Factors Influencing Community Structure along Stress Gradients: Zooplankton Responses to Acidification

Acidification effects on zooplankton community structure are examined within the context of a model proposed by Menge and Sutherland, which was originally developed from studies of the effects of physical disturbance on marine intertidal zone communities. The model proposes that with increasing environmental stress, the factors predation, competition, and physical or physiological disturbance each predominate in turn in structuring food webs. Analyses of zooplankton communities along pH gradients do not support the Menge and Sutherland model. Patterns of species richness relationships with pH follow the general predictions of Menge and Sutherland (highest diversity of intermediate stress) in data from only 3 of 10 different studies that I examined. Predation and competition links in zooplankton food webs in acidified lakes near Sudbury, Ontario, maintain a constant ratio with acidification to pH 5.0, but at higher hydrogen ion concentrations food webs become highly variable.