Lake Ecosystems Advanced article Nelson G Hairston Jr, Cornell University, Ithaca, New York, USA Article Contents . Introduction Gregor F Fussmann, McGill University, Montreal, Quebec, Canada . Lake Thermal Structure . Lake Habitats, Processes and Food Chains . The Role of Nutrients . Trophic Control of the Food Chain Online posting date: 15th May 2014 Lakes are bodies of nonmarine standing water connected ecological and biogeochemical processes and their study by water flow and aerial inputs to their surrounding falls within the discipline of ‘limnology.’ Lakes are superb landscapes (watersheds). As relatively discrete ecosys- habitats for the study of ecosystem dynamics: interactions tems, the interplay between physical, biogeochemical among biological, chemical and physical processes are frequently either quantitatively or qualitatively distinct and organismal processes in them is especially clear, and from those on land or in air. As the boundaries between can be studied, understood and put to use in effective water and land, and water and air are distinct, there is tight management. Sunlight penetrating from the lake surface coupling among many ecosystem components. See also: provides energy that warms the surface water, energy for Ecosystem Boundaries; Evolutionary Responses to Cli- photosynthesis and an environment suitable for pre- mate Change; Invasion of Introduced Species dators that hunt by sight. The depth to which light Although lakes contain 50.01% of all the water on the penetrates is determined by the amount of suspended earth’s surface, they hold 498% of the liquid surface fresh particles (phytoplankton, organic and inorganic sedi- water. Many organisms depend on fresh water for survival, ments) and coloured organic chemical compounds dis- and humans frequently depend on lakes for a great many solved in the water. Important chemicals entering from ‘goods and services’ such as drinking water, waste removal, the watershed include essential nutrients (chiefly phos- fisheries, agricultural irrigation, industrial activity and recreation. For these reasons lakes are important phorus and nitrogen) and pollutants that are taken up ecosystems. and passed through the food chain from primary produ- Lake ecosystems are influenced by their watersheds, the cers (phytoplankton and rooted plants) to consumers geological, chemical and biological processes that occur on (animals that eat plants and other animals). All organisms the land and streams that lie uphill. The movement of in lakes have adaptations that affect the strengths of their chemicals, sediments, detritus and of many organisms is interactions with their physical and biogeochemical typically unidirectional from the watershed to the lake, but environments and with other species in the food web. fish may migrate upstream, and aquatic insects may emerge Introduced species, pollutants, and other changes in the and disperse onto land. A lake and its watershed are often environment result in rapid evolution of the adaptations considered to be a single ecosystem (Likens, 1985). that determine interaction strengths. These processes are particularly obvious in discrete lake ecosystems. Lake Thermal Structure Introduction During summer, sunlight increases the temperature of lake surface water. Water at greater depths is warmed less. Lakes are inland bodies of water that lack any direct Wind at the surface causes the top several metres of lake exchange with an ocean. Lake ecosystems are made up of water to mix homogeneously to form a warm surface layer the physical, chemical and biological properties contained called the ‘epilimnion’. Below the level of wind mixing, within these water bodies. Lakes may contain fresh or salt temperature drops rapidly through a zone called the ther- water (in arid regions). They may be shallow or deep, mocline, and below this is a region of homogeneously cool permanent or temporary. Lakes of all types share many water called the ‘hypolimnion’. This two-layer physical structure is called ‘thermal stratification’. Summer strati- eLS subject area: Ecology fication does not occur in shallow-water bodies, nor in high latitude or high altitude lakes where summers are short. In How to cite: winter, surface water directly under the ice is approxi- Hairston Jr, Nelson G; and Fussmann, Gregor F (May 2014) Lake mately 08C and deeper water is slightly warmer. Wind Ecosystems. In: eLS. John Wiley & Sons, Ltd: Chichester. cannot mix water below the ice, so winter stratification DOI: 10.1002/9780470015902.a0003191.pub2 persists while the lake is frozen. Winter stratification does eLS & 2014, John Wiley & Sons, Ltd. www.els.net 1 Lake Ecosystems not occur in tropical or subtropical lakes, in many large stratified record of up to several hundreds of