Evaluation of measurements and calculation of primary production in the Dogger Bank area (North Sea) in summer 1988

Abstract

Primary production was measured during a survey in the Dogger Bank area, where both stratified and non-stratified conditions existed in July-August 1988. Daily primary production was calculated, based on I4C-fixation rates in an incubator with an artificial light source, depth profiles of in situ irradiance, and vertically heterogeneous phytoplankton distributions. Values ranged among sta- tions from 300 to 2200 mg C m-' d-l. Average primary production in the Dogger Bank area was estimated at 1.2 g C m-2 d-'. Variation in cloudiness affected primary production by a factor of 4.5. Calculations based on surface samples alone showed an average area1 primary production underesti- mate of 17 %. After correction for the uneven vertical biomass distribution, primary production was still underestimated by 12 % as a result of photoadaptation. Especially below 30 m, Ik was reduced from 310 to 174 pE m-2 S-', demonstrating the presence of shade-adapted phytoplankton at the thermocline. Calculated daily primary production rates based on in situ incubations and on incubator measurements deviated by only 5 %. Light quality did not have a significant effect on water column productivity. Size- dependent primary production based on post-incubation filtration onto 5 pm filters showed that, on average, 84 O/O of total primary production was in the > 5 pm fraction.