It goes with the flow: Size mediated plankton attenuation through the Niagara River connecting channel

The Niagara River connecting channel provides a unique system in which to examine the impact of a rapid lentic-lotic-lentic transition on a freshwater ecosystem, particularly on plankton community composition. From June to October 2014, six monitoring sites (three in the upper river and three in the lower river) were sampled monthly for water chemistry, phytoplankton and/or zooplankton composition. Zooplankton biomass decreased exponentially with distance downstream, as expected within a high flow lotic environment. The rapid attenuation of zooplankton occurring within 10 km of the river mouth was likely due to predation by large populations of planktivorous fishes in the river. Densities of phytoplankton, rotifers and bacteria also declined but were less strongly impacted by the lentic-lotic transition. Plankton communities were dominated by species common to Lake Erie, the source waterbody, and also by taxa more resilient to the highly turbulent conditions of the river. These species included siliceous diatoms, filamentous algae, small zooplankters, dreissenid veligers and loricated rotifers. Furthermore, the seasonality of species composition in the Niagara River is the same as its source water body, suggesting no impact from the river on plankton succession. Niagara Falls and the two hydroelectric facilities were not found to be a significant cause of plankton mortality down river. There is some evidence however that entrainment time in the hydroelectric reservoirs may slightly enhance phytoplankton and zooplankton densities, thus contributing to plankton populations downstream of the turbine discharges.