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

Phytoplankton communities in lakes are exposed to different within-season f requencies of heterogeneity in resource supply because of wind-induced vert ical mixing. Effects of such heterogeneity, in conjunction with herbivory, on phytoplankton community structure have rarely been simultaneously examin ed, despite the fact that each factor can have large effects on phytoplankt on composition and diversity. This study uses replicated oligotrophic mesoc osms to examine the effects of herbivory and different scales of temporal h eterogeneity in deepwater mixing. The pattern of vertical mixing alone had minor effects on phytoplankton community diversity and composition. The her bivore Daphnia caused a shift in phytoplankton composition to less edible t ypes, based mainly on morphological features (spiny shapes and trichomes on cell walls) rather than size structure alone. Phytoplankton richness depen ded jointly on mixing frequency and large Daphnia biomasses. When systems w ere well mixed, with high encounter rates between predator and prey populat ions, phytoplankton community richness was lowest. By contrast, the systems that were least often mixed had highest richness. These results are relate d to limited encounter rates with infrequent mixing and to the availability of refuges from predation. Responses to different scales of temporal heter ogeneity in these oligotrophic phytoplankton communities depend more on Dap hnia feeding than on resource pulsing.