Simultaneous measurement of the effect of food concentration on assimilation and respiration in Daphnia magna Straus

Models of the competitive success of different filter-feeding zooplankton species under limiting food conditions make assumptions about the metabolic costs of collecting and processing the food particles. Such assumptions were tested by measuring carbon assimilation and respiration of Daphnia magna Straus under varying concentrations of the green alga Scenedesmus acutus Meyen using a method that allowed determination of the incorporation of radio-carbon, release of C-14-carbon dioxide, oxygen consumption and weight increment at the same time on the same individuals. Assimilation rate increased linearly with increasing food concentration until a plateau was reached at the incipient limiting level (ILL). Respiratory rate also increased and plateaued at the ILL. A linear relationship with a slope of 0417 was found between assimilation rate and respiratory rate. The intercept of this regression was not significantly different from the respiratory rate of fasting Daphnia. This fact, the linearity of the relationship between assimilation and respiration, and the slope and the shape of the response curve of the respiratory rate to food concentration suggest that increased respiratory rates in feeding daphnids are mainly due to specific dynamic action. No evidence was found that either the filtration process or the rejection of superfluous food in Daphnia is very costly. This should be considered when models of optimal foraging are constructed for these filter-feeders.