The effects of microcrustaceans on succession and diversity of an algal microcosm community.

The effects of herbivorous microcrustaceans on algal succession and diversity were studied in replicated 200 ml freshwater microcosms. Three different experiments were conducted. Two experiments used laboratory microcosms in growth chambers. Rotenone was used to kill the microcrustaceans in one-half of the cultures. Diversity (H') and succession were monitored over a 60 day period. The third experiment used similar microcosms, but they were kept out of doors. In this experiment, microcrustaceans became extinct in some cultures because of a mechanical disturbance. In all three experiments, succession from a community dominated by green algae to one dominated by blue-green algae was significantly slower when microcrustaceans were present. Diversity was higher in grazed cultures at some times during succession, but not at all times. The dynamics of diversity during succession appear to be governed principally by the change in the relative frequency or green and blue-green algae, rather than by the dynamics of individual species. Nutrient recycling by the microcrustaceans may favor green algae, partially mitigating mortality on green algae due to grazing pressure.