The distribution of phytoplankton along trophic gradients and its mediation by available light in the pelagic zone of large eutrophic lakes
We describe the pattern and the principal factors affecting the phytoplankton biomass–nutrient relationship in the pelagic zone of large lakes. The results showed that the phytoplankton abundance and biomass of Cyanophyta, Cryptophyta, and Pyrrophyta were significantly correlated with trophic states. The total phosphorus (TP)–biomass relationship curves showed that the increment of biomass with TP is weak at high TP levels. The decrease in biomass at the high end of the curves might be a synthesis of the pattern of responses of the major taxonomic groups (except cyanobacteria) to environmental variables. Light limitation might be one of the important factors causing the decrease in the TP–biomass curve at high TP concentrations. If the mean underwater available light is lower than ∼250 µmol photons·m–2·s–1, clear-water species decline and cyanobacteria become dominant. The responses to available light of these key species play a central role in modulating the biomass–nutrient relationship. Our results contribute to the understanding of this relationship in the pelagic zone of large eutrophic lakes and have important practical implications for lake management.