
1
-0
10
10
1-
01
1-
0.
0.
Lake size range (km 2) FIGURE 6.3 Global numbers and total areas of lakes by surface area size class (data from Meybeck, 1995).
95
96
6. Physiography of Lakes and Reservoirs
TABLE 6.3 Ways That Lakes Form and Essential Characteristics of Each Typea Lake type
Formation process
Essential characteristics and examples
Tectonic
Basin formed by movement of Earth’s crust: graben, a block slips down between two others; horst, diagonal slippage Formed when ice left from retreating glacier is buried in till (solid material deposited by glacier) and then melts Glacial activity deposits a dam of rock and debris Movement of earth dams a stream or river
Can be very old and very deep; Lake Baikal, Asia, and Lake Tanganyika, Africa
Pothole or kettle
Moraine Earthslide Volcanic—caldera Dissolution lake Oxbow
Volcanic explosion causes hole that is filled with water Limestone dissolves and lake forms River bend pinches off, leaves lake behind
Small lakes/wetlands; prairie pothole region in Alberta and North and South Dakota Narrow, fill valley Similar to reservoirs; Quake Lake, Montana Often round and deep; Crater Lake, Oregon Small, steep sides Shallow, narrow, may be seasonally flooded
a
Many more types are possible (Hutchinson, 1957).
Glacial activity is responsible for the formation of many lakes in the temperate regions and for the formation of more lakes than any other process (Fig. 6.5). Several processes associated with glacial activity lead to lake formation. Glaciers scour as they move down valleys. The ice flow of these glaciers creates basins. Lakes occur where glaciers have scoured more deeply, leading to the formation of cirque (also called tarn lakes) lakes in the “amphitheaters” at the heads of the valleys. The glacier forms chains of paternoster lakes as it flows further down the valleys (Figs. 6.7 and 6.8). Glacial scour can lead to formation of extremely large lakes. The Great Slave Lake in Canada was carved to a depth of 464 m below sea level by the massive weight of the continental ice sheet and is the deepest lake in North America. The Laurentian Great Lakes of North America (e.g., Superior, Huron, and Erie) were partially formed by glacial action. Fjord lakes such as Loch Ness (the home of a legendary creature) are long glacial lakes formed in steep val-
FIGURE 6.4 Diagrams of two modes of lake formation associated with tectonic processes: (A) graben, a block drops below two others; and (B) horst, blocks tip and a lake forms along a single fault line.
40 30 20 10 0
106
Tectonic
Tectonic
104 102
Total Number in Class
Total surface area (10,000 km2)
Formation: Geological Processes
40 Glacial
30 20 10 0 6
Fluvial
100
0.01-0.1
0.1-1
1-10
10 2 -10 3
10-100
106
10 3 -10 4
10 4 -10 5
10 5 -10 6
Glacial
104 102 100 0.01-0.1 10
4
104
2
102
0
100
104-105
10-100
6
Fluvial
5
4
10
10
3
10
-1
-1
-1
6
5
0
4
0
3
0
0
00
-1
-1
2
10
.1
6
5
0
4
0
3
0
0
-0
1
10
10
1-
-1
-1
-1
.1
00
-1
01
1-
0.
0.
5
4
10
10
3
10
2
-1
10
-0
1
10
10
1-
01
1-
0.
0.
Surface area range (km2)
FIGURE 6.5
Global numbers and total areas of lakes of different geological origins by surface area size class (data from Meybeck, 1995).
leys. One of the strangest lakes associated with glaciers is the gigantic lake that has recently been described below the ice in Antarctica (Sidebar 6.1). As glaciers move, they entrain rocks and sediments into the ice. Where glaciers melt at the edges and front, they deposit these materials. As the glaciers retreat they leave this material, called glacial till, behind. If large blocks of ice remain in this till, they melt and eventually leave lakes, ponds, or wetlands called kettles or potholes (Fig. 6.7). This process formed the many lakes and ponds that provide vital habitat to waterfowl in the northern
Olkhon Island
Fault
Depth from lake surface(km)
Lake surface
0 1 0 - 1.7 million year old sediment
2 3 4
3.5 - 35 million year old sediment
5 6
FIGURE 6.6
Bedrock
0
1
2
3
4 5 Width (km)
6
7
8
9
Cross section of Lake Baikal, the south basin, in the region of maximum depth (1620 m) [redrawn from Belt (1992) and Mats (1993)].
