Impact of a controlled freshwater discharge on zooplankton distribution in a Norwegian fjord

Investigations were carried out in a 20-km long fjord branch before and during a controlled freshwater discharge (230 m³·s^-1) from a hydroelectric power plant located at the head of the fjord. Advection notably structured horizontal and/or vertical distributions of zooplankton during discharge. Brackish water species were mainly confined to the upper 2 m throughout the experimental period, and were swept out of the fjord with a surface current generated at onset of discharge. Marine species avoided the upper brackish meter; they were otherwise mainly concentrated in the upper layer, but mean depths descended slightly during discharge. Both net outward flushing with the surface current and net transport in with the compensation current were recorded among marine species. Acartia clausi behaviourally modified the effect of the advective process by performing diel vertical migrations between the outgoing surface current and incoming compensation current, and abundance in the fjord branch was not affected by the discharge.