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 crucial in maintaining processes performed by functional groups of species. Compensatory dynamics may be produced by negative interactions among species, such that the decrease in abundance of a species releases the suppression of another species and allows it to increase. We conducted a mesocosm experiment in Trout Lake, Wisconsin, USA, to test the hypothesis that compensatory shifts in species abundances play a role in overall planktonic community response to pH perturbation. In 2000-L mesocosms over a period of six weeks, we contrasted a control treatment with two acidified treatments (press, sustained pH = 4.7; and pulse, alternating pH = 4.7 and ambient pH). In the acidified treatments, we saw changes in abundance of the major zooplankton and phytoplankton species, but we observed few cases of compensatory dynamics. Nonetheless, when present, compensatory dynamics could be strong. Analyses using autoregressive models revealed negative interactions among species that could potentially lead to compensatory dynamics. However, this potential for compensatory dynamics was not realized in cases where all species were sensitive to the pH perturbations. Therefore, compensatory dynamics that buffer community responses to perturbations may be limited in communities in which many species are sensitive to the perturbation.