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Littoral Zone
Littoral Zone J A Peters and D M Lodge, University of Notre Dame, Notre Dame, IN, USA ã 2009 Elsevier Inc. All rights reserved.
Introduction The littoral zone of a lake is the nearshore interface between the terrestrial ecosystem and the deeper pelagic zone of the lake. It is the area where at least one percent of the photosynthetically active light (400–700 nm) entering the water reaches the sediment, allowing primary producers (macrophytes and algae) to flourish. The littoral zone is structurally and functionally an important part of most lakes for several reasons. First, most lakes on Earth are small and therefore, the littoral zone comprises a large proportion of total lake area (Figure 1). Second, as an interface, the littoral zone influences the movement and processing of material flowing into the lake from terrestrial runoff, groundwater, or stream connections, thus affecting the physical and biological processes in this zone and the rest of the lake ecosystem. Third, the littoral zone is generally the most productive area of the lake, especially in terms of aquatic plants and invertebrates. Finally, human uses of aquatic systems (swimming, fishing, boating, power generation, irrigation, etc.) often focus on the littoral zone. In the first section, the physical structure and nutrient dynamics within the littoral zone are described. In the second section, interactions among organisms in the littoral zone are discussed, in addition to interactions between the littoral and terrestrial ecosystems, and between the littoral and pelagic zones. In the final section, anthropogenic effects on the littoral zone are described.
Factors Influencing the Physical Structure and Nutrient Dynamics of the Littoral Zone Many characteristics determine the percentage of the lake that is littoral zone and the type of lake-bottom substrates found there. Littoral zone area and substrate type, in turn, influence the processing of incoming nutrients, minerals, and organic matter and therefore the functioning of the entire lake. Physical Structure of the Littoral Zone
Zonation In general, the littoral zone can be divided into upper, middle, and lower zones, extending from the shoreline area sprayed by waves to the bottom of the littoral zone, below which light does not penetrate
(Figure 2). Emergent vegetation is rooted in the upper littoral zone; floating vegetation is found in the middle littoral; and submergent vegetation often grows in the lower littoral. The littoriprofundal, which is inhabited by algae and autotrophic bacteria, is a transitional zone below the lower littoral zone. Below this transitional zone, fine particles permanently settle into the profundal zone because wind or convection current energy is not sufficient at these depths to resuspend the particles. The littoral zone depth commonly corresponds to the summer epilimnion depth in stratified lakes. Habitats Compared with the homogeneous distribution of sediments in the profundal zone, the habitats and sediments of the littoral zone are distributed as heterogeneous patches (Figure 3). Sizes of particles in the sediments range from very fine organic and inorganic particles (muck or silt) to large cobble and boulders. Macrophytes and fallen trees often provide vertical substrates within the littoral zone (refer to ‘see also’ section). The abundance and distribution of habitats within the littoral zone mediates the abundance, diversity, and interactions of biota. For example, cobble substrates provide a refuge for crayfish from fish predation; in contrast, fine organic substrates favor the growth of macrophytes that provide refuge for invertebrates, zooplankton, and juvenile fish. Invertebrate abundance and composition differ among different kinds of substrate. Overall benthic invertebrate diversity is greater in the heterogeneous littoral region compared with the homogeneous profundal region. As explained later, the types of habitats found in the littoral zone depend on lake morphometry, the surrounding landscape, wind patterns, and nutrient loads to the lake. Lake morphometry The morphometric characteristics that influence the kinds of habitats within the littoral zone include lake area, depth, shoreline sinuosity, and underwater slope. The origin of a lake largely determines lake morphometry. For example, lakes formed through tectonic or volcanic activities are usually very large, steep-sided lakes with minimal littoral areas, whereas glacial lakes and reservoirs often have complex basin shapes and large littoral areas. Lakes with greater lake area to depth ratios, more sinuous shorelines, more complex bathymetry, and shallow sloped basins will have a larger percent littoral
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zone compared with pelagic zone. For instance, shallow lakes with large surface areas have large littoral zones because the light is able to penetrate to the sediment in a high proportion of the lake area. Lake morphometry characteristics also influence the types of substrates found within the littoral zone. Steep sloped littoral areas typically have rocky/cobble substrates, and areas with a gradual slope can be dominated by fine sediments with or without macrophytes (Figure 3). Lakes with a high shoreline sinuosity have more bays with macrophytes growing on sand or muck compared with circular shaped lakes, because wave action is reduced in protected bays, allowing the accumulation of fine
Number of lakes of the world
107 106 105 104 103 102 101 1
0.01
0.1
1
10
100
1000
Pelagic dominated Littoral dominated Ratio of pelagic to littoral zone by area Figure 1 Number of lakes of the world dominated by littoral or pelagic zones. Modified from Wetzel RG (2001) Limnology: Lake and River Ecosystems. New York: Elsevier, Academic Press.
organic sediments, nutrients and minerals, and the establishment of macrophytes. Surrounding landscape The topography and geology of the land surrounding a lake influence the movement of water, associated nutrients, minerals, and organic matter into the littoral zone. The relative contribution of surface runoff and groundwater to a lake depends on water infiltration and transmission rates of surrounding soils, the productivity of terrestrial vegetation, and the slope and the drainage density of the watershed. Elevation and hydrologic flow define the position of a lake in the landscape. High in the landscape, lakes tend to be small seepage lakes, which are fed primarily by precipitation and groundwater. Larger drainage lakes, which are fed by surface water, groundwater, and precipitation, tend to be lower in the landscape (Figure 4). Lakes lower in the landscape tend to have larger, more productive littoral areas because of greater watershed inputs of nutrients, minerals, and dissolved or particulate organic material, from both surface water and stream connections. This material input increases buffering capacity (ability to reduce affects of acidification) and the abundance and diversity of macrophytes and the invertebrates like snails that live on macrophytes. Also, lakes lower in the landscape usually have a more complex basin bathymetry, which also increases littoral area. Wind patterns Substrates found in the littoral zone are a function of wind patterns such as fetch and exposure. Fetch is the distance the wind blows across the lake. The windward and lee sides of the lake will have distinctly different substrate characteristics. The stronger the wave action caused by the wind, the more fine particles will be suspended and eventually deposited in the profundal zone of the lakes, and the more the littoral zone substrate will be characterized by rocks. Wave action will also be reduced by sinuous shorelines and macrophytes as described earlier.
High wave action, coarse substrates Invertebrates: mayflies, stoneflies, coleoptera Upper littoral Occasional Occasionalwaves, waves,high highplant plantbiomass, biomass, Invertebrates: snails, odonata, caddisflies
Middle littoral Lower littoral Littoriprofundal
No Nowaves, waves,detritus, detritus,fine finesubstrate substrate Invertebrates: Invertebrates:bivalves, bivalves,oligochaetes, oligochaetes,chironomids chironomids
Figure 2 Zonation of the littoral zone. Associated wave conditions, substrate, and invertebrates are listed. The upper littoral zone can have emergent vegetation or cobble substrates depending on the type of lake and the wave action.
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N E
W S
0
100 Meters
Mirror lake West Thornton. Graifton Co., New Hampshire Depth contours in meters Gyttja
Cobbles
Bedrock
Boulders in finer matrix
Sand: local cobbles, gravel, and organic debris
Thin organic debris or mud
Figure 3 Example of habitat heterogeneity in the littoral zone and the influence of slope on substrate composition. From Moeller RE (1978) The hydrophytes of Mirror Lake: A study of vegetational structure and seasonal biomass dynamics, Ph.D. dissertation, 212 pp. Ithaca, NY: Cornell University.
