MEPS 156:289-297 (1997)  -  doi:10.3354/meps156289

As part of the German research project PRISMA, a drift experiment conducted in the German Bight in April 1991 showed that the diatom spring bloom was ending and a Phaeocystis bloom was beginning. Due to formation of Phaeocystis colonies, zooplankton biomass declined. Phosphate, nitrate and silicate were depleted and phosphate was below limiting values. The data are interpreted using a vertically resolved phosphorus-based primary production model. The model adequately explains the phosphate development and the amplitude and timing of the algal standing stock. It is inferred that primary production in this area is high, about 1.3 g C m^-2 d^-1, due to excellent production conditions at the end of April. The contributions of various processes are represented by the pathways of phosphorus in the upper 30 m of the water column. Phosphate uptake amounted to 4.8 mmol m^-2 for the 6 d of the experiments. Sinking of phytoplankton plus diffusion and sinking of detritus to the bottom were the sources for bottom detritus (0.6 mmol m^-2). The bottom detritus was mainly supplied by phytoplankton mortality (0.3 mmol m^-2) due to ciliate grazing. Inorganic phosphate was supplied via benthic remineralization and upward diffusion (0.9 mmol m^-2). The overall trend of phosphate and phytoplankton indicates that nearly the same water body was sampled over the course of the drift experiment. Due to the 6-hourly observations, interruptions due to advection in most of the measured parameters are visible. For the phytoplankton concentrations, the short-term and overall differences are shown by a comparison between the central drift station and 3 additional stations placed around the central station in a triangle and located 3 nautical miles away. The drift buoy moved in and out of the bloom area. The water column model neglects horizontal advection, so it would be better employed for mesocosm experiments. In the future fully 3-dimensional models should be implemented.

North Sea · German Bight · Drift experiment · Plankton dynamics · Primary production model · Phosphorus budget