Influence of different sized Daphnia species on chlorophyll concentration and summer phytoplankton community structure in eutrophic Wisconsin lakes

We conducted field observations on 13 eutrophic Wisconsin lakes dominated by either the larger bodied Daphnia pulicaria or the smaller bodied Daphnia galeata mendotae. While Daphnia numerical densities were not significantly different between groups of lakes, pulicaria lakes had much higher Daphnia biomasses and filtration potentials than galeata lakes. Although we found significant differences in chlorophyll (Chl) a between both groups of lakes during June, on a seasonal basis populations of different sized Daphnia were not associated with significant differences in Chi a. Filtration potential per se was the major determinant of Chi a, regardless of which Daphnia species dominated. However, in pulicaria lakes, the clear-water phase started earlier, lasted longer, and was usually characterized by greater Secchi disc readings than in galeata lakes. For large blue-green algae such as Aphanizomenon, D. pulicaria appeared to delay bloom conditions, but ultimately did not prevent the alga from growing. Our results suggest that high densities of large-sized Daphnia are a desirable goal of biomanipulation because they can attain filtration potentials high enough to increase summer water clarity in eutrophic lakes.