Nutrient Dynamics in the Littoral Zone
Sources of nutrient inputs Detritus (dead organic matter) and associated nutrient inputs into the littoral zone are either allochthonous (derived from terrestrial sources) or autochthonous (aquatic sources). Allochthonous sources include groundwater, precipitation, fluvial inputs, terrestrial plant litter fall, and materials from soil erosion. Nutrients can also be transported into the littoral zone by animals moving between the terrestrial and the littoral zone for food resources (i.e., amphibians, waterfowl, or mammals such as beaver, etc.) or as food resources (i.e., small mammals) for aquatic organisms such as fish. Autochthonous sources of nutrients come from the death of aquatic organisms (plants and animals), and secretion, excretion, and egestion from living animals and plants. The distribution of detritus influences the availability of dissolved organic matter and nutrients for biotic
uptake. Detritus deposited in the profundal zone may become permanently lost from littoral food webs, whereas detritus deposited within the littoral zone can contribute to internal loading of dissolved organic matter and nutrients (i.e., phosphorus and nitrogen) for primary and secondary production. Much of the energy that drives ecosystem metabolism comes from allochthonous or autochthonous detritus, and shallower lakes with a greater percent of littoral area have a net deposition rate of detrital organic matter that is always greater than that of deeper lakes. Retention capacity of the littoral zone The retention time of water, nutrients, and detritus is influenced by the size and configuration of the littoral zone. Deep lakes have longer water retention times (up to hundreds or thousands of years) compared with shallow lakes (often less than a year) and the pelagic zone
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High in landscape
Low in landscape Figure 4 Lakes in the landscape. Hydrologic connectedness ranges from isolated lakes to those connected by large rivers. Magnuson JJ, Kratz TK, Benson BJ (eds.) (2006) Long-Term Dynamics of Lakes in the Landscape: Long-Term Ecological Research on North Temperate Lakes, 51 pp. Oxford, UK: Oxford University Press.
has longer retention times compared with the littoral zone. The amount of time water is retained within the littoral zone influences the dynamics of nutrients within the lake. The longer it takes for water to pass through the littoral zone, the greater the amount of nutrients that will be used by plants and animals in the littoral zone. Although deep lakes have a greater retention time of water, they usually have a small littoral zone that continuously loses detritus and nutrients to the profundal zone as detritus sinks through the metalimnion. In some stratified lakes, half of the total phosphorus can be lost to the hypolimnion (profundal zone) during the summer and only partially returned by the mixing of the lake in the spring and fall. Shallow lakes, on the other hand, do not have this constant nutrient loss because they have a greater proportion of the epilimnion volume in contact with the lake bottom. Thus in shallow lakes, nutrients are recycled within the littoral zone at a greater rate and less loss to the profundal zone occurs. The littoral zone has therefore been described as a ‘metabolic sieve’ or ‘trap’ because of its ability to strain
incoming water and nutrients before passing it on to the pelagic and profundal zone. In many cases, most of the dissolved organic matter and nutrients that are not used in the littoral zone will ultimately be lost to sedimentation and burial in the profundal zone. Major nutrients in the littoral zone There are many nutrients and minerals (silica, calcium, iron, manganese, sulfur, etc.) that influence the type of chemical and biological processes that occur in the littoral zone. For example, high concentrations of ions such as calcium and magnesium increase the buffering capability of lakes. Iron and manganese bind to phosphorus (often the nutrient most limiting primary production) in aerobic conditions making it unavailable for biotic uptake. Calcium is used by snails and other invertebrates for shell or exoskeleton maintenance, while sponges and diatoms require silica for spicule and test construction. Not only do littoral biota require nutrients and minerals, but in turn organisms such as bacteria, macrophytes, benthic invertebrates, and benthivorous fish alter the availability and composition of nutrients within the littoral zone. Autotrophic and heterotrophic bacteria can use and produce many different nutrients and gases including oxygen, carbon dioxide, iron, several nitrogen and sulfur products, and methane, depending on whether the conditions are aerobic or anaerobic. Macrophytes modify the chemical composition in the littoral zone by altering the oxygen and carbon dioxide concentrations and pH levels in the surrounding sediments and overlying water. Benthic invertebrates and fish increase nutrient release, such as phosphorus, through sediment resuspension.
The Biota of the Littoral Zone Biota of the littoral zone includes both permanent and transient species (Figure 5). Transient species – those that move in and out of the littoral zone from the surrounding terrestrial ecosystem and pelagic zone create linkages between the littoral zone and surrounding environs. Both the biota and associated linkages are discussed in this section. For ease of constructing the food web, organisms in Figure 5 are grouped as either being terrestrial or aquatic. However, certain species within each group actually belong in both the terrestrial ecosystem and the littoral zone (i.e., amphibians and waterfowl) or in both the littoral and pelagic zones (i.e., zooplankton and fish). The important point is that many species, including some of those discussed below, use multiple food resources and zones within the lake, which can have cascading effects throughout terrestrial, littoral, and pelagic food webs.
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Terrestrial
Littoral
Waterfowl: Dabbling ducks mallards, swans, geese Other birds: Gulls, terns, eagles
Waterfowl: Diving ducks mergansers
Mammals: Otter, beaver, racoon, mice, etc.
Fish: Planktivores, herbivores, benthivores piscivores
Pelagic
Fish: Piscivores planktivores
Amphibians: Frogs, toads, newts, salamanders, etc. Reptiles: Snakes, turtles
Zooplankton Invertebrates: Insects, snails, crayfish, mussels, worms, leeches, etc.
Insects: Damselfly, dragonfly, mayflies, etc. Macrophytes/periphyton
Bacteria Figure 5 Linkages between the littoral zone with the terrestrial ecosystem as well as pelagic zone. Arrows represent energy flow. Only the interactions with the littoral zone are shown. There are interactions between biota on land and in the pelagic zone that are not depicted in this figure.
Bacteria
Bacterial production is up to 120 times greater in the littoral zone than in the pelagic zone. Bacteria are one of the main biotic components that allow the littoral zone to act as a ‘metabolic sieve.’ The main function of bacteria in the littoral zone is to break down allochthonous and autochthonous organic material. As bacteria process detritus, different nutrients and gases, such as particulate and dissolved organic matter, nitrogen, phosphorus, methane and sulfur, etc., are produced and in many cases become available to other biota in the littoral zone. Lakes with large, shallow littoral zones will have increased bacteria metabolism and faster detrital processing. Periphyton
Periphyton is a mixture of autotrophic and heterotrophic microorganisms embedded in a matrix of organic detritus (refer to ‘see also’ section). Periphyton covers most submerged substrates, ranging from sand to macrophytes to rock. The metabolic importance of periphyton at the whole lake scale is constrained by the morphometry and substrate characteristics of the littoral zone. In oligotrophic lakes, even those
with few macrophytes for periphyton to grow on, periphyton can be an important component of whole-lake primary production. For example, in one oligotrophic lake, the littoral zone comprised only 15% of the lake, but the periphyton accounted for 70–85% of the lake primary production. In eutrophic lakes, however, phytoplankton is more abundant and shading by phytoplankton reduces periphyton and macrophyte abundance. Periphyton is a common food source for invertebrates and some amphibians.
