Temporal coherence of zooplankton abundance in a tropical reservoir

Population densities in different sites are frequently observed to fluctuate synchronously, a phenomenon termed temporal coherence. The two main causes of temporal coherence are dispersal and the effects of a regional factor (Moran effect) that influence each of the populations similarly. If synchronous patterns are observed, it is possible to infer that there is a regional process (e.g., climate) exerting a uniform influence over the entire region. Here, we evaluate patterns of temporal coherence in total densities of zooplankton groups. Data were gathered at 11 sites in the Corumba Reservoir (Central Brazil) between 1996 and 2000 (n = 27 months). These sites were distributed in the main channel, arms, and tributaries of the reservoir. Reservoir-wide correlations (as estimated by the average Spearman rank correlation between temporal trajectories of abundances) were low (-0.01, 0.06, 0.23, and 0.14 for cladocerans, copepods, rotifers, and testate amoebae, respectively). In general, high temporal coherence was detected only between geographically adjacent sites and/or between sites with similar limnological characteristics. Contrary to many recent studies, these results illustrate that, in a small geographic area (i.e., a single reservoir of approximately 65 km(2)), local processes may override the effects of regional processes or dispersal. Moreover, they demonstrate that the lack of regional trajectories (i.e., time series of population densities with asynchronous patterns of fluctuation) should be considered in interpreting results obtained in long-term studies or monitoring programs based on a single site per ecosystem.