ANNUAL DISTRIBUTION AND ACTIVITY OF PHYTOPLANKTON IN THE SKAGERRAK KATTEGAT FRONTAL REGION

Data from 15 cruises conducted in the northern Kattegat and southern Skagerrak at different times of the year during the period 1984 to 1993 were collated and analysed in an attempt to identify the effect that the Skagerrak/Kattegat frontal system may have on the annual distribution of phytoplankton biomass and activity in this region. The location of the front is transient and, on some cruises, it was not observed. However, when present, it occurred most often within the latitudinal boundaries of 57-degrees 25' N and 58-degrees 00' N (here referred to as the 'frontal region'). Phytoplankton biomass (expressed as depth-integrated chlorophyll to 30 m) was shown to be significantly higher in the frontal region than in surrounding waters. During periods when light limitation of primary production was predicted, the biomass maximum associated with the front was found in the weakly stratified waters bordering the front. During summer, the biomass maximum associated with the front was generally recorded at the most weakly stratified stations (where nutrient-rich bottom water extends highest into the photic zone). Total annual primary production for the entire study area was estimated to be approximately 190 g C m-2 yr-1. The data set was not sufficient to identify significant differences in primary production in and outside the frontal region. During the period from about April to October, the phytoplankton distribution was characterised by the presence of a subsurface chlorophyll maximum. This maximum was usually associated with the upper layer of the nutrient-rich bottom waters (i.e. the bottom of the pycnocline) and phytoplankton in the surface waters showed signs of nutrient depletion during the same period. Seven of the cruises included in the study were carried out during May but in different years. Significant differences in the distribution of phytoplankton in the water column were shown between these years but no significant difference in primary production could be demonstrated.