Herbivore Effects on Phytoplankton Succession in a Eutrophic Lake

Herbivory can potentially affect the speed and direction of plant succession by favoring the development of a community dominated by grazing—resistant species. This idea was tested experimentally by examining the effects of the planktonic herbivore, Daphnia, on phytoplankton succession in a naturally eutrophic lake. Phytoplankton succession was characterized by two major transitions in community structure. Algal dominance shifted from small diatoms and chlorophytes during the spring bloom to cryptophyte flagellates during the clear—water phase. After the clear—water phase, dominance shifted to filamentous blue—greens (cyanobacteria). Algal species positions in the successional sequence were repeatable from year to year, despite interannual shifts in the timing of species' peak abundances associated with a drastic change in the food web. In addition, evidence is presented to suggest that Daphnia—induced changes in water clarity may have fostered increases in epilimnetic mixing, which, in turn, may have stimulated brief blooms of large—celled algae after clear—water periods. Daphnia manipulation in large enclosures, and whole—lake observations before and after a fish kill, showed that intense grazing promoted the transition from edible, spring—bloom species to similarly edible, cryptophyte flagellates. In contrast, Daphnia grazing retarded further succession to grazing—resistant, filamentous blue—greens. Thus, the effects of herbivory on algal succession were not predictable from the relative susceptibilities of these algal species to grazing mortality. These results underscore the importance of indirect effects in the herbivore—plant interactions of planktonic communities. The observation that a single species of herbivore had opposite effects during two successional transitions implies that caution should be exercised when extrapolating grazer effects beyond the time scale of an experiment.