The food web structure of arctic lakes is simplified and contains a limited number of species overall and often a truncated number of trophic levels. For example, in many lakes the top predators are zooplankton or even insects, such as the larvae of dytiscid beetles. Even when fish are present it is rare to have the classic (yet simply) structure of piscivorous top predators feeding only on forage fish which feed only on zooplankton. Typically fish feed on a variety of prey, and their presence can strongly impact the structure of the food web. If a lake is shallow enough to freeze to the bottom, or if the outlet of the lake is too steep to be negotiated by fish, the fish-less lake food web will be distinguished by very abundant and large-bodied zooplankton. Lakes with fish tend to have a small-bodied zooplankton community or sometimes a mixture of the two communities in larger lakes containing greater habitat diversity.
The energy flow in arctic lake food webs is most often driven by benthic production. Starting as early as the 1950s it was noticed that zooplankton existed in unusual abundances where the pelagic (open water) primary production was insufficient to support such high biomass. In turn, the level of fish biomass in arctic lakes requires energy subsidies from benthic organisms such as mollusks and chiro-nomids in addition to open-water zooplankton food. Despite the well-known importance of benthic organisms in arctic lakes, only —2-3 complete energy budgets of arctic lakes exist, meaning that to date we know very little about the energy flows and ecosystem dynamics in lakes of the Arctic.
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