Mineral limitation of zooplankton: Stoichiometric constraints and optimal foraging

Nutritional deficiencies are a very common phenomenon, and consumers genera lly face food that is not optimally suited for their needs. Especially herb ivores are habitually confronted with food of inferior quality, usually a r esult of too-low nutrient concentrations in plant material. Waterfleas of t he genus Daphnia are good model organisms to study the effect of inferior q uality food, and how animals deal with this. We tested the effect of algae to which we had given different phosphorus contents on both life history an d feeding parameters of Daphnia magna. Phosphorus content of the algae stro ngly affected both the growth rate and the feeding activity of the daphniid s. Feeding activity increased with declining food quality (increase in C:P ratio of the algae), whereas growth rates were maximal at intermediate C:P levels. We conclude that the direct limitation of phosphorus is a very impo rtant factor determining food quality for zooplankters. Daphniids counterba lanced lower P content of their food by spending more C (energy) on acquiri ng this limiting resource. This implies that when Daphnia are given phospho rus-limited food both the addition of phosphorus and the addition of carbon (energy) should increase the growth rate of the animals (co-limitation). T he influence of the phosphorus content of the food on the feeding activity of Daphnia offers a mechanistic explanation for the observed homeostasis in daphniids.