From 1955 to the mid 1980s the loads of both nitrogen and phosphorus from the River Rhine to the Dutch coastal area, the Wadden Sea included, increased. Since 1985 the phosphorus loads has decreased significantly, while the nitrogen load remained about the same. Annual primary production in the western Dutch Wadden Sea has increased from c. 40 g C m super(-2) (1950) to 150 (mid 1960s) and over 500 g C m super(-2) (1986). The biomass of macrozoobenthos has more than doubled since 1970. Simultaneously, the meat yield of cultured blue mussels (Mytilus edulis), has increased since the 1960s. Previously, it was indicated that the increase in primary production of the phytoplankton over the period 1950 to 1986 was stimulated by the load of dissolved inorganic phosphate from Lake IJssel, a reservoir supplied by Rhine water. Since 1990, however, primary production has been higher than was expected from decreased phosphate loads from Lake IJssel. It is argued that this lack of response may have been caused by increased concentrations of dissolved inorganic phosphate at sea originating from increased inflow from a.o. the Strait of Dover, which compensate for the decrease in phosphate from the rivers, possibly in combination with a significant improvement of the light conditions of the water in the Wadden Sea.
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