The zooplankton of tropical streams: is it determinism or stochasticity that drives the spatial and temporal patterns in community structure?

The comprehension of the ecological structure and the functioning of natural and/or impacted lotic ecosystems is the purpose of most studies concerning rivers and streams. This study aimed to investigate the main factors influencing the zooplankton community organization in nine impacted tropical streams of three different sub-basins of the Cinzas River basin, Paraná state, Brazil. We tested the hypothesis that macro-factors such as productivity, current velocity, habitat structure, and stream order and stretches (headwater, middle and mouth), are the main factors structuring the zooplankton community in tropical streams. Zooplankton was represented by 101 taxa, mainly testate amoebae, followed by rotifers, cladocerans and copepods. Results showed that the greatest differences in physical and chemical characteristics and those related to zooplankton community structure were observed among the three sub-basins studied. However, we found that these differences were not related to the environmental heterogeneity, but were rather influenced mainly by spatial factors related to stochasticity in the structuring of planktonic communities. Thus, our results suggest that connectivity between environments within the same sub-basin, associated with random processes driven by dispersal, may determine the existence or not of spatial patterns, among stream order or stretches, and temporal patterns, between seasons, in the community attributes here analyzed.