97
98
6. Physiography of Lakes and Reservoirs
FIGURE 6.7
Formation of some types of glacial lakes. (A) A cross section of a glacier moving down a valley. (B) After the glacier has retreated, it leaves cirque, moraine dammed, and pothole lakes.
FIGURE 6.8
Paternoster lakes (a string of glacial lakes) in a snowy mountain valley in the Jewel Basin of Montana.
Formation: Geological Processes
99
prairies of North America, although many have been filled for agricultural purposes. If the forward flow of a glacier is approximately equal to its backward melting rate, a wall of material is formed called a terminal moraine, which can impound water flow and lead to formation of lakes. Materials deposited along the sides of glaciers form lateral moraines. Glacial lakes tend to be smaller than tectonic lakes, but a few very large glacial lakes (e.g., the North American Great Lakes) make up a considerable area when considered on a global basis (Fig. 6.5). A catastrophic mode of lake formation is the release of large volumes of water from behind glacial ice dams. Some of these outbursts happen on a moderate scale now; larger ones occurred during the last ice age. These outbursts occurred as a result of pooling of glacial water as large ice sheets receded, followed by collapse of the ice dam. Such outbursts created massive floods that scoured out existing lakes and created new lakes below any spillways that existed in the channels. Kehew and Lord (1987) suggest that such outbursts established the courses of most major rivers in the midcontinental United States and Canada. Lake Missoula was formed in western Montana behind the retreating ice sheet and was responsible for several massive floods Sidebar 6.1. downstream in the Columbia River basin. A Large Lake beneath the Ice The lakes in the Grand Coulee in eastern in Antarctica Washington state are below the spillways In 1974 and 1975, an airborne radio-echo surwhere the floods dug out massive quantities vey of Antarctic ice depths led to the discovof sediment and deeply incised the channels. ery of a lake under the ice. The ice sitting on Volcanic activities can lead to formation the lake’s surface is flat relative to the surof lakes. Explosions of volcanoes or pockets rounding ice sitting on land, and remote satelof steam can leave behind depressions in the lite measurements of ice elevation have alcraters that fill with water. These lakes are lowed determination of the size of the lake called caldera or maar lakes. An example of (Kapitsa et al., 1996). The lake is estimated to a volcanic lake is the exceptionally clear and cover about 15,000 km2, is 125 m deep, and deep Crater Lake in Oregon (Fig. 1.1). The rests below about 4 km of ice. Preliminary callake was formed when the volcano Mount culations suggest that the residence time of Mazama exploded about 6000 years ago. This the water in the lake is tens of thousands of eruption must have been a catastrophic event years, and that the lake basin is about 1 million for Native Americans living in the region duryears old. However, there is significant water ing the time since several meters of ash were exchange between the lake and the ice sheet deposited throughout western North Amer(Siegert et al., 2000). Scientists have taken ica. The resulting crater filled with water and cores through the ice sheet to 3950-m depth a subsequent eruption formed a volcanic cone (about 120 m above the lake). The ice at 3310 in the lake known as Wizard Island. m is about 420,000 years old and was formed Water can dissolve sedimentary rocks by refrozen lake water (Jouzel et al., 1999). and lead to depressions that form lakes. In Analyses of the refrozen lake water from the karst regions, sinkholes form where limeice cores indicate the presence of a microbial stone is dissolved and the cavity collapses to community (Priscu et al. 1999, Karl et al., 1999) form a lake. Similar processes can occur and some of these bacteria may still be viable where old subterranean salt deposits are dis(Karl et al., 1999). Sampling the lake without solved or where sandstone is washed away. contaminating it will be technically difficult but These dissolution lakes are generally small. is certain to yield interesting results. Activities of rivers can form fluvial lakes, including oxbow lakes where meanders
100
6. Physiography of Lakes and Reservoirs
pinch off (see Chapter 5). A levee lake is another fluvial type that is formed next to rivers where periodic floods scour and fill depressions parallel to river channels. The lowland areas surrounding the Amazon River contain many lakes of this sort. They are connected to the Amazon during times of high flow. Fluvial lakes provide an important habitat for many organisms and are involved intimately with the ecology of the river. Fluvial lakes tend to be smaller than either glacial or tectonic lakes and are less important globally than the other two lake types (Fig. 6.5). Additional processes that can form lakes include erosion by wind (aeolian lakes), crater formation by meteoric impacts, formation of depressions by alligators (Alligator mississippiensis) or bison (Bos bison), accretion of corals leading to lakes in the centers of coral atolls, and dam building by beavers (Castor).
