Ecological studies on the phytoplankton of Korsfjorden, western Norway. The dynamics of a spring bloom seen in relation to hydrographical conditions and light regime

The investigation was carried out at one station in Korsfjorden, a typical deep silled fjord of western Norway. During 14 cruises from 4 February to 30 June 1977 ATP, chlorophll a , phaeopigment a , and in situ14 C-assimilation were measured in the net (>30 μm), nano and ultraplankton (<5 μm). Samples were collected from five light depths within euphotic zone. The impact of hydrographical conditions and light regime on the bloom dynamics was also studied. In the periods 4 February -7 March and 13 April-30 June, ultraplankton contributed >60% to the total primary production while net and nanoplankton dominated from 7 March to 13 April. The diatoms Skeletonema costatum, Chaetoceros compressits and C. debilis , and Rhizosolenia hebetata var. semispina made up the main part of the biomass on 21 March, 28 March and 4 April respectively. A shade adapted diatom society was located at the top of the nutricline in late June with S. costatum , Chaetoceros spp ., and Thalassiosira spp . as the dominant species. The highest assimilation number of eight for the net plankton and four for the ultraplankton were found at the depth of 32% light intensity on 28 March and 24 May respectively. Linear relationships were found between chlorophyll a and ATP for the different size fractions with regression slopes ranging from 4.3 to 5.8. The total primary production for the period of investigation was calculated to 74 g C m −2 . Light regime and water column stability were decisive factors for the outburst of the first diatom bloom in late March. Grazing on net plankton diatoms increased during late March-early April. Changes in the longshore wind-stress component were found to be essential for the understanding of the bloom dynamics.