Nutrient and plankton development in Rosfjorden and enclosed ecosystems captured from changing water bodies during POSER

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Abstract
During POSER in spring 1979, nutrient and plankton development were investigated in an open south Norwegian fjord and in different enclosures. Enclosures were plastic tubes, 1 m in diameter and up to 40 m long, fastened to a central float. The water in the fjord exchanged several times. This could be well documented also by changes in nutrient, phyto- and zooplankton concentrations. Water with different plankton and nutrient concentrations was enclosed to exclude advective processes. The experiment was divided into 3 main periods. In the 1st period the water in the fjord was haline, nutrient-rich and slightly stratified, due to a bloom of Skeletonema costatum. During this phase the phytoplankton disappeared in the fjord and in the enclosures within 6 d [days], likely due to low light intensities. In the 2nd period, following a water exchange, the water was less haline, poorer in nutrients and richer in phytoplankton (mainly Thalassiosira nordenskioldii) and in zooplankton. Part of this water was enclosed until silicate and phytoplankton concentrations became depleted and the major zooplankton species, Calanus finmarchicus, had developed to the 3rd copepodite-stage. In the 3rd period, following a new upwelling of haline and nutrient-rich water up to 10 m depth, 1 enclosure was filled with well-stratified water, nutrient-poor in the upper layer, but nutrient-rich below. In spite of sufficient nutrients the phytoplankton biomass decreased in this enclosure as well as in the fjord. This was partly caused by increasing numbers of copepodites, which reached more than 50% of the phytoplankton biomass during the last 8 d of observation.