In situ filtration responses of Daphnia galeata to changes in food quality

In the stoichiometric study of phyto-zooplankton interactions, a controversy exists about how Daphnia species regulate their feeding rate when submitted to low-quality food (i.e. high dietary carbon:phosphorus [C:P] ratio). In this study, we gathered data over 3 years on in situ clearance rates of a Daphnia galeata population, by conducting grazing experiments from April 1998 to October 2000 in the Esch-sur-Sure reservoir (Grand-Duchy of Luxembourg). Observed clearance rates (2.5-13.5 mL individual(-1) day(-1), mean 7.0 mL individual(-1) day(-1)) were correlated with population and environmental variables. Mean body size of Daphnia individuals was the best predictor of clearance rate (r(2) = 0.639), followed by water temperature (r(2) = 0.262) and P concentration in the seston (r(2) <= 0.241). When combined with body size, seston C:P or nitrogen:P ratios provided the best predictive regression model of clearance rate (r(2) = 0.852-0.897). The clearance rate was always correlated negatively with P concentration in the food. Thus, Daphnia reacts to a decrease of food quality (i.e. higher C:P ratio) by an increase of its feeding rate. However, the filtration response to food quality was inhibited by high population density. We suggest that, when food quality is low and population density high, this inhibition may favour fitness of future conspecific generations. In conclusion, our results emphasize the effect of P availability on transfer rate of organic matter between lacustrian autotrophs and herbivores and pose questions about consequent effects on nutrient biogeochemical cycles.