Biomass pyramids in lake plankton: influence of Cyanobacteria size and abundance

The ratio of zooplankton to phytoplankton (Z:P) standing stock biomass in freshwater lakes has been suggested to decline in highly productive systems. An increasingly large proportion of inedible phytoplankton, especially in eutrophic systems dominated by Cyanobacteria, is one possible mechanism for declining Z:P. We tested this hypothesis by calculating the biomass in phytoplankton and zooplankton samples collected from 173 culturally eutrophic lakes and estimating the change in the functional relationship between zooplankton and phytoplankton biomass after ignoring inedible Cyanobacteria. We found up to 2 orders of magnitude less zooplankton biomass than would be predicted at a given total phytoplankton biomass and that removing Cyanobacteria led to zooplankton biomass approaching the level expected based on remaining phytoplankton biomass. Z:P increased with the percentage of edible phytoplankton biomass, indicating greater zooplankton biomass in lakes with the least Cyanobacteria. The lower Z:P found in these eutrophic lakes likely results from 89% of the phytoplankton biomass being composed of Cyanobacteria, whose cells were significantly larger than other phytoplankton. These results suggest that zooplankton biomass is limited by a declining proportion of edible phytoplankton in the most productive lakes and illustrate how eutrophication leads to declining resource use efficiency by consumers.