Daphnia fitness over a food gradient:: is body size the single trait predicting exploitative ability?

Phytoplankton dynamics in freshwater ecosystems can be faced as a meaningful natural stressor for its main grazers, and hence constrain bottom-up interactions within the trophic structure of the food-web. Filter-feeding zooplankters, such as Daphnia, cope with resources fluctuations by adjusting their life-history as a function of balances in the energy allocation rules. Additionally, Daphnia populations often withstand interspecific competition within coexisting related species, and it is generally assumed that body size is a strategic trait conditioning exploitative ability and the related competitive advantage of species. The life-history responses of one Daphnia magna population and three Populations of Daphnia cf longispina to a discrete gradient of algae concentration were assessed. This experiment was focused on (i) how these distinct Populations regulate life-history when feeding over low, intermediate and high food-levels; and on (ii) whether the large-bodied D. magna can be better in exploiting resources over the gradient than can the smaller D. cf longispina populations. We found evidences that body size might not be the single trait influencing the exploitative ability of related species. The D. cf longispina populations were often better than D. magna in exploiting different levels of resources, and remarkable differences in fitness were found between these similar-sized individuals within food-level; indeed, these differences were frequently of higher scale than those found between the larger D. magna and any of the D. cf longispina populations.