LAKE HABITATS AND MORPHOMETRY A lake can be divided into several subhabitats. Lake habitats in general are referred to as lentic or lacustrine (i.e., habitats with deep, nonflowing waters). The open water of a lake, particularly that above sediments that do not receive enough light to maintain photosynthetic organisms, is the pelagic habitat. The profundal zone is the benthic habitat below the pelagic waters. The profundal zone is influenced by materials that settle from the pelagic waters and usually has sediment composed of fine silt or mud. The shallow zone of a lake, where enough light reaches the bottom to allow the growth of photosynthetic organisms, is referred to as the littoral zone. The relative occurrence of these different subhabitats is determined by the size and shape of the lake. Morphometry, or the shape and size of lakes and their watersheds, is one of the first ways that people classify lakes. The bathymetric map (a depth-contour map of a lake bottom) of a lake provides important information on geomorphologic properties (Fig. 6.9). Generally the first measurement made is of the area of the lake (A) and the second is of depth (z). The maximum depth (zmax), mean depth (z苶), and volume (v) are also of interest. The volume is the product of area and mean depth: v A z苶 In general, lakes with a low mean depth are more productive than deeper lakes. Greater productivity of shallow lakes is a consequence of wind mixing the nutrients up from the bottom more readily, more extensive shallow habitat for primary producers that use the lake bottom, and other morphometric considerations. If the volume of a lake and the amount of water entering and leaving the lake are known, then the retention time or water residence time of the water in the lake can be determined. The average retention time can be calculated as follows: Retention time volume/discharge into lake The retention time can vary widely from several hours for a small pond with a large inflow to thousands of years for very large lakes. The water residence time is important in determining the residence time of pollutants in a lake,
Lake Habitats and Morphometry
FIGURE 6.9
Bathymetric maps of Belton Reservoir, Texas (left), and Flathead Lake, Montana (right). The reservoir is dendritic and shallow, with the deepest portion near the dam (lower right). Flathead Lake was formed by a combination of tectonic and glacial processes and is deep with a regular shoreline and shallow outlet (lower left).
