PRIMARY PRODUCTION OF PHYTOPLANKTON AT A FRONTAL ZONE LOCATED AT THE NORTHERN SLOPE OF THE DOGGER-BANK (NORTH-SEA)

In July 1988 a survey was made in the Dogger Bank area of the North Sea. As a result of wind stress the area was found to be frequently well mixed. At the northerly slope a transition zone was observed between the stratified central North Sea and the well-mixed Dogger Bank area. Low nutrient concentrations were observed in surface waters; especially for nitrate (<0,1 μM). High concentrations of phosphate (>0,5 μM), nitrate (>1 μM), ammonium (>2 μM) and silicate (>2 μM) only prevailed below the thermocline. Chlorophyll a values were below 1 μg l-1 near the surface. Enhanced values (up to 4 μg l-1) were observed in the deeper layer at the transition zone and just below the thermocline at well-stratified locations. At the transition zone high specific C-fixation rates (up to 100 mg C mg-1 chl a d-1) at the surface indicated the presence of enhanced productivity. The compensation depth for primary production was found to coincide with a specific C-fixation rate of 5 mg C mg-1 chl a d-1. At greater depths, phytoplankton was only found where tidally induced vertical mixing allowed a regular exposure to higher light intensities. Storms resulted in a rapid redistribution of chlorophyll a and enhancement of the C-fixation rate in the upper layer of the water column. © 1990 Springer-Verlag.