ADVECTION OF EUPHAUSIIDS IN A NORWEGIAN FJORD SYSTEM SUBJECT TO ALTERED FRESH-WATER INPUT BY HYDROELECTRIC POWER PRODUCTION

Due to hydro-electric power production, the freshwater input to the Sandsfjord system has increased during winter, while the freshwater input to Jøsenfjorden has been removed. In studies conducted during autumn and winter, both Meganyctiphanes norvegica and Thysanoessa raschii were most common in Jøsenfjorden, and T.raschii was almost wanting in the Sandsfjord system. The euphausiids generally swam to the surface layer at night, but avoided low salinity water in the uppermost metres of the Sandsfjord system. Still, in most locations ∼50% of M.norvegica was found in the upper 10 m. Their vertical distribution overlapped with freshwater-driven currents, and estuarine circulation was apparently prominent in governing the horizontal distribution within the Sandsfjord system. At the entrance to the fjord, M. norvegica was distributed deeper than within the fjord, reducing the importance of transport with estuarine circulation relative to currents driven by wind at the coast. Estimates of advective rates of M.norvegica suggest that exchange with coastal waters was more important for the standing stock than local growth within the fjords. In Jtfsenfjorden, maxima of T.raschii were always found in a zone of shallowing bottom topography (bottom depth ∼130 m), where this species possibly maintained its horizontal position by means of vertical migrations. © 1990 Oxford University Press.