years. temperate-zone lakes, or in many salt lakes. Paleolimnologists investigate this record to make infer- Lakes that stratify during summer and winter mix from ences about historical changes in the lake ecosystem and its top to bottom during spring and autumn in events called watershed. In addition to dead material, the lake sediment ‘turnover’. Lakes that do not stratify in winter, and those can contain a number of dormant life history stages of that do not stratify in summer, mix continuously plankton. Many of these organisms produce reproductive throughout their unstratified periods. Tropical lakes may stages that have a delayed development that become active stratify every day and mix every night. and join the plankton community only after a dormancy Summer thermal stratification divides the lake environ- period. Such resting stages (single cells, eggs or dormant ment into two distinct parts: an illuminated and warm immature stages, depending on the species) are heavier epilimnion where phytoplankton (algae and photo- than water and sink to the bottom of lakes where they synthetic cyanobacteria) carries out photosynthesis, and a accumulate as a ‘propagule bank’ and spend between dark and cool hypolimnion where this production is weeks and decades of suspended life before they re-emerge. decomposed. Decomposition also dominates under the ice Resting stages can allow plankton populations to bridge of lakes that freeze over. When the amount of decomposing temporarily unfavourable conditions such as periods of organic matter is large, anoxic conditions (no dissolved cold temperature, low food availability or drought (Orsini oxygen) can prevail. et al., 2013). See also: Palaeoecology Submersed rooted plants (macrophytes) growing at the lake margin define the littoral zone, and provide habitat for attached algae (epiphytes), insects and other inverte- Lake Habitats, Processes and Food brates, and fishes that use this area for breeding, cover and Chains foraging. Some fish consume rooted plants, but most eat invertebrates or other fish. The littoral zone captures In the pelagic zone (open-water region) of the lake, phy- much of the chemicals, sediments and detritus washing toplankton carries out photosynthesis at the base of the in from the watershed and processes these materials food web. These unicellular or simple colonial algae or before they reach the pelagic zone. As macrophytes cyanobacteria sink only very slowly and are easily resus- require light to grow up from the lake bottom each spring, pended by wind-driven water movements. Very small thedistancethelittoralzoneextendsintothelakedepends bacteria are consumed by unicellular zooplankton; larger on how steeply the lake bottom drops off near shore, and phytoplankton is consumed by larger zooplankton. Some how turbid the lake water is with phytoplankton or sus- taxa are generalists that filter most algae that are encoun- pended sediments. See also: Macrophytes: Ecology of tered (e.g. Daphnia) and can have a major impact on phy- Aquatic Plants toplankton densities in lakes. Other taxa tend to select the The profundal zone is the bottom water and sediments of more nutritious phytoplankton to consume. See also: Algal deep lakes where there is insufficient light for photo- Ecology; Phytoplankton synthesis. In this region, bacteria and fungi obtain energy Fine detritus (dead plankton) suspended in the pelagic by decomposing detritus, or by chemoautotrophy (break- zone is colonised by heterotrophic bacteria, which is then ing energy-rich chemical bonds in inorganic molecules). consumed by protists (ciliates and flagellates) and gen- Insect larvae (typically dipterans) and annelid worms eralist grazers. The protists are in turn consumed by other (oligochaetes) live in the soft bottom sediments and con- protists or by copepods. This return of energy to the pelagic sume detritus. All organisms that live in, on, or in asso- food chain is called the ‘microbial loop’. Its ultimate ciation with the lake bottom are called ‘benthos’. importance in lake ecosystems remains a point of debate. There are exchanges among all lake habitats. Nutrients Other detritus produced in the epilimnion may be trapped and dissolved organic carbon (DOC) molecules released by at the thermocline or sink into the hypolimnion where it is macrophytes in the littoral zone diffuse to the pelagic zone decomposed by bacteria. where they are used by algae and bacteria. Detritus from Grazing zooplankton is consumed by predatory inver- the epilimnion sinks to the hypolimnetic profundal zone tebrates (rotifers, cyclopoid copepods, some cladocerans where nutrients are released. Nutrients in the hypolimnion and insect larvae of the genus Chaoborus) or vertebrates are returned to the epilimnion via diffusion, turbulent