HERBIVORE EFFECTS ON PHYTOPLANKTON SUCCESSION IN A EUTROPHIC LAKE

Herbivory can potentially affect the speed and direction of plant succ ession by favoring the development of a community dominated by grazing -resistant species. This idea was tested experimentally by examining t he effects of the planktonic herbivore, Daphnia, on phytoplankton succ ession in a naturally eutrophic lake. Phytoplankton succession was cha racterized by two major transitions in community structure. Algal domi nance shifted from small diatoms and chlorophytes during the spring bl oom to cryptophyte flagellates during the clear-water phase. After the clear-water phase, dominance shifted to filamentous blue-greens (cyan obacteria). 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 f ood web. In addition, evidence is presented to suggest that Daphnia-in duced changes in water clarity may have fostered increases in epilimne tic 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, sprin g-bloom species to similarly edible, cryptophyte flagellates. In contr ast, Daphnia grazing retarded further succession to grazing-resistant, filamentous blue-greens. Thus, the effects of herbivory on algal succ ession were not predictable from the relative susceptibilities of thes e algal species to grazing mortality. These results underscore the imp ortance of indirect effects in the herbivore-plant interactions of pla nktonic communities. The observation that a single species of herbivor e had opposite effects during two successional transitions implies tha t caution should be exercised when extrapolating grazer effects beyond the time scale of an experiment.