Effects of acidification on ecological interactions in zooplankton communities

The effects of lake acidification on biological interactions in zooplankton communities were studied using (1) a literature test of a model of stress effects and (2) field experiments.;Acidification experiments were conducted using zooplankton communities in 16 m;Detailed monitoring of zooplankton populations during enclosure allows preliminary identification of causal mechanisms of community change, but confirmation of these mechanisms requires experimentation under more controlled conditions. To separate the effects of pH, algal composition, and algal biomass on abundance, size, reproduction and condition, Daphnia pulex and Bosmina longirostris were tested in a field experiment using water and natural phytoplankton assemblages from a circumneutral (pH 6.43) and a more acid (pH 5.75) lake in south-central Ontario. pH and phytoplankton composition affected survival of both species through increased egg production, lipid reserves, body size or abundance in treatments containing algae and/or water from the circumneutral lake compared to treatments containing water and phytoplankton from the more acid lake. No significant improvement in zooplankton condition was observed in acid water treatments containing concentrated phytoplankton from the acid lake. This suggests that the poorer performance of zooplankton in acid water resulted from low algal quality, not inadequate algal biomass. The negative effect of acid conditions on Bosmina suggests that the population increase observed in most acidified lakes is not due to a positive response to low pH or ambient phytoplankton, but to altered biotic interactions possibly involving reduced competition.;Literature dealing with pH effects on zooplankton community structure was reviewed in order to assess the generality of a model proposed by Menge and Sutherland (1987), which was originally developed from studies of the effects of physical disturbance on marine intertidal zone communities. The model proposes that the relative importance of disturbance, predation and competition in food webs varies in a predictable manner along a stress gradient. Species richness analyses of twelve zooplankton data sets from the acidification literature and food web analysis of zooplankton in acidified lakes near Sudbury, Ontario, support the general predictions of the model; the relative importance of predation declines, and that of competition increases, with increasing hydrogen ion concentrations.