Macrophytes
Macrophytes require specific substrate types to thrive, and their growth provides a unique habitat for other organisms (refer to ‘see also’ section). Macrophytes grow best in a mixture of sand and muck, and are often found in areas with upwelling groundwater. Once macrophytes become established within the littoral zone they modify the microclimate through the reduction of wave energy and the creation of thermal gradients that prevents water from mixing. These conditions promote particle sedimentation. The degree of microclimate modification
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depends on the characteristics of the sediment structure, nutrient availability, and diffusion of oxygen through the sediment. Macrophytes are integral to nutrient cycling in the littoral zone as both sources and sinks of nutrients. Traditionally, limnologists have considered macrophytes a nutrient source, since they may incorporate nutrients from the anoxic sediment and then release them into the water column upon senescence. Others have found that nutrients removed from sediments or surrounding water column by plants are largely retained by plants until the plants decay. In addition to their role in nutrient cycling, macrophytes provide important habitat for organisms such as bacteria, periphyton, zooplankton, invertebrates, amphibians, fish, and waterfowl. Invertebrates and small fish use macrophytes as a habitat refuge from predation by invertebrates (e.g., dragonfly or damselfly nymphs), fish (e.g., Esox), and amphibians, and as a place to reproduce. For many invertebrates (e.g., insects, crustaceans) and vertebrates (e.g., waterfowl, moose), macrophytes are a major food source. Invertebrates
Invertebrates are very diverse, and include: zooplankton, crayfish, insects, worms, and leeches. Invertebrates living on the bottom of lakes are referred to as zoobenthos, and are far more abundant and diverse in the littoral zone than in other lake zones. Therefore the ratio of the abundance of zoobenthos to zooplankton is inversely related to lake size. However, the absolute diversity and abundance of zoobenthos increases with lake size. Invertebrate diversity is also positively related to habitat complexity, macrophyte abundance, conductivity, and the presence of stream connections. Habitat availability within the littoral zone influences the type of invertebrates that will colonize (Figure 2). For instance, ephemeroptera (mayflies) and plecoptera (stoneflies) generally prefer substrates that have higher wave action and coarser substrates, while lightly disturbed fine sediments are colonized by chironomids (midge larvae), bivalves (clams), and oligochaetes (worms). Substrate, macrophyte abundance, and detritus are the three main factors controlling the diversity and distribution of invertebrates, but water depth, wave exposure, and water clarity, (which influence the first three main factors) may consequently also affect invertebrate abundance and distribution. Fish
Like invertebrate diversity, fish diversity is positively related with lake size and habitat complexity. The use by fish of different littoral zone habitats also often
varies seasonally and with the age or size of the fish. As mentioned above, macrophytes are both a refuge and a hunting ground for predatory fish. Some fish species (e.g., Perca spp.) also use macrophytes as substrate for egg-laying. Many other fish species lay eggs within cobble substrates, and some use cobble as a refuge (e.g., sculpin, darters, juvenile burbot). Fish are often classified by their primary food source. Piscivores mainly eat other fish, planktivores consume plankton, benthivores feed on zoobenthos, and herbivores eat macrophytes and periphyton. Some fish species may change what they eat as they mature into adulthood. For instance, many fish species eat plankton as a juvenile and smaller fish as an adult. Even adult fish may have very broad diets as they move between littoral and pelagic zones (Figure 5). Terrestrial – Littoral Links
It is impossible to separate the processes that occur within a lake from the surrounding watershed and even the air above the watershed. Many organisms move resources and energy between the surrounding watershed and the littoral zone. Food resources from the littoral zone are an important source of energy for many terrestrial and semi-terrestrial organisms. Fish, for example, are eaten by many different terrestrial and amphibious species including waterfowl, hawks, herons, egrets, mammals, reptiles, and humans. Aquatic invertebrates found within the littoral zone provide an important source of protein for terrestrial and semi-terrestrial organisms. The reproductive success of ducks is closely related to the availability of chironomids and other insects emerging from their benthic larval form. Waterfowl, such as geese, feed on aquatic plants and can remove up to 50% of the standing stock of macrophytes in some areas. Food resources from the terrestrial ecosystem are also important for littoral species. Depending on the time of the year and the species of fish, up to half the food consumed can be allochthonous insects and small mammals. Another type of resource that is moved from land to the littoral zone is large woody debris used by beavers to construct their lodges. The woody debris used by beavers also provides habitat for many fishes. Finally, transient organisms such as waterfowl, mink, otter, beaver, muskrat, snakes, and turtles, among others, move nutrients in and out of the littoral zone via feeding and excretion and egestion. The riparian habitat is another resource that is important for species that use the littoral zone. For example, snakes and turtles sun themselves on logs and rocks found along the shoreline. Waterfowl and some mammals use the low lying shoreline habitat to
Air/Water and Land/Water Interfaces, Wetlands and the Littoral Zone _ Littoral Zone 85
make their nests, while eagles and some diving ducks use the trees surrounding lakes for their nesting sites. Hawks also use trees surrounding the lake as a perch to search for food. Riparian habitat is important for amphibians (e.g., newts and frogs) during different times within their lives. Trophic cascades – food web interactions that strongly alter the abundance of three or more trophic levels – are well documented in the pelagic and littoral zone of lakes. They also cross the littoralland interface (Figure 6). Plants near ponds with fish have more visits from pollinators than plants near ponds without fish. This is because in ponds with fish, larval dragonflies are reduced by fish predation, and thus the abundance of adult dragonflies is also decreased. Adult dragonflies have direct and indirect effects on insect pollinators. They directly prey upon the pollinators and indirectly reduce the number of pollinator visits just by being present. Littoral – Pelagic Links
The littoral and the pelagic zones are also strongly linked, especially by the diel horizontal migration of zooplankton, and by fish movements. Zooplankton sometimes move up to 30 m horizontally twice each day between zones. Zooplankton that normally reside in the pelagic zone will move into macrophyte habitats during the day to avoid pelagic predators
such as Chaoborus (phantom midge larvae) and visually feeding planktivores like small fishes. In some lakes, this movement can benefit zooplankton through reduced mortality from fish predation, food availability in the littoral zone (some zooplankton can become browsers in the littoral zone compared to being filter feeders in the pelagic zone), and enhanced growth. Predation by planktivores is often reduced by migration into the littoral zone, but in some lakes, littoral invertebrates (e.g., dragonfly larvae) pose a substantial risk of predation within the littoral zone. Thus, zooplankton movement depends on the complex interactions occurring in both the pelagic and littoral zones, which differ among lakes. Fish movements also link the littoral and pelagic zones. The dependency of fish on littoral production differs by fish type, with planktivores, benthivores, and even piscivores relying on littoral food production to some degree (Figure 7). Fish that are often categorized as pelagic planktivores can derive up to 30% of their energy from the littoral zone, while fish categorized as piscivores sometimes derive almost all their energy, at least indirectly (e.g., from other fish that consume littoral-derived foods), from the littoral zone (Figure 7). Without the littoral zone, the production of many fish, including fish that may rarely venture into the littoral zone, would decline dramatically. Pollinator
Dragonfly
−
+
+
+ −
Larval dragonfly + −
Fish
Figure 6 Example of a trophic cascade that links the terrestrial ecosystem with the littoral zone. Fish reduce the abundance of dragonflies, which leads to increased pollinators, and thereby facilitating the reproduction of terrestrial plants. Solid arrows indicate direct interactions; dashed arrows denote indirect interactions. þ are positive interactions, represents negative interactions. Modified from Knight et al. (2005) Trophic cascades across ecosystems. Nature 437: 880–883.
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Air/Water and Land/Water Interfaces, Wetlands and the Littoral Zone _ Littoral Zone
% Reliance on littoral food chain
100
80
60
40
20
Pla be nkt nth ivo ivo re/ re
vo re Be nth i
Pis civ ore
Pla nk t iv ore
0
Figure 7 The range of reliance different types of fishes have on littoral zone resources. Modified from Vadeboncoeur et al. (2005) Effects of multi-chain omnivory on the strength of trophic control in lakes. Ecosystems 8: 682–693.
