Annual phytoplankton primary production in the Marsdiep tidal inlet increased from ca 150 gC·m-2 in the period 1964 to 1976, to ca 300 gC·m-2 in 1981/1982 and 1985. This increase is considerable, but comparable to that observed in the outer Ems estuary, eastern Wadden Sea, from 240 gC·m-2·a-1 in 1972/1973 to 400-500 gC·m-2·a-1 in 1976/1980. Although the increase is most probably due to eutrophication, as illustrated by the regular increase in phosphate in the Marsdiep area since 1950, it is difficult to explain why this affected primary production not earlier than the late seventies. Primary production has probably not increased in the more turbid inner parts of the Wadden Sea, where light is the limiting factor, and P values were already higher than in the inlet areas.
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