EFFECT OF FRESH-WATER DISCHARGE, INTRUSIONS OF COASTAL WATER, AND BATHYMETRY ON ZOOPLANKTON DISTRIBUTION IN A NORWEGIAN FJORD SYSTEM

Freshwater discharge from a hydroelectric power plant had a strong impact on plankton transport and distribution within the fjord system. Most zooplankters submerged beneath the outgoing brackish current, and subsequent advection by the compensation current caused high concentrations near the power plant outlet. Currents driven by wind-induced changes in the density field in the adjacent coastal waters dominated transport between the fjord system and outer regions. Normal plankton advection rates appeared to be on the same order of magnitude as local zooplankton growth within the fairly enclosed system. However, during strong, short-term intrusions of coastal water, plankton composition and distribution changed markedly in the course of a few days. The bathymetry seemed to be decisive for the distribution of Acartia, which was recurrently found in highest numbers at the shallowest sampling location.