The Anthropogenic Influences on the Littoral Zone Humans derive many ecosystem goods (e.g., harvested fish and waterfowl) and services (e.g., water purification, water supply) from the littoral zones of lakes. In turn, humans have immense impacts on the structure and function of the littoral zone. First, increased nutrient loading from activities such as logging, agriculture, and development causes eutrophication. Eutrophication leads to increased primary production in the littoral zone of many lakes, which can cause undesirable algal blooms, as well as increases in undesirable fish and cloudy water. Second, human-mediated spread of invasive species (e.g., zebra mussels, rusty crayfish, round gobies) alters nutrient cycling and food web composition in the littoral zone, causing changes that are generally undesirable to humans. Third, fossil fuel combustion in industry and automobiles causes acid deposition and climate change. In many cases, acidification of lakes causes decreased abundance and diversity of macrophytes, invertebrates, and fish, while increasing filamentous green algal production, all of which has cascading effects through the food web. Acidification also causes the release of metals toxic to fish, e.g., aluminum and mercury. Some of these metals bioaccumulate in fish, which then makes the fish hazardous to humans. Climate change is expected to cause warmer lake waters, and in many parts of the world, will reduce runoff, increase water residence times, lower water
levels, and increase evaporation. Concentrations of many ions will therefore increase, causing changes in nutrient and detritus availability as well as primary and secondary production within the littoral zone. Warming could also lead to poleward range expansion of many littoral species, further changing food web dynamics. Finally, fluctuations in water level are often increased by irrigation and dams. At high water levels, flooding and erosion of riparian habitat occurs. This increases the dissolved organic matter input and turbidity in the littoral zone. On the other hand, water drawdown has both positive and negative effects on the littoral zone, depending on the lake basin morphometry. Steep sided littoral zones are not as affected as shallow sloping ones. At low water levels macrophytes are reduced, the percent of sandy/fine grained habitat increases, benthic invertebrate diversity and abundance decreases and fish refuges and spawning habitat can be reduced. The response of the littoral zone to all these anthropogenic impacts is influenced by the structure and function of the littoral zone as well as the interaction between the littoral zone and the terrestrial ecosystem and the pelagic and profundal zones. The degree to which a lake responds or the length of time a lake can resist being effected by one of the humanmediated stressors described above depends on the size of the littoral zone, the position of the lake within the landscape, the abundance and distribution of different habitats within the littoral zone, and different biota present within that zone. Therefore, the littoral zone is important for whole lake functioning as well as the response of the whole lake to human beings. See also: Acidification; Algae of River Ecosystems; Algae; Amphibians; Amphipoda; Annelida, Hirudinea (Leeches); Aquatic Insects – Ecology, Feeding, and Life History; Aquatic Insects, Classification; Aquatic Plants: A General Introduction; Aquatic Plants and Attached Algae; Benthic Invertebrate Fauna, Lakes and Reservoirs; Benthic Invertebrate Fauna, River and Floodplain Ecosystems; Benthic Invertebrate Fauna, Small Streams; Benthic Invertebrate Fauna, Tropical Stream Ecosystems; Benthic Invertebrate Fauna, Wetland Ecosystems; Benthic Invertebrate Fauna; Birds; Branchiopoda (Anostraca, Notostraca, Laevicandata, Spinicaudata, Cyclestherida); Bryozoa; Chrysophytes Golden Algae; Cnidaria (Coelenterata); Coarse Woody Debris in Lakes and Streams; Coleoptera (Beetles) in Aquatic Ecosystems; Comparative Primary Production; Cyanoprokaryota and Other Prokaryotic Algae; Decapoda; Diatoms; Diptera (Biting Flies); Diptera (NonBiting Flies); Effects of Climate Change on Lakes; Ephemeroptera (Mayflies); Eutrophication of Lakes and
Air/Water and Land/Water Interfaces, Wetlands and the Littoral Zone _ Littoral Zone 87 Reservoirs; Fish, Characteristics; Fish, Populations; Fish, Productivity; Fish, Systematics and Evolution; Flatworms (Turbellarians); Flow in Wetlands and Macrophyte Beds; Gastrotricha; Green Algae; Hemiptera (True Bugs); Hydrachnida (Water Mites); Invasive Species; Isopoda (Aquatic Sowbugs); Mammals; Megaloptera (Alderflies, Dobsonflies); Microbial Food Webs; Mollusca; Nematoda; Nematomorpha (Horsehair Worms); Odonata (Dragonflies and Damselflies); Ostracoda; Other Phytoflagellates and Groups of Lesser Importance; Photosynthetic Periphyton and Surfaces; Phytoplankton Nutrition and Related Mixotrophy; Phytoplankton Population Dynamics: Concepts and Performance Measurement; Phytoplankton Productivity; Plecoptera (Stoneflies); Porifera (Sponges); Role of Zooplankton in Aquatic Ecosystems; Shallow Lakes and Ponds; Subterranean Aquatic Ecosystems - Groundwater Ecology; Tardigrada (Water Bears); Trichoptera (Caddisflies).
Further Reading Burks RL, Lodge DM, Jeppesen E, and Lauridsen TL (2002) Diel horizontal migration of zooplankton: Costs and benefits of inhabiting the littoral. Freshwater Biology 47: 343–365. Gasith A and Gafny S (1990) Effects of water level fluctuation on the sturucture and function of the littoral zone. In: Tilzer MM and Serruya C (eds.) Large Lakes: Ecological Structure and Function, pp. 156–171. New York: Srpinger.
Jeppesen E, Sondergaard M, Sondergaard M, and Christoffersen K (eds.) (1998) The Structuring Role of Submerged Macrophytes in Lakes. New York: Springer. Kalff J (2002) Limnology: Inland Water Ecosystems. Upper Saddle River, NJ: Prentice-Hall. Knight TM, McCoy MW, Chase JM, et al. (2005) Trophic cascades across ecosystems. Nature 437: 880–883. Likens GE (ed.) (1985) An Ecosystem Approach to Aquatic Ecology. New York: Springer. Lodge DM, Barko JW, Strayer D, et al. (1988) Spatial heterogeneity and habitat interactions in lake communities. In: Carpenter SR (ed.) Complex Interactions in Lake Communities. New York: Springer. Magnuson JJ, Kratz TK, and Benson BJ (eds.) (2006) Long-Term Dynamics of Lakes in the Landscape: Long-Term Ecological Research on North Temperate Lakes. Oxford, UK: Oxford University Press. O’Sullivan PE and Reynolds CS (eds.) (2004) The Lakes Handbook. Oxford, UK: Blackwell Science. Pieczynska E (1993) Detritus and nutrient dynamics in the shore zone of lakes: A review. Hydrobiologia 251: 49–58. Scheffer M (1998) Ecology of Shallow Lakes. New York: Chapman and Hall. Vadeboncoeur Y, McCann KS, VanderZanden MJ, and Rasmussen JB (2005) Effects of multi-chain omnivory on the strength of trophic control in lakes. Ecosystems 8: 682–693. Weatherhead MA and James MR (2001) Distribution of macroinvertebrates in relation to physical and biological variables in the littoral zone of nine New Zealand lakes. Hydrobiologia 462: 115–129. Wetzel RG (1989) Land-water interfaces: Metabolic and limnological regulators. International Association of Theoretical and Applied Limnology Proceedings 24: 6–24. Wetzel RG (1989) Limnology: Lake and River Ecosystems. New York: Academic Press.
