Plankton biomass partitioning in a eutrophic subtropical lake: comparison with results from temperate lake ecosystems

Abstract

Plankton were sampled for 6 years in a subtropical eutrophic lake in FL, USA, and absolute and relative carbon biomass was determined for bacteria, phytoplankton, heterotrophic and phototrophic nanoflagellates, ciliates, rotifers and crustacean zooplankton. We compared the results with findings from a comprehensive study of carbon biomass partitioning in eutrophic German lakes with elucidate common patterns and differences. Similarities between the temperate and subtropical systems included: similar seasonal dynamics, with maximal carbon biomass of nanoflagellates and metazoan zooplankton in spring and phytoplankton in summer to autumn, yearly averaged carbon occurring mainly in the phytoplankton and phytoplankton accounting for a much greater proportion of carbon than bacteria. There also were differences: the Florida lake had lower absolute and relative carbon biomass in crustacean zooplankton, stronger dominance of protozoa in total grazer carbon biomass, a lower ratio of zooplankton to phytoplankton carbon and almost a monoculture of predation-resistant copepods (versus a relatively balanced distribution of carbon among cladocerans, copepods and rotifers in the temperate lakes). The subtropical lake also had 4-fold higher relative biomass of small filamentous cyanobacteria in its phytoplankton, which we attribute to light limitation. Although the Florida and German studies did not measure biomass of planktivorous fish, the differences observed here are consistent with a recent hypothesis that fish predation exerts stronger top-down control on the pelagic food web in subtropical lakes than in temperate lakes of similar trophic status.