how quickly the biota can be washed out, and the general influence of tributaries entering a lake. For example, Lake Tahoe has a 700-year residence time (Table 6.2) so it is very sensitive to nutrient pollution (see Chapter 17). Another important aspect of morphometry is the irregularity or degree of convolution of the shore. An index used to quantify this is called the shoreline development (DL). This index compares the minimum possible circumference of the lake, given a specific surface area (i.e., a perfect circle), to the actual circumference of the lake and its surface area. A value of 1 for shoreline development is a perfect circle and a larger value means that the shoreline is highly dissected. High shoreline development is generally related to small values of mean depth and mode of formation, and it is indicative of a high degree of watershed influence. Shoreline development is calculated as follows: L DL 2兹 A 苶0 Where L is the length of the shore, and A0 is the surface area of the lake. Lakes with high shoreline development are often naturally productive relative to those with low shoreline development (Example 6.1). The idea of a dissected shoreline resulting in increased productivity leads to consideration of the watershed of a lake as a determinant of productivity. A lake with a relatively large watershed will have much land from which nutrients can be washed. Such a lake is likely to be more productive than a lake with a small watershed. Land-use practices also play a major role in determining nutrient inputs. Reservoirs are common features of today’s landscape, so understanding how they vary from natural lakes is important. Damming can form
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EXAMPLE 6.1. Compare Morphology of Two Lakes—Crater Lake and Milford Reservoir Crater Lake, Oregon, and Milford Reservoir, Kansas, are lakes of contrasting properties, despite the fact that they are similar in surface area. Crater Lake (Fig. 1.1) is a deep oligotrophic lake in the crater of a large volcano; its scenic grandeur has earned it national park status. Milford Reservoir is a typical Midwest U.S. lake; it is eutrophic but widely used for recreation, including a vibrant fishery. Much of the difference in levels of productivity in these two systems can be related to their contrasting morphometric properties, but the level of agricultural activity in the watershed of Milford Reservoir is also much greater. The following are vital characteristics of the two lakes:
Area (km2) Mean depth (m) Watershed area (km2) Shoreline length (km) Inflow (m3/s)
Crater Lake
Milford Reservoir
55 364 81 42 4.3
65 7.4 64,465 122 27.2
Calculate volume, water replacement time, and shoreline development. Also calculate the ratio of the area of the watershed to the lake volume and speculate how these features may alter trophic state. Volume for Crater Lake area mean depth 55 0.364 20.3 km3 Volume for Milford Reservoir 65 0.0074 0.48 km3 Water replacement for Crater Lake volume/discharge 203/ 0.136 1500 years Water replacement for Milford Reservoir 0.48 km3/(0.858 km3/y1) 0.56 years Shoreline development for Crater Lake DL 42/(2兹 55 苶) 1.6 Shoreline development for Milford Reservoir DL 122/(2兹 65 苶) 4.3 Watershed area/ lake volume for Crater Lake 4.0 Watershed area/ lake volume for Milford Reservoir 134,000
All these parameters but one suggest that the watershed will have a much greater influence on the water in Milford Reservoir and that nutrients and light should be greater in Milford Reservoir. The only caveat is that the water replacement is so slow in Crater Lake that once a nutrient enters the system, it could be recycled for some time. The high shoreline development index, watershed area to volume, and low mean depth suggest that Milford Reservoir should indeed be more eutrophic than Crater Lake.
Stratification
natural lakes, and presumably reservoirs are not much different, except that natural lakes usually do not release deep waters downstream. Occasionally, the capacity of natural lakes is increased and outflow is regulated by adding a dam. Unlike natural lakes, reservoirs are deep near the dam and generally become shallower near the deltas of the rivers that feed them. Reservoirs are often limited by the surrounding topography, so they have a lower mean depth than many natural lakes. Low mean depth can lead to increased mixing and associated suspended solids. Reservoirs fill the drainage basins of rivers and streams, and each arm of a reservoir moves up into a former stream channel. Thus, a typical reservoir has a dendritic or tree-like shape (Figs. 6.1 and 6.9). A dendritic shape results in a high value for the shoreline development index. The shallow mean depth and high shoreline development index indicate that many reservoirs are very productive unless turbidity limits light for photosynthetic production.
