Phytoplankton activity in enclosed and free marine ecosystems in a southern Norwegian fjord during spring 1979

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Abstract
Primary production was estimated during enclosure experiments in a southern Norwegian fjord during POSER [plankton observations with simultaneous enclosures in Rosfjorden] in spring 1979. The measurements were supplemented by analyses of phytoplankton from the fjord, enclosed natural plankton communities, and nutrient enriched monocultures of the dominant diatoms Thalassiosira nordenskioldii, Chaetoceros debilis and Skeletonema costatum. Phytoplankton activity in enclosed and free marine ecosystems agreed well. Five phases were characterized by different temperature and salinity regimes in the fjord. A sudden decrease in diatom biomass in upwelled continental coastal water could not be explained on the basis of the ecosystem factors investigated. In the fjord and in enclosures with mixed plankton communities sustained at low P043- and silicate concentrations, reduced phytoplankton activity was observed. Doubling times of biomass were long compared with generation times of the dominant species growing at optimal nutrient conditions. Assimilation rate (.mu.g C .mu.g Chla-1 h-1) [Chla = Chlorophyll a] in mixed plankton communities was normally below 1; in diatom cultures it exceeded 1. During a period of low water temperatures (1.degree.-3.degree. C), assimilation efficiency divided by incident light was significantly linearly correlated with temperature.