Influence of reservoirs on phytoplankton dispersion and functional traits: a case study in the Upper Parana River, Brazil

A confluence zone in the Upper Parana River Floodplain was sampled to verify the influence of upstream reservoirs on the dispersion and colonisation of phytoplankton. We hypothesized that the dilutive effect and high discharge limit phytoplankton colonisation in the main stem and that the phytoplankton functional structure in floodplain lakes is influenced by upstream reservoirs. The recruitment of functional group (FG) M from Rosana Reservoir was evidenced with low establishment in the main stem and colonisation in the floodplain lake. The lower connectivity with the Parana River during limnophase reflected in water column total mixing and dominance of the FGs C and P in floodplain lake and greater stability of the water column, with dominance of the FG H (1) in a secondary channel. In the potamophase, the Parana River negatively influenced the phytoplankton biovolume in the associated environments. In this period, FG P was dominant in the secondary channel and FG X (2) in the floodplain lake. Our results emphasize that despite the contribution of inocula to the Parana River from the upstream inflow and from tributaries, the phytoplankton depletion was due to retention and seston sedimentation in the Porto Primavera Reservoir, high discharge and dilution in this confluence zone.