Plankton community structure and production along a freshwater-influenced Norwegian fjord system

Abstract.

Plankton community structure and production was investigated along a freshwater-influenced Norwegian fjord system, from the innermost branch, across the sill, to more open waters. The water column shifted from being highly stratified, with a brackish surface layer (6 psu) separated from >30 psu water by a strong pycnocline, to a more open system outside the front. The zooplankton community in the brackish surface layer was dominated by ciliates, heterotrophic dinoflagellates, the cladoceran Evadne nordmanni and the copepod Eurytemora affinis. In and below the pycnocline and off the sill the diversity of the zooplankton increased, and 20 copepod and 36 protozoan taxa were observed. Mesozooplankton was sampled both with a WP-2 net (mesh size 200 µm) and with 10 l Niskin bottles. The biomass from the net samples was only 35–72% compared to the estimates from the Niskin bottles. Especially copepod nauplii, Oithona spp. and Microsetella norvegica were severely underestimated. Calanus finmarchicus dominated the copepod community. However, the bottle sampling demonstrated that also Oithona spp. and M. norvegica contributed significantly to copepod biomass and production. Protozooplankton contributed with 54–87% of the total zooplankton production and of potential grazing along the fjord, stressing the importance of the smaller grazers in the food web. A simple modeling approach shows that overall exchange processes between the fjord and coastal waters and in-fjord production of copepods are equally important forces determining in-fjord copepod biomass. In the surface layer, however, only E. affinis, E. nordmanni and the protozooplankton had growth rates that could compensate for advective loss.