Introduction The littoral zone of a lake is the nearshore interface between the terrestrial ecosystem and the deeper pelagic zone of the lake. It is the area where at least one percent of the photosynthetically active light (400–700 nm) entering the water reaches the sediment, allowing primary producers (macrophytes and algae) to flourish. The littoral zone is structurally and functionally an important part of most lakes for several reasons. First, most lakes on Earth are small and therefore, the littoral zone comprises a large proportion of total lake area (Figure 1). Second, as an interface, the littoral zone influences the movement and processing of material flowing into the lake from terrestrial runoff, groundwater, or stream connections, thus affecting the physical and biological processes in this zone and the rest of the lake ecosystem. Third, the littoral zone is generally the most productive area of the lake, especially in terms of aquatic plants and invertebrates. Finally, human uses of aquatic systems (swimming, fishing, boating, power generation, irrigation, etc.) often focus on the littoral zone. In the first section, the physical structure and nutrient dynamics within the littoral zone are described. In the second section, interactions among organisms in the littoral zone are discussed, in addition to interactions between the littoral and terrestrial ecosystems, and between the littoral and pelagic zones. In the final section, anthropogenic effects on the littoral zone are described.
Factors Influencing the Physical Structure and Nutrient Dynamics of the Littoral Zone Many characteristics determine the percentage of the lake that is littoral zone and the type of lake-bottom substrates found there. Littoral zone area and substrate type, in turn, influence the processing of incoming nutrients, minerals, and organic matter and therefore the functioning of the entire lake. Physical Structure of the Littoral Zone
Zonation In general, the littoral zone can be divided into upper, middle, and lower zones, extending from the shoreline area sprayed by waves to the bottom of the littoral zone, below which light does not penetrate
(Figure 2). Emergent vegetation is rooted in the upper littoral zone; floating vegetation is found in the middle littoral; and submergent vegetation often grows in the lower littoral. The littoriprofundal, which is inhabited by algae and autotrophic bacteria, is a transitional zone below the lower littoral zone. Below this transitional zone, fine particles permanently settle into the profundal zone because wind or convection current energy is not sufficient at these depths to resuspend the particles. The littoral zone depth commonly corresponds to the summer epilimnion depth in stratified lakes. Habitats Compared with the homogeneous distribution of sediments in the profundal zone, the habitats and sediments of the littoral zone are distributed as heterogeneous patches (Figure 3). Sizes of particles in the sediments range from very fine organic and inorganic particles (muck or silt) to large cobble and boulders. Macrophytes and fallen trees often provide vertical substrates within the littoral zone (refer to ‘see also’ section). The abundance and distribution of habitats within the littoral zone mediates the abundance, diversity, and interactions of biota. For example, cobble substrates provide a refuge for crayfish from fish predation; in contrast, fine organic substrates favor the growth of macrophytes that provide refuge for invertebrates, zooplankton, and juvenile fish. Invertebrate abundance and composition differ among different kinds of substrate. Overall benthic invertebrate diversity is greater in the heterogeneous littoral region compared with the homogeneous profundal region. As explained later, the types of habitats found in the littoral zone depend on lake morphometry, the surrounding landscape, wind patterns, and nutrient loads to the lake. Lake morphometry The morphometric characteristics that influence the kinds of habitats within the littoral zone include lake area, depth, shoreline sinuosity, and underwater slope. The origin of a lake largely determines lake morphometry. For example, lakes formed through tectonic or volcanic activities are usually very large, steep-sided lakes with minimal littoral areas, whereas glacial lakes and reservoirs often have complex basin shapes and large littoral areas. Lakes with greater lake area to depth ratios, more sinuous shorelines, more complex bathymetry, and shallow sloped basins will have a larger percent littoral
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zone compared with pelagic zone. For instance, shallow lakes with large surface areas have large littoral zones because the light is able to penetrate to the sediment in a high proportion of the lake area. Lake morphometry characteristics also influence the types of substrates found within the littoral zone. Steep sloped littoral areas typically have rocky/cobble substrates, and areas with a gradual slope can be dominated by fine sediments with or without macrophytes (Figure 3). Lakes with a high shoreline sinuosity have more bays with macrophytes growing on sand or muck compared with circular shaped lakes, because wave action is reduced in protected bays, allowing the accumulation of fine
Number of lakes of the world
107 106 105 104 103 102 101 1
0.01
0.1
1
10
100
1000
Pelagic dominated Littoral dominated Ratio of pelagic to littoral zone by area Figure 1 Number of lakes of the world dominated by littoral or pelagic zones. Modified from Wetzel RG (2001) Limnology: Lake and River Ecosystems. New York: Elsevier, Academic Press.
organic sediments, nutrients and minerals, and the establishment of macrophytes. Surrounding landscape The topography and geology of the land surrounding a lake influence the movement of water, associated nutrients, minerals, and organic matter into the littoral zone. The relative contribution of surface runoff and groundwater to a lake depends on water infiltration and transmission rates of surrounding soils, the productivity of terrestrial vegetation, and the slope and the drainage density of the watershed. Elevation and hydrologic flow define the position of a lake in the landscape. High in the landscape, lakes tend to be small seepage lakes, which are fed primarily by precipitation and groundwater. Larger drainage lakes, which are fed by surface water, groundwater, and precipitation, tend to be lower in the landscape (Figure 4). Lakes lower in the landscape tend to have larger, more productive littoral areas because of greater watershed inputs of nutrients, minerals, and dissolved or particulate organic material, from both surface water and stream connections. This material input increases buffering capacity (ability to reduce affects of acidification) and the abundance and diversity of macrophytes and the invertebrates like snails that live on macrophytes. Also, lakes lower in the landscape usually have a more complex basin bathymetry, which also increases littoral area. Wind patterns Substrates found in the littoral zone are a function of wind patterns such as fetch and exposure. Fetch is the distance the wind blows across the lake. The windward and lee sides of the lake will have distinctly different substrate characteristics. The stronger the wave action caused by the wind, the more fine particles will be suspended and eventually deposited in the profundal zone of the lakes, and the more the littoral zone substrate will be characterized by rocks. Wave action will also be reduced by sinuous shorelines and macrophytes as described earlier.
High wave action, coarse substrates Invertebrates: mayflies, stoneflies, coleoptera Upper littoral Occasional Occasionalwaves, waves,high highplant plantbiomass, biomass, Invertebrates: snails, odonata, caddisflies
Middle littoral Lower littoral Littoriprofundal
No Nowaves, waves,detritus, detritus,fine finesubstrate substrate Invertebrates: Invertebrates:bivalves, bivalves,oligochaetes, oligochaetes,chironomids chironomids
Figure 2 Zonation of the littoral zone. Associated wave conditions, substrate, and invertebrates are listed. The upper littoral zone can have emergent vegetation or cobble substrates depending on the type of lake and the wave action.
Air/Water and Land/Water Interfaces, Wetlands and the Littoral Zone _ Littoral Zone 81
N E
W S
0
100 Meters
Mirror lake West Thornton. Graifton Co., New Hampshire Depth contours in meters Gyttja
Cobbles
Bedrock
Boulders in finer matrix
Sand: local cobbles, gravel, and organic debris
Thin organic debris or mud
Figure 3 Example of habitat heterogeneity in the littoral zone and the influence of slope on substrate composition. From Moeller RE (1978) The hydrophytes of Mirror Lake: A study of vegetational structure and seasonal biomass dynamics, Ph.D. dissertation, 212 pp. Ithaca, NY: Cornell University.
