The ISIS in situ incubator was used to evaluate the dynamics of a phytoplankton bloom in the Kattegat in late October 1979. The nutrient status of the water column indicated a typical fall regenerative situation with relatively high concentrations of ammonium and phosphate and only low concentrations of nitrate. The photic zone was well mixed during the bloom due to preceding rough weather. Clear, sunny and calm conditions prevailed the day prior to and during the bloom. Primary production was measured at four depths and amounted to 90.4 mg C m-2 h-1 integrated from the surface to 12 m. The highest fixation rate occurred at 1 m depth and was 16.4 mg C m-3 h-1. The productivity index was 3.7 mg C (mg chlorophyll a)-1 h-1 at 1 m and 5.3 mg C (mg chlorophyll a)-1 h-1 at 4 m depth. The light response relationship showed light saturation at about 3.8路1023 quanta m-2 h-1. It is concluded that primary production during this experiment was controlled by a combination of excess light energy and temperature but not by nutrient limitation
