Compensatory dynamics in planktonic community responses to pH perturbations

Compensatory population dynamics, in which species that decline in response to an environmental perturbation are replaced by similar species, may be c rucial in maintaining processes performed by functional groups of species. Compensatory dynamics may be produced by negative interactions among specie s, such that the decrease in abundance of a species releases the suppressio n of another species and allows it to increase. We conducted a mesocosm exp eriment in Trout Lake, Wisconsin, USA, to test the hypothesis that compensa tory shifts in species abundances play a role in overall planktonic communi ty response to pH perturbation. In 2000-L mesocosms over a period of six we eks, we contrasted a control treatment with two acidified treatments (press , sustained pH = 4.7; and pulse, alternating pH = 4.7 and ambient pH). In t he acidified treatments, we saw changes in abundance of the major zooplankt on and phytoplankton species, but we observed few cases of compensatory dyn amics. Nonetheless, when present, compensatory dynamics could be strong. An alyses using autoregressive models revealed negative interactions among spe cies that could potentially lead to compensatory dynamics. However, this po tential for compensatory dynamics was not realized in cases where all speci es were sensitive to the pH perturbations. Therefore, compensatory dynamics that buffer community responses to perturbations may be limited in communi ties in which many species are sensitive to the perturbation.