Print PageThe ratio of heterotroph to autotroph (H/A) biomass in freshwater ecosystems has been shown to decrease with increasing standing stocks of pelagic primary producers. This transition has often been characterized as a shift from an inverted trophic pyramid in unproductive systems, where heterotrophy may be supplemented by high turnover rates in autotrophs or alternative sources of carbon such as bacteria or DOC; to a normal pyramid in highly productive systems, where autotrophs form a much larger standing stock of biomass than grazers. This contrast is often attributed to increasing biomass of larger and less palatable groups of phytoplankton (i.e., Cyanobacteria) in more eutrophic systems. We tested this hypothesis by calculating the biomass from phytoplankton and zooplankton samples collected from 173 eutrophic lakes and estimating the slope of the relationship between zooplankton and phytoplankton biomass. We then estimated the edible fraction of phytoplankton as non-Cyanobacteria nanoplankton (<35 μm) and compared the H/A ratio with and without this fraction.
Results/Conclusions
We found that ~95% of the phytoplankton biomass in these systems is made up of Cyanobacteria and that their cells are 2-3 times larger than other phytoplankton, on average. We also found that removing Cyanobacteria biomass leads to a 20% increase in the slope of zooplankton versus phytoplankton biomass. Finally, we found that even in these extremely productive systems the H/A ratio increases with an increasing percentage of edible biomass. These results provide evidence that the contrast between biomass ratios in unproductive and productive systems may, at least in part, be attributed to the inedibility of a large portion of the autotrophic biomass due to size and grazing resistance. In addition to providing a possible mechanism for the disproportionally large biomass of phytoplankton observed in highly eutrophic lakes, these results also may have implications for the inefficacy of top-down management in these systems.
