Effects of river nutrient load reduction on the Eutrophication of the North Sea, simulated with the ecosystem model ERSEM

The results of the ecosystem model ERSEM showed, that a reduction in the nutrient load by 50% for N and P cannot be linearly transferred to a similar reduction in primary production in comparison to the standard run for the year 1988. While the reduction scenario results in decreased winter concentrations of nitrogen and phosphorus of up to 40% the decrease in net primary production reached only up to 20% in small areas in the coastal zone. The phytoplankton groups indicated different reactions to the changed nutrient availability. Generally, there were significant changes in the strength and timing of the nutrient limitation in all phytoplankton groups in the model, but the diatom concentration did not change much. Differences did occur for the flagellates, with sporadically higher flagellate concentration in comparison to the standard run. This result is important, because the increase in algal biomass due to eutrophication was related mainly to an increase in flagellates, which are not decreasing accordingly in the reduction scenario. The reduction scenarios demonstrated that changes in the discharges of the major rivers hardly affect the central North Sea, but lead to significant regional differences in the net primary production. Greatest differences with regard to primary production were found downstream of the river Rhine and Elbe. This leads to changes in the mass flows in the coastal area with an increased importance of the microbial loop. One possible reason for the muted reaction of primary production to decreasing nutrient inputs can be seen in the temporal coincidence of maximum river inputs and the phytoplankton spring bloom. Due to the high nutrient uptake during the spring bloom, inorganic nutrients are bound in the phytoplankton and form a potential for remineralisation. With a more efficient microbial loop, the system becomes less dependent on riverine nutrient inputs in summer.