STRATIFICATION The factors influencing density of water that were discussed in Chapter 2 and the heating effects of light discussed in Chapter 3 have profound effects on mixing in lakes. These effects influence the biogeochemistry, biology, and physical geology of lakes. A primary factor creating stratification of lakes is the difference in density resulting from temperature or salinity variation. The classical understanding of lake stratification is based on consideration of cold-temperate lakes, so this seasonal sequence of stratification is considered first. During the early spring in a cold-temperate lake, the water is isothermal, or approximately the same temperature from top to bottom (Fig. 6.10). An isothermal lake can be completely mixed by wind, leading to spring mixing. The entire lake will continue to mix as long as the wind continues to blow. As the spring season progresses, the surface of the water is warmed by solar energy. The surface waters of the lake heat the most because the infrared radiation (heat) is absorbed quickly with depth. If you have ever swum in cold water on a calm, sunny spring day, you are familiar with the phenomenon of the top several centimeters of the water being much warmer than the deeper water. Such stratification is only temporary because the wind can mix a shallow layer of warm water into the lake. When a series of calm, warm days occurs, the lake stratifies. The surface waters of the lake heat enough so that the wind cannot completely mix the warmer, less dense water into the cooler water below. The top of the stratified lake is called the epilimnion. The zone of rapid temperature transition is the metalimnion or thermocline. The bottom of the lake at fairly constant temperature is called the hypolimnion (Fig. 6.11). The stratified layers will stay distinct until a prolonged period of cool weather occurs. The period with distinct layers is called summer stratification. Prolonged summer stratification is a combined function of the very slow rate of diffusion of heat across the metalimnion and the continued heating of the epilimnion. Because there is minimal mixing across the metalimnion, no eddy diffusion of heat occurs; only molecular diffusion occurs. The slow rates of molecular diffusion were discussed in Chapter 3.
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5
14
2
6 7 8
11 10
ice 18 17
10 16
9
4
7
11
3
15
8
0 2 4 6 8 10 12 14
Depth (m)
13
4
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Temperature 0
0
5 10 15 20
5 10 15 20
0
Dec
Jan
Feb
Mar
(0C) 5 10 15 20
0
5 10 15 20
0 2
Depth (m)
4 6 8 10 12 14
Spring mixing Summer stratification Winter stratification Fall mixing A depth contour plot of lake temperature over the course of a year in a dimictic cold-temperate lake (Esthwaite Water, an English lake). The thick black line at the top right corner of contour plot indicates ice cover. (Bottom) Two-dimensional representations of the temperature versus depth at each phase of stratification (data from Mortimer, 1941).
FIGURE 6.10
The epilimnion is very stable relative to the ability of the wind to mix a lake. There can be some mixing of the top of the hypolimnion (entrainment) with extreme winds, but even hurricane-force winds will not fully mix a well-stratified lake (Fig. 6.12). The stratification will break down only when the autumn weather can cool the epilimnion to approximately the same temperature as the hypolimnion. Cool air coupled with continued heat losses from surface evaporation decrease the temperature of the
Temperature (0C) 0
0
2
4
6
8
10
12
14
16
18
Epilimnion
5 Depth (m)
104
Metalimnion (thermocline)
10
15 Hypolimnion
20
25 Temperature as a function of depth for Triangle Lake, Oregon, on October 1, 1983, and positions of epilimnion, metalimnion, and hypolimnion (data from R. W. Castenholz).
FIGURE 6.11
Stratification
Temperature (oC) 0
6
8
10
12
14
16
18
20
3
Depth (m)
5 8 10
After hurricane Before hurricane
13 15
FIGURE 6.12
Stability of the thermocline in Linsley Pond, Connecticut, before and after a hurricane on September 21, 1938, with wind speeds up to 100 km h1 (from G. E. Hutchinson, A Treatise on Limnology, Vol. 1, Geography, Physics and Chemistry. Copyright © 1957 John Wiley & Sons, Inc. Reprinted by permission of John Wiley & Sons, Inc.).
