Relative effects of Daphnia and Ceriodaphnia on phosphorus-chlorophyll relationships in small urban lakes

Empirical models based on zooplankton biomass were used to predict mean summer chlorophyll a (Chl a) and to examine how zooplankton influenced the total phosphorus (TP) - Chl a relationship. Four years of data were analyzed for three lakes having similar TP concentrations but varied abundances of Daphnia and Ceriodaphnia. Mean TP did not correlate significantly with mean Chl a during the study period, although mean Daphnia density was a good predictor of Chl a concentration (p > 0.001). Both residuals from the TP - Chl a relationship (p > 0.001) and Secchi depth (p > 0.007) were negatively correlated with Daphnia abundance. Ceriodaphnia abundance was positively correlated with Chl a (p > 0.002) and Secchi depth (p > 0.001). Mean size of Daphnia during spring was the best predictor of the Daphnia-Ceriodaphnia shift in mid-summer. Early establishment of a large-sized Daphnia cohort may prevent their summer elimination by Chaoborus and intensify competition with Ceriodaphnia. These results imply an important link between Daphnia and Ceriodaphnia thereby limiting the utility of Chl a - TP model predictions in these small, urban lakes. This linkage and the differential effect of these two zooplankton species on planktonic algae deserve further consideration in similar lakes where phytoplankton and zooplankton tend to be tightly coupled.