Nutrient Dynamics in the Littoral Zone
Sources of nutrient inputs Detritus (dead organic matter) and associated nutrient inputs into the littoral zone are either allochthonous (derived from terrestrial sources) or autochthonous (aquatic sources). Allochthonous sources include groundwater, precipitation, fluvial inputs, terrestrial plant litter fall, and materials from soil erosion. Nutrients can also be transported into the littoral zone by animals moving between the terrestrial and the littoral zone for food resources (i.e., amphibians, waterfowl, or mammals such as beaver, etc.) or as food resources (i.e., small mammals) for aquatic organisms such as fish. Autochthonous sources of nutrients come from the death of aquatic organisms (plants and animals), and secretion, excretion, and egestion from living animals and plants. The distribution of detritus influences the availability of dissolved organic matter and nutrients for biotic
uptake. Detritus deposited in the profundal zone may become permanently lost from littoral food webs, whereas detritus deposited within the littoral zone can contribute to internal loading of dissolved organic matter and nutrients (i.e., phosphorus and nitrogen) for primary and secondary production. Much of the energy that drives ecosystem metabolism comes from allochthonous or autochthonous detritus, and shallower lakes with a greater percent of littoral area have a net deposition rate of detrital organic matter that is always greater than that of deeper lakes. Retention capacity of the littoral zone The retention time of water, nutrients, and detritus is influenced by the size and configuration of the littoral zone. Deep lakes have longer water retention times (up to hundreds or thousands of years) compared with shallow lakes (often less than a year) and the pelagic zone
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High in landscape
Low in landscape Figure 4 Lakes in the landscape. Hydrologic connectedness ranges from isolated lakes to those connected by large rivers. Magnuson JJ, Kratz TK, Benson BJ (eds.) (2006) Long-Term Dynamics of Lakes in the Landscape: Long-Term Ecological Research on North Temperate Lakes, 51 pp. Oxford, UK: Oxford University Press.
has longer retention times compared with the littoral zone. The amount of time water is retained within the littoral zone influences the dynamics of nutrients within the lake. The longer it takes for water to pass through the littoral zone, the greater the amount of nutrients that will be used by plants and animals in the littoral zone. Although deep lakes have a greater retention time of water, they usually have a small littoral zone that continuously loses detritus and nutrients to the profundal zone as detritus sinks through the metalimnion. In some stratified lakes, half of the total phosphorus can be lost to the hypolimnion (profundal zone) during the summer and only partially returned by the mixing of the lake in the spring and fall. Shallow lakes, on the other hand, do not have this constant nutrient loss because they have a greater proportion of the epilimnion volume in contact with the lake bottom. Thus in shallow lakes, nutrients are recycled within the littoral zone at a greater rate and less loss to the profundal zone occurs. The littoral zone has therefore been described as a ‘metabolic sieve’ or ‘trap’ because of its ability to strain
incoming water and nutrients before passing it on to the pelagic and profundal zone. In many cases, most of the dissolved organic matter and nutrients that are not used in the littoral zone will ultimately be lost to sedimentation and burial in the profundal zone. Major nutrients in the littoral zone There are many nutrients and minerals (silica, calcium, iron, manganese, sulfur, etc.) that influence the type of chemical and biological processes that occur in the littoral zone. For example, high concentrations of ions such as calcium and magnesium increase the buffering capability of lakes. Iron and manganese bind to phosphorus (often the nutrient most limiting primary production) in aerobic conditions making it unavailable for biotic uptake. Calcium is used by snails and other invertebrates for shell or exoskeleton maintenance, while sponges and diatoms require silica for spicule and test construction. Not only do littoral biota require nutrients and minerals, but in turn organisms such as bacteria, macrophytes, benthic invertebrates, and benthivorous fish alter the availability and composition of nutrients within the littoral zone. Autotrophic and heterotrophic bacteria can use and produce many different nutrients and gases including oxygen, carbon dioxide, iron, several nitrogen and sulfur products, and methane, depending on whether the conditions are aerobic or anaerobic. Macrophytes modify the chemical composition in the littoral zone by altering the oxygen and carbon dioxide concentrations and pH levels in the surrounding sediments and overlying water. Benthic invertebrates and fish increase nutrient release, such as phosphorus, through sediment resuspension.
The Biota of the Littoral Zone Biota of the littoral zone includes both permanent and transient species (Figure 5). Transient species – those that move in and out of the littoral zone from the surrounding terrestrial ecosystem and pelagic zone create linkages between the littoral zone and surrounding environs. Both the biota and associated linkages are discussed in this section. For ease of constructing the food web, organisms in Figure 5 are grouped as either being terrestrial or aquatic. However, certain species within each group actually belong in both the terrestrial ecosystem and the littoral zone (i.e., amphibians and waterfowl) or in both the littoral and pelagic zones (i.e., zooplankton and fish). The important point is that many species, including some of those discussed below, use multiple food resources and zones within the lake, which can have cascading effects throughout terrestrial, littoral, and pelagic food webs.
Air/Water and Land/Water Interfaces, Wetlands and the Littoral Zone _ Littoral Zone 83
Terrestrial
Littoral
Waterfowl: Dabbling ducks mallards, swans, geese Other birds: Gulls, terns, eagles
Waterfowl: Diving ducks mergansers
Mammals: Otter, beaver, racoon, mice, etc.
Fish: Planktivores, herbivores, benthivores piscivores
Pelagic
Fish: Piscivores planktivores
Amphibians: Frogs, toads, newts, salamanders, etc. Reptiles: Snakes, turtles
Zooplankton Invertebrates: Insects, snails, crayfish, mussels, worms, leeches, etc.
Insects: Damselfly, dragonfly, mayflies, etc. Macrophytes/periphyton
Bacteria Figure 5 Linkages between the littoral zone with the terrestrial ecosystem as well as pelagic zone. Arrows represent energy flow. Only the interactions with the littoral zone are shown. There are interactions between biota on land and in the pelagic zone that are not depicted in this figure.
Bacteria
Bacterial production is up to 120 times greater in the littoral zone than in the pelagic zone. Bacteria are one of the main biotic components that allow the littoral zone to act as a ‘metabolic sieve.’ The main function of bacteria in the littoral zone is to break down allochthonous and autochthonous organic material. As bacteria process detritus, different nutrients and gases, such as particulate and dissolved organic matter, nitrogen, phosphorus, methane and sulfur, etc., are produced and in many cases become available to other biota in the littoral zone. Lakes with large, shallow littoral zones will have increased bacteria metabolism and faster detrital processing. Periphyton
Periphyton is a mixture of autotrophic and heterotrophic microorganisms embedded in a matrix of organic detritus (refer to ‘see also’ section). Periphyton covers most submerged substrates, ranging from sand to macrophytes to rock. The metabolic importance of periphyton at the whole lake scale is constrained by the morphometry and substrate characteristics of the littoral zone. In oligotrophic lakes, even those
with few macrophytes for periphyton to grow on, periphyton can be an important component of whole-lake primary production. For example, in one oligotrophic lake, the littoral zone comprised only 15% of the lake, but the periphyton accounted for 70–85% of the lake primary production. In eutrophic lakes, however, phytoplankton is more abundant and shading by phytoplankton reduces periphyton and macrophyte abundance. Periphyton is a common food source for invertebrates and some amphibians.
