Influence of two Daphnia species on summer phytoplankton assemblages from eutrophic lakes

We conducted grazing experiments to test whether larger-bodied Daphnia pulicaria have a different effect from smaller-bodied Daphnia galeata mendotae on the composition of summer algal assemblages in eutrophic lakes. Three separate cubitainer experiments were run for 5 days in a replicated factorial design utilizing two algal community types and the two Daphnia species. Inorganic phosphorus and nitrogen were added to prevent nutrient limitation of the algae. Both edible and inedible size fractions of chlorophyll a increased in cubitainers without Daphnia spp. Grazer addition usually resulted in a reduction in edible chlorophyll; reductions were greater in D.pulicaria cubitainers. Grazing by Daphnia spp. on presumed inedible chlorophyll was variable. Algal size was not always a good predictor of grazeability. The results of this study indicate that D.pulicaria, because of its greater filtration potential and ability to ingest larger particles, provides a stronger control on inedible-sized algae when compared to equal numerical densities of D.g.mendotae. However, Aphanizomenon increased as a response to heavy grazing pressure by D.pulicaria on other algal species. This suggests that biomanipulation efforts that promote large-bodied Daphnia may not produce desirable results if nutrient inputs remain high.