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 zo oplankton herbivore fitness. Previous studies have restricted diets to a si ngle 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 p otentially 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 fluctuat ing food relates to growth rates expected for single, constant food types. We tested whether grazers could integrate over daily variation in their foo d and thus realize body growth rates higher than would be expected from gro wth determined on single constant food types alone. We found that Daphnia m agna grow near maximal rates when exposed to high-quality food for half or less of the day. These results suggest that nutritional gradients associate d with vertical migration bear close examination. They also strongly sugges t that some form of diel P storage in zooplankton, a form of luxury consump tion, occurs, and allows for integration over temporal variation in supply of this essential dietary ingredient.