The effect of a sharp pycnocline on plankton dynamics in a freshwater influenced Norwegian fjord

Plankton community structure and productivity was investigated in a freshwater influenced Norwegian fjord in July 1995. A thin surface layer of 6 - 7 psu was separated from underlying > 30 psu water by a sharp pycnocline. In association with the pycnocline, accumulations of plankton were observed and the pycnocline had major consequences for the structure of the plankton community. Few mesozooplankton species could cope with the range in salinity, while most protozooplankton taxa were found throughout the euphoric zone. A carbon budget was established for the surface, pycnocline/upper euphotic zone, and the deeper part of the euphotic zone. The copepod biomass was low in the surface layer and protozooplankton accounted for most of the zooplankton production and potential grazing. In and below the pycnocline, the importance of mesozooplankton increased and copepods and cladocerans together contributed with about half of the zooplankton production. The present investigation illustrates the importance of salinity as a forcing factor for the structure and dynamics of the pelagic food web.