NUTRIENT ENRICHMENT AND GRAZER EFFECTS ON PHYTOPLANKTON IN LAKES

I derived predictions about how the magnitude of Daphnia effects on total phytoplankton biomass should vary across a gradient of enrichment (expressed as algal carrying capacity) using two, simple predator-prey models. These predictions were then compared with data from a survey of field experiments in temperate lakes. Algal responses to Daphnia manipulation were quantified as an Algal Response Factor (ARF), defined as total algal biomass in the low-Daphnia treatment divided by total algal biomass in the high-Daphnia treatment. Total phosphorus concentration (TP) was used as an index of algal carrying capacity, ranging from 10 to 460-mu-g/L over the 22 experiments surveyed. The Algal Response Factor ranged from 1 to 40 and was a positive, linear function of TP: ARF = -0.14 + 0.08(TP), r2 = 0.81; log ARF = -0.81 + 0.83(log TP), r2 = 0.75. Thus, algal carrying capacity, as quantified by TP, explains much of the variation in Daphnia effects on total algal biomass across lakes. The survey results supported the prediction of the simpler model, a two-species, Lotka-Volterra model of pure exploitation. Incorporating the additional complexity of inedible algae into this model did little to improve its predictive power. The conclusion that inedible algae are not of major importance to the prediction of Daphnia effects across an enrichment gradient was supported by a survey of Daphnia effects on the proportionate biomass of inedible algae. There was no evidence that Daphnia grazing typically favors dominance by inedible algae at equilibrium in highly enriched lakes. My results suggest that the well-established positive relationship between TP and equilibrium algal biomass in lakes is more likely to be a consequence of increases in Daphnia's death rate with enrichment, rather than decreases in Daphnia's feeding and assimilation rates. When zooplanktivorous fish are rare, Daphnia should be able to prevent phytoplankton biomass from responding to nutrient addition at equilibrium. As a consequence, the success of biomanipulation in eutrophic lakes should critically depend upon the effectiveness of strategies aimed at reducing zooplanktivory.