A test of food web hypotheses by exploring time series of fish, zooplankton and phytoplankton in an oligo-mesotrophic lake

The strength of trophic cascades in the pelagic area of lakes at low productivity has been discussed intensively, but predictions of trophic coupling differ strongly. Many studies suggest that trophic cascades are weak in oligotrophic lakes, but some models discussed that trophic interactions might be strong at low nutrient concentration. Here, we used time series over 9 (phytoplankton and zooplankton) or 6 (fish) years from the oligo-mesotrophic Lake Stechlin (Germany) to explore correlative relationships between biomasses of these trophic levels. The fourth trophic level of piscivorous fish was almost absent in the pelagic area. The biomass of planktivorous coregonid fishes was not at all correlated to total zooplankton biomass, which was dominated by calanoid copepods. However, there was a strong negative correlation of adult coregonid biomass to the proportion of Daphnia in zooplankton, and a strong positive correlation of adult fish to proportion of calanoid copepods. Zooplankton and phytoplankton biomasses were not correlated except for a significantly negative correlation between Daphnia biomass and biomass of Cyanobacteria. Overall, our results suggest that planktivorous fish may modify the zooplankton structure, but not the zooplankton biomass, in lakes of low productivity. However, this top-down effect by fish does not cascade further down to the phytoplankton biomass or community structure, confirming earlier hypotheses that trophic cascades are weak in oligotrophic lakes