Prediction of Daphnia Production along a Trophic Gradient

To predict Daphnia secondary productivity along a trophic gradient indexed as total phosphorus(TP) concentration, we estimated energy transfer efficiencies from food quality for Daphnia such as eicosapentaenoicacid (EPA) or docosahexaenoic acid (DHA) content. Eleven flow-through Daphnia magna growthexperiments were conducted with seston from 9 lakes, ponds and river waters. Primary productivities wereestimated from food supply rates in the flow-through experiments, producing energy transfer efficiencies fromseston to D. magna. We found DHA content was the best predictor of energy transfer efficiencies among theessential fatty acids. An asymptotic saturation model explained 79.6% of the variability in energy transfer efficiencies.Based on empirical data in this study and empirical models from literature, we predict that Daphniaproductivity would peak in mesotrophic systems by decreasing food quality and increasing food quantity alongtrophic gradient.