Fish limit bottom-up nutrients by top-down control

Posted on December 15, 2011

In warm, productive lakes the nutrients excreted by fish can play an important role in sustaining phytoplankton populations.  In unproductive systems on the other hand, fish are less abundant and recycle much lower quantities of nutrients.  Nevertheless, because fish can exert strong top-down control on zooplankton populations even in cold, high latitude lakes, it’s possible that in these environments fish still have a substantial influence (albeit an indirect one) on nutrient recycling.  This idea was tested by comparing the ecology of six lakes (three with fish and three without fish) in Brooks Mountain Range, Arctic Alaska  The lakes were sampled in the ice-free summer between June and October, when nutrient inputs from the catchment are minimal.  Mean levels of nitrogen and phosphorus excretion by zooplankton were 1.9-2.7 times higher in lakes where fish were absent than in lakes where fish were present.  While zooplankton excretion satisfied 19-130% of the nitrogen and 37-200% of the phosphorus required by phytoplankton, fish excretion satisfied no more than about 5% of the nutrient requirement.  The presence of predatory fish (grayling, trout and char) reduced zooplankton biomass by about 80%.  Therefore, by cropping the zooplankton population, fish significantly reduced the supply of recycled nutrients available to lake phytoplankton.   In cold lakes internal nutrient recycling should be considered alongside watershed runoff as an important source of potentially limiting nutrients.

Reference:  Johnson, C.R., Luecke, C., Whalen, S.C. & Evans, M.A.  2010.  Direct and indirect effects of fish on pelagic nitrogen and phosphorus availability in oligotrophic Arctic Alaskan lakes.  Canadian  Journal of Fisheries & Aquatic Sciences 67, 1635–1648.