surface. The cooled surface water is denser than the water immediately below, so it sinks. The wind can mix the lake once the entire lake is isothermal, and the fall mixing period begins. The lake will continue to cool and mix until formation of an ice cover on the surface of the lake. The surface of the lake can freeze when the temperature of the entire lake is below 3.9°C. If the lake is warmer, cool water from the surface will continue to sink and mix with the less dense water below it. If the lake temperature is 3.9°C, the water is at its densest, so cooler water will sit on the surface of the lake as long as the wind does not mix it. The surface of the lake can freeze if there is a cold, calm night. The low-density ice and cold, low-density water will sit on the surface of the lake once the surface has frozen. No more wind reaches the surface of the water so no more mixing occurs. Winter stratification is the second period during the year when a cold-temperate lake does not mix (Fig. 6.10), and it lasts as long as cold weather maintains the ice cover. Duration of ice cover provides one of the best long-term records of the effects of global warming on freshwater systems. There are good data on dates of formation and breakup of ice cover for many lakes and rivers for the past 100 years, and records exist for Lake Suwa in Japan almost continuously since 1450. These data suggest that freeze dates have become later by 5.8 days over the past 100 years and breakup dates 6.5 days earlier (Magnuson et al., 2000). Much terminology is used to describe the mixing regimes of lakes (Table 6.1). Lakes that mix twice a year are called dimictic. Lakes that only mix once during the year are called monomictic and are common in temperate and subtropical regions where winters are not cold enough to freeze lake surfaces but are cool enough to allow the lakes to become isothermal. Lakes that never mix are called amictic. Polymictic lakes stratify or mix several times a year and are found mainly in tropical regions. Meromictic
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METHOD 6.1. How Do Limnogists Sample Water from Lakes? The main consideration in determining how to sample water from a lake is what information is desired. If the information can be obtained with a submersible sensor (such as for temperature, dissolved oxygen, pH, and conductivity) results are obtained most easily without removing samples. However, for many chemical and biological parameters, water must be removed from known depths with a minimum of contamination or turbulence. Several devices are available that allow water to be sampled from depth. Many different types of limnological equipment are lowered into the water on ropes or cables. The devices are generally triggered with a weight that is dropped down the line to the sampling equipment. This weight is called a messenger. Van Dorn bottles and Kemmerer bottles are devices used to remove
lakes rarely if ever mix because they contain dissolved compounds in the hypolimnion that stabilize density layers. Several conditions cause meromictic lakes. Seasonal temperature regimes can be constant enough (mainly in tropical areas) that lakes rarely mix. The temperature difference in tropical lakes does not need to be as great to form a stable stratification as in the temperate zone because the water temperatures are higher and a greater relative difference in density occurs for each degree difference in water temperature (Fig. 2.3). For example, the density difference is greater between 20 and 25°C than between 10 and 15°C water. Salinity differences can also cause stable stratification. In this case, more saline water can sit below cooler surface waters when the salinitycaused density difference is greater than the temperature-related differences. Several conditions can cause such salinity differences. In tropical lakes that are stratified for long periods of time, the nutrients enter the surface waters from rivers. These nutrients enter the biomass of the planktonic food web, and when organisms die they sink. The sinking organisms slowly release nutrients and a portion is transported to the hypolimnion. Slowly, the salinity of the hypolimnion increases and the stratification is stabilized. In arid regions, evaporation can lead to increases in dissolved salt concentrations. Fresh river water flowing into the lake will remain on top of the denser saline water. A fresh surface lake is a common occurrence in closed basins where saline lakes form. Such was the case in the Dead Sea, where a stable stratification was maintained for nearly 300 years until water diversions for human uses reduced inflow to the lake and the dilute surface layer disappeared in the 1970s (Gavreili, 1997). Fjord lakes can also have saline waters below freshwater. In this case, a glacial valley is formed below sea level. As the glacier recedes, saline marine water floods the valley. The floor of the valley rebounds from the weight of the glacier and if a raised portion exists at the end of the valley (e.g., a terminal moraine), the saline ocean water can be isolated. Freshwater flows on top of the saline water and a lake forms with saline water
Water Movement and Currents in Lakes
water from lakes (Lind, 1974). The Van Dorn bottle consists of a tube with covers over each end. The messenger releases a catch so the stretched rubber connectors can pull the covers onto the tube, sealing the water into it. Kemmerer bottles operate similarly, but rather than using rubber connectors to pull ends onto a tube, gravity is used (Fig. 6.13). Additional devices include pumps to remove water from depth, bottles with strings attached to stoppers that can be unplugged at depth, and pipes that allow water to flow up to containers that displace surface waters (Fig. 6.13). The choice of device depends on the type of sample that is required. Toxic materials generally are to be avoided, and if chemical analysis on metals is to be done metal samplers should not be used. The violent closure of some samplers can harm some organisms that are susceptible to pressure shock. Zooplankton may avoid an opaque sampler more than a clear one because of their predation avoidance behaviors.
