Impact of freshwater runoff on physical oceanography and plankton distribution in a Western Norwegian fjord: an experiment with a controlled discharge from a hydroelectric power plant

Investigations were carried out in a 20-km long fjord branch prior to, during, and partly after a 51-h controlled discharge from a hydroelectric power plant. The freshwater runoff (230 m³ s^-1) generated an estuarine circulation which was most prominent along the mid-axis of the fjord. High velocities were recorded both in the outgoing surface current, with a maximum of 1 m s^-1 (10 km downstream of the power plant), and in a compensatory current (registration at 10-m depth) with a maximum of 0·6 m s^-1 (3 km downstream). Velocities were low at 5-m depth. During discharge, salinity increased in the surface layer and decreased at a depth of several metres because of more extensive mixing. Phytoplankton was partly flushed out in the upper layers throughout the fjord branch, but abundance increased in deeper layers in an outer station, and the horizontal patchiness increased. The vertical centre of zooplankton biomass descended significantly during running of the plant. Biomass maxima in the ingoing compensation current indicate net zooplankton import during running of the power plant, but no change in total zooplankton biomass in the fjord branch was found during this experiment.