Plankton distribution and activity in the North Sea/Skagerrak-Kattegat frontal area in April 1984

47 fixed stations off the Northwest Danish coast were s a m ~ l e d svstematicallv and repeatedly between 5 and 14 April 1984, for distribution of temperature, salinity, and chlorophyll a fluorescence throuahout the water column. Low chloro~hvll concentrations were recorded at stations in . . North Sea water. In the Skagerrak/Kattegat stations sampled, dense phytoplankton concentrations (chl a up to >20 mg m-3) were observed in association with a strong halocline (surface to bottom AS up to >l5 %o). This halocline weakened somewhat in the region northhortheast of Skagen and at 'upwelling' stations near the Swedish coast. Surface patches of elevated chlorophyll concentrations were frequently associated with these weakenings of the halocline. Determination of combined nitrite/ nitrate revealed elevated values in surface waters in these less stratified regions. A strong front occurred in the approaches toward North Sea water to the west of Skagen. Over the frontal region, the water column salinity difference decreased from > l 2 to <0.1 %O over the course of 39 km. The distribution of chlorophyll in relation to the hydrographic features of this front was determined 4 times during 1 wk. Chlorophyll distribution patterns were apparently related to hydrographic features and were observed to be extremely dynamic. Primary production and light measurements were taken on 1 frontal crossing. These indicate that daily production rates may have been on the order of 25 X higher in this frontal region than in surrounding waters at the time of sampling. Copepod nauplii and egg distributions were ca I O X higher in Skagerrak/Kattegat waters than in the North Sea and did not show a clear association with the front. The highest nauplii/egg concentrations were observed in association with dense chlorophyll patches located at the pycnocline. Egg production rates of the copepod Calanus finmarchicus were shown to be positively correlated with chlorophyll concentration at the depth of collection.