Context-dependent effects of Daphnia pulex on pond ecosystem function: observational and experimental evidence.

Large-bodied zooplankton of the genus Daphnia are thought to be keystone species in freshwater pelagic habitats, potentially able to exert strong grazing effects and enhance phosphorus limitation of algae. I examined the degree to which Daphnia pulex differ from small-bodied zooplankton in their effects on algal biomass, seston C:P and N:P, total nitrogen and total phosphorus. This was done with both survey data from natural ponds and an in situ experiment in which D. pulex was compared to a small zooplankton assemblage under low and high nutrient conditions and in two different ponds. D. pulex effects on algae were only evident under high nutrient conditions. In natural ponds, D. pulex dominance resulted in a significantly weaker chlorophyll-total phosphorus relationship, with the divergence between D. pulex and small zooplankton-dominated systems being greatest in highly enriched ponds. In the experiment, D. pulex exerted stronger top-down control in enriched treatments only and tended to graze algae to lower levels in the more productive pond. Dynamics of C:P over the course of the experiment did not reveal strong effects of zooplankton composition. However, data on the final date of the experiment provided some evidence that D. pulex can enhance phosphorus limitation of algae; total phosphorus was lower and C:P higher in D. pulex treatments. Survey results revealed no effects of D. pulex on seston C:P or N:P, suggesting that this species may not be an important factor governing phosphorus limitation of algae in natural ponds.