During the period July 1970 to July 1971 phosphorus concentrations (phosphate, dissolved organic phosphorus and particulate phosphorus) have been measured over the whole Dutch Wadden Sea including the Eems-Dollard area and the northern part of the Ijsselmeer. Phosphate values were in general higher than in the North Sea. Highest concentrations occur in summer in the inner parts as a result of accumulation and mineralization of organic material. In this way steep gradients are built up between these inner parts and the tidal inlets, with maxima of2.5 ugat P/l. In winter differences are small and the phosphate distribution is more homogeneous. Very low phosphate values were only found in spring and summer in the tidal inlet between the islands Vlieland and Terschelling where phosphorus probably becomes limiting for plankton growth. Since 1950 phosphate concentrations increased three times (0.5 to 1.7 ugat P/l average). Over the same period the river Rhine shows a similar increase. However, particulate phosphorus concentrations only doubled in the Wadden Sea (2.4 to 5.1 ugat P/100 mg carbonate-free suspended matter, on the average). It has been assumed that the difference between the two factors is due to a considerable increase in microbial activity .
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