Role of intra-community biotic interactions in structuring riverine zooplankton under low-flow, summer conditions

While the importance of biotic interactions in shaping zooplankton communities has long been established in lentic environments, river plankton is traditionally thought to be primarily abiotically constrained. The role of interactions among zooplankters has thus only occasionally been examined and mainly regarded as secondary or negligible in rivers. This study addresses the question whether and under which conditions biotic interactions within the zooplankton can become the chief drivers of community structure and dynamics in riverine environments. A short-term, high-frequency sampling campaign was performed in the potamal reach of the Po River (Italy). To test for the presence of association patterns among zooplankters, which might be suggestive of potential trophic interactions (predation/competition), taxa were aggregated into functional groups according to their feeding ecology, and time series of their abundances were analysed by means of a variance ratio test, which revealed the occurrence of compensatory dynamics among functional groups under low and stable, although truly advective, discharge conditions. Evidence on the importance of predator-prey interactions and intra-population regulation mechanisms also came from further analyses of the dominant predators gut contents and of the main prey population dynamics. Our results show that, under certain conditions, zooplankton exhibit internal, self-regulatory mechanisms also in the main current of large rivers. The view of riverine zooplankton as a mere assemblage of taxa exclusively abiotically controlled is therefore oversimplified.