Macrophytes
Macrophytes require specific substrate types to thrive, and their growth provides a unique habitat for other organisms (refer to ‘see also’ section). Macrophytes grow best in a mixture of sand and muck, and are often found in areas with upwelling groundwater. Once macrophytes become established within the littoral zone they modify the microclimate through the reduction of wave energy and the creation of thermal gradients that prevents water from mixing. These conditions promote particle sedimentation. The degree of microclimate modification
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depends on the characteristics of the sediment structure, nutrient availability, and diffusion of oxygen through the sediment. Macrophytes are integral to nutrient cycling in the littoral zone as both sources and sinks of nutrients. Traditionally, limnologists have considered macrophytes a nutrient source, since they may incorporate nutrients from the anoxic sediment and then release them into the water column upon senescence. Others have found that nutrients removed from sediments or surrounding water column by plants are largely retained by plants until the plants decay. In addition to their role in nutrient cycling, macrophytes provide important habitat for organisms such as bacteria, periphyton, zooplankton, invertebrates, amphibians, fish, and waterfowl. Invertebrates and small fish use macrophytes as a habitat refuge from predation by invertebrates (e.g., dragonfly or damselfly nymphs), fish (e.g., Esox), and amphibians, and as a place to reproduce. For many invertebrates (e.g., insects, crustaceans) and vertebrates (e.g., waterfowl, moose), macrophytes are a major food source. Invertebrates
Invertebrates are very diverse, and include: zooplankton, crayfish, insects, worms, and leeches. Invertebrates living on the bottom of lakes are referred to as zoobenthos, and are far more abundant and diverse in the littoral zone than in other lake zones. Therefore the ratio of the abundance of zoobenthos to zooplankton is inversely related to lake size. However, the absolute diversity and abundance of zoobenthos increases with lake size. Invertebrate diversity is also positively related to habitat complexity, macrophyte abundance, conductivity, and the presence of stream connections. Habitat availability within the littoral zone influences the type of invertebrates that will colonize (Figure 2). For instance, ephemeroptera (mayflies) and plecoptera (stoneflies) generally prefer substrates that have higher wave action and coarser substrates, while lightly disturbed fine sediments are colonized by chironomids (midge larvae), bivalves (clams), and oligochaetes (worms). Substrate, macrophyte abundance, and detritus are the three main factors controlling the diversity and distribution of invertebrates, but water depth, wave exposure, and water clarity, (which influence the first three main factors) may consequently also affect invertebrate abundance and distribution. Fish
Like invertebrate diversity, fish diversity is positively related with lake size and habitat complexity. The use by fish of different littoral zone habitats also often
varies seasonally and with the age or size of the fish. As mentioned above, macrophytes are both a refuge and a hunting ground for predatory fish. Some fish species (e.g., Perca spp.) also use macrophytes as substrate for egg-laying. Many other fish species lay eggs within cobble substrates, and some use cobble as a refuge (e.g., sculpin, darters, juvenile burbot). Fish are often classified by their primary food source. Piscivores mainly eat other fish, planktivores consume plankton, benthivores feed on zoobenthos, and herbivores eat macrophytes and periphyton. Some fish species may change what they eat as they mature into adulthood. For instance, many fish species eat plankton as a juvenile and smaller fish as an adult. Even adult fish may have very broad diets as they move between littoral and pelagic zones (Figure 5). Terrestrial – Littoral Links
It is impossible to separate the processes that occur within a lake from the surrounding watershed and even the air above the watershed. Many organisms move resources and energy between the surrounding watershed and the littoral zone. Food resources from the littoral zone are an important source of energy for many terrestrial and semi-terrestrial organisms. Fish, for example, are eaten by many different terrestrial and amphibious species including waterfowl, hawks, herons, egrets, mammals, reptiles, and humans. Aquatic invertebrates found within the littoral zone provide an important source of protein for terrestrial and semi-terrestrial organisms. The reproductive success of ducks is closely related to the availability of chironomids and other insects emerging from their benthic larval form. Waterfowl, such as geese, feed on aquatic plants and can remove up to 50% of the standing stock of macrophytes in some areas. Food resources from the terrestrial ecosystem are also important for littoral species. Depending on the time of the year and the species of fish, up to half the food consumed can be allochthonous insects and small mammals. Another type of resource that is moved from land to the littoral zone is large woody debris used by beavers to construct their lodges. The woody debris used by beavers also provides habitat for many fishes. Finally, transient organisms such as waterfowl, mink, otter, beaver, muskrat, snakes, and turtles, among others, move nutrients in and out of the littoral zone via feeding and excretion and egestion. The riparian habitat is another resource that is important for species that use the littoral zone. For example, snakes and turtles sun themselves on logs and rocks found along the shoreline. Waterfowl and some mammals use the low lying shoreline habitat to
Air/Water and Land/Water Interfaces, Wetlands and the Littoral Zone _ Littoral Zone 85
make their nests, while eagles and some diving ducks use the trees surrounding lakes for their nesting sites. Hawks also use trees surrounding the lake as a perch to search for food. Riparian habitat is important for amphibians (e.g., newts and frogs) during different times within their lives. Trophic cascades – food web interactions that strongly alter the abundance of three or more trophic levels – are well documented in the pelagic and littoral zone of lakes. They also cross the littoralland interface (Figure 6). Plants near ponds with fish have more visits from pollinators than plants near ponds without fish. This is because in ponds with fish, larval dragonflies are reduced by fish predation, and thus the abundance of adult dragonflies is also decreased. Adult dragonflies have direct and indirect effects on insect pollinators. They directly prey upon the pollinators and indirectly reduce the number of pollinator visits just by being present. Littoral – Pelagic Links
The littoral and the pelagic zones are also strongly linked, especially by the diel horizontal migration of zooplankton, and by fish movements. Zooplankton sometimes move up to 30 m horizontally twice each day between zones. Zooplankton that normally reside in the pelagic zone will move into macrophyte habitats during the day to avoid pelagic predators
such as Chaoborus (phantom midge larvae) and visually feeding planktivores like small fishes. In some lakes, this movement can benefit zooplankton through reduced mortality from fish predation, food availability in the littoral zone (some zooplankton can become browsers in the littoral zone compared to being filter feeders in the pelagic zone), and enhanced growth. Predation by planktivores is often reduced by migration into the littoral zone, but in some lakes, littoral invertebrates (e.g., dragonfly larvae) pose a substantial risk of predation within the littoral zone. Thus, zooplankton movement depends on the complex interactions occurring in both the pelagic and littoral zones, which differ among lakes. Fish movements also link the littoral and pelagic zones. The dependency of fish on littoral production differs by fish type, with planktivores, benthivores, and even piscivores relying on littoral food production to some degree (Figure 7). Fish that are often categorized as pelagic planktivores can derive up to 30% of their energy from the littoral zone, while fish categorized as piscivores sometimes derive almost all their energy, at least indirectly (e.g., from other fish that consume littoral-derived foods), from the littoral zone (Figure 7). Without the littoral zone, the production of many fish, including fish that may rarely venture into the littoral zone, would decline dramatically. Pollinator
Dragonfly
−
+
+
+ −
Larval dragonfly + −
Fish
Figure 6 Example of a trophic cascade that links the terrestrial ecosystem with the littoral zone. Fish reduce the abundance of dragonflies, which leads to increased pollinators, and thereby facilitating the reproduction of terrestrial plants. Solid arrows indicate direct interactions; dashed arrows denote indirect interactions. þ are positive interactions, represents negative interactions. Modified from Knight et al. (2005) Trophic cascades across ecosystems. Nature 437: 880–883.
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Air/Water and Land/Water Interfaces, Wetlands and the Littoral Zone _ Littoral Zone
% Reliance on littoral food chain
100
80
60
40
20
Pla be nkt nth ivo ivo re/ re
vo re Be nth i
Pis civ ore
Pla nk t iv ore
0
Figure 7 The range of reliance different types of fishes have on littoral zone resources. Modified from Vadeboncoeur et al. (2005) Effects of multi-chain omnivory on the strength of trophic control in lakes. Ecosystems 8: 682–693.
