Seasonal cycling of phosphorus in the southern bight of the North Sea

We have investigated the seasonal cycle of nutrients and the phosphorus speciation, i.e. dissolved inorganic and organic phosphorus (DIP and DOP) and particulate inorganic and organic phosphorus (PIP and POP), for 10 stations in the Belgian coastal zone. The Belgian part of the southern North Sea is strongly influenced by the river plumes of the Rhine, Meuse and Scheldt. In winter, high nutrient concentrations are observed, whereas in April-May these have all been consumed during the spring bloom and silica or phosphorus limitation develops. The phosphate concentrations increase rapidly again in summer-fall, whereas nitrate and silicate return to their winter values much later. This shows the efficient phosphorus recycling that takes place in the water column. The DOP concentration exhibits two peaks during a seasonal cycle: one in April-May when the phosphate concentration is at its lowest and a second one in fall when the POP content decreases. This indicates two periods of increased phosphorus recycling activity. The seasonal cycle of the DOP is different from that of dissolved organic nitrogen (DON).