on the bottom and freshwater on the top. Saline springs on the bottom of lakes have also formed stable layers. Some of the dry-valley lakes in Antarctica have such stable layers (see Chapter 15). The biological and biogeochemical effects of stratification on lake organisms are strong. Molecular diffusion rates that dominate movement of dissolved materials across the metalimnion are slow enough that a significant depletion of O2 in the hypolimnion will lead to anoxia during the summer. In turn, O2 loss from the hypolimnion means that biogeochemical cycling and lake productivity are altered. Anoxia and the biogeochemistry are discussed in detail in Chapters 11–13. Given the very complex chemical and physical characteristics in many stratified lakes, several methods for sampling lake waters from different depths have been developed (Method 6.1).
WATER MOVEMENT AND CURRENTS IN LAKES The movement of wind is generally the main cause of waves across lakes, although motorboat activity can cause significant wave action. Wave action is important partially because it is associated with surface mixing and erosion of the shoreline. Lakeshore erosion can lead to habitat destruction and large financial losses associated with property damage; many environmental engineering firms specialize in controlling erosion. Wave action can influence which species can successfully inhabit the different depths of the shallow benthos (littoral zone). The two main determinants of wave height are the strength (speed and duration) of the wind and the length of lake on which the wind acts. The influence of the wave also is dependent on the geometry and materials that make up the shoreline. The length of lake on which the wind acts is called the fetch (Fig. 6.14). The longer the fetch, the higher the waves (Fig. 6.14). A perfectly round lake would be affected similarly by wind from any direction. On an irregularly shaped lake, certain wind directions lead to the largest waves. In smaller
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FIGURE 6.13 (A) A simple water sampler made from a weighted bottle and stopper, (B) a sampler that collects water from depth by displacing water at the surface, and (C) a Kemmerer sampler (photograph courtesy of Wildlife Supply Company). Only samplers (such as type C) that close at depth are suitable for collecting dissolved gas samples.
FIGURE 6.14 How fetch of an irregularly shaped lake varies with wind direction (A, B) and relationship between maximum wave height and fetch (C) (equation for C from Wetzel, 1983).
Summary
ponds and lakes, features such as hills or trees can prevent the wind from making very large waves. For example, deforestation can lead to a deeper epilimnion (by about 2 m in some small Canadian lakes) because of increased mixing during spring warming (France, 1997a). Even though the processes of surface wave formation are the most apparent to human observers, the wind causes other water movements in lakes that can also be important. As wind moves water at the surface of a lake forward, it must be replaced by water from below. This process leads to spiral circulation patterns called Langmuir circulation cells (Fig. 6.15). The spiraling water moves in alternating directions, leading to lines of downwelling water alternating with lines of upwelling water. These lines form along the direction of wind. Floating materials aggregate at the water surface along the downwelling lines and form streaks in the same direction as the wind is blowing. These circulation cells are several meters wide. In addition to the smaller scale waves and Langmuir cells, movement of water can also occur within the whole lake’s volume. When a sustained wind occurs, it causes water to pile up on the downwind side of the lake, and when the wind suddenly ceases the surface of the lake can rock. This rocking of a lake’s entire surface is called a seiche. An interesting phenomenon occurs in stratified lakes that are subjected to a sustained unidirectional wind. The force of the wind causes the water in the epilimnion to move across the lake to the downwind side (Fig. 6.16), and the depth of the epilimnion is greater downwind than upwind. Under extreme winds, the hypolimnion can come to the surface on the upwind side. When the wind ceases, the less dense water of the epilimnion moves back across the
Convergence
Spiral water currents
Divergence
Wind direction
Top view
Floating streaks
Water surface
Divergence
Convergence
Divergence
Convergence
Divergence
Convergence
Side view FIGURE 6.15
Langmuir circulation cells on a lake (top view and side view).
