Herbivory in variable environments:: an experimental test of the effects of vertical mixing and Daphnia on phytoplankton community structure

Phytoplankton communities in lakes are exposed to different within-season frequencies of heterogeneity in resource supply because of wind-induced vertical mixing. Effects of such heterogeneity, in conjunction with herbivory, on phytoplankton community structure have rarely been simultaneously examined, despite the fact that each factor can have large effects on phytoplankton composition and diversity. This study uses replicated oligotrophic mesocosms to examine the effects of herbivory and different scales of temporal heterogeneity in deepwater mixing. The pattern of vertical mixing alone had minor effects on phytoplankton community diversity and composition. The herbivore Daphnia caused a shift in phytoplankton composition to less edible types, based mainly on morphological features (spiny shapes and trichomes on cell walls) rather than size structure alone. Phytoplankton richness depended jointly on mixing frequency and large Daphnia biomasses. When systems were well mixed, with high encounter rates between predator and prey populations, phytoplankton community richness was lowest. By contrast, the systems that were least often mixed had highest richness. These results are related to limited encounter rates with infrequent mixing and to the availability of refuges from predation. Responses to different scales of temporal heterogeneity in these oligotrophic phytoplankton communities depend more on Daphnia feeding than on resource pulsing.