Functional chlorophyll a, potential primary production and estimated primary production in situ of the phytoplankton in the Wadden Sea show a spring peak and a broad summer peak. The spring peak is most pronounced in the inlet areas. The composition of the phytoplankton during the spring peak differs from year to year: in 1973 Phaeocystis was the dominant species whereas in other years diatoms were dominant. In situ primary production estimated from potential primary production, depth of the euphotic zone and daylength, gave figures of 100 and 120 g C/m² year for the western and eastern part of the Wadden Sea, respectively. Values for the Eems estuary and Dollard were lower, 55 and 13 g C/m² year, respectively. For the northern IJsselmeer a higher value of 400 g C/m² year was found. Light will be the most important limiting factor for phytoplankton primary production in the Wadden Sea. Although functional chlorophyll a and potential primary production in the eastern Wadden Sea are 1.6 to 1.7 times as high as in the western part, production in situ is only 1.2 times as high due to the higher turbidity .The very high turbidity in the Dollard, caused by pollution, results in a very low in situ production. Grazing by Mytilus was suggested to effect low chlorophyll and production values in an area south of Terschelling where extensive mussel culture areas occur.
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