Effects of zooplankton structure and phosphorus concentration on phytoplankton biomass in a freshwater pelagic food chain

Abstract

In Lake Lugano (Switzerland and Italy), temporal variation in the structure of herbivore zooplankton (a shift to greater dominance by large-bodied species and higher biomass) and phosphorus (soluble reactive phos- phorus, SRP) concentrations provided useful settings to examine the effects of these changes on phytoplankton biomass. Results from this study indicate that, within an overlapping range of SRP concentrations (14.2–24.5 μg P L –1, which indicate a P-enriched lake), phytoplankton biomass (measured as Chlorophyll-a, CHL) was substan- tially lower after the shift to larger-bodied zooplankton. In addition, after the shift, CHL displayed an unforeseen negative association with SRP, which was parelleled by a decrease in consumer control (indicated by the ratio between herbivore and phytoplankton production) toward lower SRP concentrations. The suppression of phytoplankton indicates that the shift to greater biomass and larger-bodied zooplankton increased consumer-control on phytoplankton biomass, even at relatively high SRP concentrations. The negative relationship between SRP and CHL after the shift might have stemmed from reduced phytoplankton edibility and/or reduced quality at low SRP concentrations, although establishing whether the association is causal will require further research. The results suggest that zooplankton structure and SRP concentrations influenced phytoplankton biomass, although through complex and unanticipated effects.