Diel integration of food quality by Daphnia:: Luxury consumption by a freshwater planktonic herbivore

The chemical content of living algal food is an important determinant of zooplankton herbivore fitness. Previous studies have restricted diets to a single food type presented over multiple body mass turnover times; these have established that phosphorus content is one factor closely associated with growth of some zooplankton. In nature, however, algal P content and other potentially important dimensions to food quality vary in space and time. It would be unlikely for a grazing animal to experience a single constant food type over a major portion of its life. Many grazers likely experience diel variation in food quality associated with their vertical migration through nutritional gradients. It is not known how long-term growth under fluctuating food relates to growth rates expected for single, constant food types. We tested whether grazers could integrate over daily variation in their food and thus realize body growth rates higher than would be expected from growth determined on single constant food types alone. We found that Daphnia magna grow near maximal rates when exposed to high-quality food for half or less of the day. These results suggest that nutritional gradients associated with vertical migration bear close examination. They also strongly suggest that some form of diel P storage in zooplankton, a form of luxury consumption, occurs, and allows for integration over temporal variation in supply of this essential dietary ingredient.