The Anthropogenic Influences on the Littoral Zone Humans derive many ecosystem goods (e.g., harvested fish and waterfowl) and services (e.g., water purification, water supply) from the littoral zones of lakes. In turn, humans have immense impacts on the structure and function of the littoral zone. First, increased nutrient loading from activities such as logging, agriculture, and development causes eutrophication. Eutrophication leads to increased primary production in the littoral zone of many lakes, which can cause undesirable algal blooms, as well as increases in undesirable fish and cloudy water. Second, human-mediated spread of invasive species (e.g., zebra mussels, rusty crayfish, round gobies) alters nutrient cycling and food web composition in the littoral zone, causing changes that are generally undesirable to humans. Third, fossil fuel combustion in industry and automobiles causes acid deposition and climate change. In many cases, acidification of lakes causes decreased abundance and diversity of macrophytes, invertebrates, and fish, while increasing filamentous green algal production, all of which has cascading effects through the food web. Acidification also causes the release of metals toxic to fish, e.g., aluminum and mercury. Some of these metals bioaccumulate in fish, which then makes the fish hazardous to humans. Climate change is expected to cause warmer lake waters, and in many parts of the world, will reduce runoff, increase water residence times, lower water
levels, and increase evaporation. Concentrations of many ions will therefore increase, causing changes in nutrient and detritus availability as well as primary and secondary production within the littoral zone. Warming could also lead to poleward range expansion of many littoral species, further changing food web dynamics. Finally, fluctuations in water level are often increased by irrigation and dams. At high water levels, flooding and erosion of riparian habitat occurs. This increases the dissolved organic matter input and turbidity in the littoral zone. On the other hand, water drawdown has both positive and negative effects on the littoral zone, depending on the lake basin morphometry. Steep sided littoral zones are not as affected as shallow sloping ones. At low water levels macrophytes are reduced, the percent of sandy/fine grained habitat increases, benthic invertebrate diversity and abundance decreases and fish refuges and spawning habitat can be reduced. The response of the littoral zone to all these anthropogenic impacts is influenced by the structure and function of the littoral zone as well as the interaction between the littoral zone and the terrestrial ecosystem and the pelagic and profundal zones. The degree to which a lake responds or the length of time a lake can resist being effected by one of the humanmediated stressors described above depends on the size of the littoral zone, the position of the lake within the landscape, the abundance and distribution of different habitats within the littoral zone, and different biota present within that zone. Therefore, the littoral zone is important for whole lake functioning as well as the response of the whole lake to human beings. See also: Acidification; Algae of River Ecosystems; Algae; Amphibians; Amphipoda; Annelida, Hirudinea (Leeches); Aquatic Insects – Ecology, Feeding, and Life History; Aquatic Insects, Classification; Aquatic Plants: A General Introduction; Aquatic Plants and Attached Algae; Benthic Invertebrate Fauna, Lakes and Reservoirs; Benthic Invertebrate Fauna, River and Floodplain Ecosystems; Benthic Invertebrate Fauna, Small Streams; Benthic Invertebrate Fauna, Tropical Stream Ecosystems; Benthic Invertebrate Fauna, Wetland Ecosystems; Benthic Invertebrate Fauna; Birds; Branchiopoda (Anostraca, Notostraca, Laevicandata, Spinicaudata, Cyclestherida); Bryozoa; Chrysophytes Golden Algae; Cnidaria (Coelenterata); Coarse Woody Debris in Lakes and Streams; Coleoptera (Beetles) in Aquatic Ecosystems; Comparative Primary Production; Cyanoprokaryota and Other Prokaryotic Algae; Decapoda; Diatoms; Diptera (Biting Flies); Diptera (NonBiting Flies); Effects of Climate Change on Lakes; Ephemeroptera (Mayflies); Eutrophication of Lakes and
Air/Water and Land/Water Interfaces, Wetlands and the Littoral Zone _ Littoral Zone 87 Reservoirs; Fish, Characteristics; Fish, Populations; Fish, Productivity; Fish, Systematics and Evolution; Flatworms (Turbellarians); Flow in Wetlands and Macrophyte Beds; Gastrotricha; Green Algae; Hemiptera (True Bugs); Hydrachnida (Water Mites); Invasive Species; Isopoda (Aquatic Sowbugs); Mammals; Megaloptera (Alderflies, Dobsonflies); Microbial Food Webs; Mollusca; Nematoda; Nematomorpha (Horsehair Worms); Odonata (Dragonflies and Damselflies); Ostracoda; Other Phytoflagellates and Groups of Lesser Importance; Photosynthetic Periphyton and Surfaces; Phytoplankton Nutrition and Related Mixotrophy; Phytoplankton Population Dynamics: Concepts and Performance Measurement; Phytoplankton Productivity; Plecoptera (Stoneflies); Porifera (Sponges); Role of Zooplankton in Aquatic Ecosystems; Shallow Lakes and Ponds; Subterranean Aquatic Ecosystems - Groundwater Ecology; Tardigrada (Water Bears); Trichoptera (Caddisflies).
Further Reading Burks RL, Lodge DM, Jeppesen E, and Lauridsen TL (2002) Diel horizontal migration of zooplankton: Costs and benefits of inhabiting the littoral. Freshwater Biology 47: 343–365. Gasith A and Gafny S (1990) Effects of water level fluctuation on the sturucture and function of the littoral zone. In: Tilzer MM and Serruya C (eds.) Large Lakes: Ecological Structure and Function, pp. 156–171. New York: Srpinger.
Jeppesen E, Sondergaard M, Sondergaard M, and Christoffersen K (eds.) (1998) The Structuring Role of Submerged Macrophytes in Lakes. New York: Springer. Kalff J (2002) Limnology: Inland Water Ecosystems. Upper Saddle River, NJ: Prentice-Hall. Knight TM, McCoy MW, Chase JM, et al. (2005) Trophic cascades across ecosystems. Nature 437: 880–883. Likens GE (ed.) (1985) An Ecosystem Approach to Aquatic Ecology. New York: Springer. Lodge DM, Barko JW, Strayer D, et al. (1988) Spatial heterogeneity and habitat interactions in lake communities. In: Carpenter SR (ed.) Complex Interactions in Lake Communities. New York: Springer. Magnuson JJ, Kratz TK, and Benson BJ (eds.) (2006) Long-Term Dynamics of Lakes in the Landscape: Long-Term Ecological Research on North Temperate Lakes. Oxford, UK: Oxford University Press. O’Sullivan PE and Reynolds CS (eds.) (2004) The Lakes Handbook. Oxford, UK: Blackwell Science. Pieczynska E (1993) Detritus and nutrient dynamics in the shore zone of lakes: A review. Hydrobiologia 251: 49–58. Scheffer M (1998) Ecology of Shallow Lakes. New York: Chapman and Hall. Vadeboncoeur Y, McCann KS, VanderZanden MJ, and Rasmussen JB (2005) Effects of multi-chain omnivory on the strength of trophic control in lakes. Ecosystems 8: 682–693. Weatherhead MA and James MR (2001) Distribution of macroinvertebrates in relation to physical and biological variables in the littoral zone of nine New Zealand lakes. Hydrobiologia 462: 115–129. Wetzel RG (1989) Land-water interfaces: Metabolic and limnological regulators. International Association of Theoretical and Applied Limnology Proceedings 24: 6–24. Wetzel RG (1989) Limnology: Lake and River Ecosystems. New York: Academic Press.