Lake food chains not so simple

Posted on September 22, 2010


Open-water (pelagic) food webs in lakes are traditionally thought of as simple food chains, with zooplankton being supported by phytoplankton.  However, this account doesn’t easily explain the fact that nutrient-poor lakes sometimes sustain very large zooplankton populations.  Evidence collected from a small subarctic lake in Swedenhas revealed strong seasonal variations in food web structure.  The stable isotope signatures of a dominant zooplankton species (Cyclops scutifer) and its food resources revealed that in summer this species depends on organic matter imported from the catchment in addition to plant plankton produced by photosynthesis within the lake.  But in winter (October-May), when the lake is permanently covered by ice and light levels are very low, the same species relies on nutrients derived from mats of algae growing on the lake bottom.  Organic matter accumulates as a result of rapid algal growth in summer, encouraged by the shallow depth and clear water in the lake.  The organic matter is colonized by bacteria, which act as a food source for flagellates, ciliates and rotifers, and these are eaten in turn by zooplankton.  Most of the  zooplankton grazing seems to occur in the open water or at the sediment surface rather than in the sediment itself.  These findings show that open water food webs can be strongly subsidized by nutrients from terrestrial  ecosystems and adjacent lake-bed habitats.

Reference: 

Karlsson, J. &  Sawstrom, C.  2009.  Benthic algae support zooplankton growth during winter in a clear-water lake.  Oikos 118, 539-544.

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Posted in: food webs, lakes, plankton