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-Su ˆre 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.

We investigated the behaviour and life histories of large zooplankton in the Esch-sur-Suˆre reservoir (Luxembourg). We found that the decrease in size at maturity, as well as diurnal refuge in deep waters, were the adaptive responses (concurrently or alternatively) adopted by large zooplankters to cope with the increasing predation risk throughout the summer. Daphnia galeata initiated a more severe trade-off to cope with high summer predation relative to other cladoceran species. Diaphanosoma brachyurum and Daphnia cucullata seem less susceptible to fish predation as indicated by the low alteration of their size structure and their vertical distribution. The copepod Eudiaptomus gracilis took refuge in deep waters and achieved important modifications of its mature size.

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We explored rates and stoichiometry (C : N : P ratios) of sinking particles in a temperate reservoir during a 2-yr period. Plankton was sampled weekly, and a sediment trap placed below the metalimnion collected sinking particles. There were no significant relationships between the stoichiometry of entrapped material and seston or zooplankton stoichiometry. However the differences in the entrapped C : P and N : P ratios between consecutive trap samplings were negatively correlated with the time variations of the zooplankton C : P and N : P ratios. Zooplankton C : P and N : P ratios were positively correlated with the percentage of copepod biomass in total zooplankton biomass Ͼ250 m and negatively correlated with the percentage of cladocerans. Zooplankton biomass Ͼ250 m reduced the fraction of N and P primary production lost to sinking (export ratio). The residuals of the N export ratio versus zooplankton biomass relationship were negatively correlated with the zooplankton N : P ratio, whereas there was a positive relationship with the residuals of the P export ratio relationship. These observations support the hypothesis that the regulation of elemental homeostasis in the herbivorous zooplankton consumers occurs at least partly at the assimilation/egestion level. Elements ingested in excess-P for the herbivorous copepods and N for many cladocerans-are concentrated into sinking feces, whereas the deficient elements are captured into biomass.In lakes, the vertical flux of small particulate matter essentially comprises two components: the sinking of ungrazed phytoplankton cells (e.g., Reynolds et al. 1982) and the sinking of the feces or fecal pellets of planktonic primary consumers (Sarnelle 1999). Both types of particles will settle from the upper layer below the thermocline if their sinking rates are higher than their mineralization rates. A simple heuristic model (Elser et al. 1995) explicitly represents the processes involved (Eq.

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