Coastal planktonic community unaffected by Boreal hydropower complex in Québec, Canada

Comprehensive studies of the impact of hydropower on coastal environments are rare. This study examines the impact of commissioning the hydropower plants of the Romaine complex on the freshwater discharge of the Rivière Romaine near its estuary and on the Chenal de Mingan ecosystem in the summers of 2015, 2017 and 2019. Continuous temperature, salinity and chlorophyll a data were obtained from two instrumented buoys, and nutrients as well as the phytoplankton and zooplankton communities were sampled five times a year at 11 stations. The results demonstrate the major influence of offshore waters on temperature and salinity in the study area, and the decreasing influence of the Rivière Romaine with distance from its mouth. Nutrient concentrations in the estuary did not covary with river discharge or with nutrient concentrations in the river. Importantly, impoundment of the reservoirs of the complex had no measurable effect on nutrient stoichiometry in the Chenal de Mingan. Overall, the chlorophyll a concentrations ranged from 0.1 to 7.6 µg L^-1 in the channel, the community was dominated by diatoms, and phytoplankton growth was either nitrate limited or under predation pressure. The zooplankton community has been composed of the same groups of species and has been dominated by cyclopoids and calanoids since 2015. Our study underlines the importance of including regional meteorological trends in the analysis to avoid biased conclusions on the impact of hydropower projects. The study concluded that modulation of the Rivière Romaine discharge and related changes in water quality did not lead to measurable change in plankton production in the Chenal de Mingan.