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 (<0,1µM). High concentrations of phosphate (>0,5µM), nitrate (>1µM), ammonium (>2µM) and silicate (>2µM) only prevailed below the thermocline. Chlorophylla values were below 1µg l^–1 near the surface. Enhanced values (up to 4µg 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 chla 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 chla 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 chlorophylla and enhancement of the C-fixation rate in the upper layer of the water column.Publication No. 10 of the project Applied Scientific Research Netherlands Institute for Sea Research (BEWON)