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Wind stops A
Epilimnon
D
Hypolimnion Hypolimnion begins to rock
Wind E
B
Hypolimnion rocks to opposite extreme, then returns to original position
Wind C
F
Mixing zones
Seiche continues but slowly dissipates
FIGURE 6.16
Formation of an internal seiche and entrainment associated with wind. Dashed arrows show water flow. (A) The lake under calm conditions; (B) the wind deepens the epilimnion on the right; (C) a strong wind mixes some of the epilimnion with the hypolimnion; (D) the wind stops and the hypolimnion begins to oscillate; and (E and F) the amplitude of the seiche diminishes over time.
surface of the lake, and the hypolimnion moves back toward its original position. Like a pendulum, the surface of the hypolimnion rocks back farther than its original position. Thus, an internal seiche is created where the surface of the lake appears still, but the plane that forms the top of the hypolimnion continues to oscillate for hours or days after the wind ceases. Aside from the intrinsic elegance of the seiche as a physical phenomenon, this type of water movement has an important biological implication. Even though the hypolimnion is very stable, seiches can lead to a moderate amount of mixing of hypolimnetic and epilimnetic water. The movement of this water up to the epilimnion is called entrainment. Entrainment causes nutrient-rich water from the hypolimnion to reach the epilimnion (Fig. 16.16C), causing stimulation of primary production. Nutrient mixing can be significant biologically because the mixing rate far exceeds the rate of molecular diffusion that usually predominates between the hypolimnion and the epilimnion. Seiches can also influence rooted plants and benthic invertebrates by altering temperature and nutrient regimes. The lake model exercises discussed by Wetzel and Likens (1991) are highly recommended for students who want a clearer understanding of the processes of stratification and seiches.
SUMMARY 1. A variety of processes form lake basins, including tectonic, glacial, fluvial, volcanic, and damming processes. Glacial lakes are the most numerous
Questions for Thought
2.
3. 4. 5.
6.
7.
8.
worldwide, but some of the largest, deepest, and oldest lakes are formed tectonically. Fluvial lakes can be very important to riverine ecology. Lake basin morphology is described with various parameters, including mean depth, area, maximum depth, volume, shoreline development (DL), and watershed area relative to lake surface area. Shallow lakes with large watersheds and highly dissected shorelines are generally the most productive. Waves are greatest where the wind has the longest length of lake (fetch) to act on. Wind causes Langmuir circulation patterns, which lead to streaks of floating material on the water surface but also mix the lake to depth. Stratification can alter the water circulation in lakes and thus alter biogeochemical, ecosystem, and community properties. Mixing can occur often (polymictic), once a year (monomictic), twice a year (dimictic), or rarely (amictic or meromictic), depending on climate and type of stratification. Thermal stratification occurs when warm surface water sits above denser, cooler waters. The warm surface layer of a thermally stratified lake is the epilimnion, the zone of steep temperature transition is the metalimnion, and the deepest stable zone is the hypolimnion. High concentrations of dissolved substances can also lead to stratified layers in lakes. Such chemically driven stability can exceed temperature-driven stability because density differences can be greater than are possible with natural temperature differences. A sustained wind that suddenly stops can cause oscillation of the lake surface (an external seiche) or the hypolimnion (internal seiche). This rocking can lead to breakdown of stratification. Mixing of deeper waters into the surface is called entrainment.
QUESTIONS FOR THOUGHT 1. Why is it sometimes difficult to assign a single geological explanation for a lake’s origin? 2. Why is a lake with a high value for DL likely to have smaller waves than a lake of comparable surface area with a DL close to 1? 3. Why do more lakes occur farther from the equator? 4. In which order (from greatest to least) should lakes be ranked with respect to the ratio of maximum depth divided by the mean depth: tectonic, glacial, and fluvial? 5. Langmuir circulation cells concentrate particles slightly more dense than water below the surface of the water: Where will these particles be concentrated and why? 6. Under what conditions would thermal stratification lead to anoxia in the hypolimnion? 7. Explain why some rivers flowing into lakes flow down into the hypolimnion, some flow across the surface, and others flow into the metalimnion.
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