High remaining productivity in the Dutch western Wadden Sea despite decreasing nutrient inputs from riverine sources

Annual primary production data for the western Dutch Wadden Sea between the mid 1960s and 1986 have more than tripled, when compared with 1950, the increase appears more than tenfold. There are indications that the increase in primary production of the phytoplankton over that period has been fuelled by the phosphate loading from Lake IJssel, a Rhine tributary. This also suggests that, on an annual basis, the primary production in the western Dutch Wadden Sea over the period 1950 to 1986 was mainly limited by phosphate. Despite a significant decrease in the phosphorus loads since the early 1980s, the primary production in recent years remained higher than expected based on the emission reduction. This unexpectedly high primary production of phytoplankton might have been caused by a series of other changes in inputs, including the influence of the influx from the English Channel. Clear conclusions, however, are at present not possible due to the complex influence of nutrients, irradiance, mixing and water circulation on the phytoplankton dynamics.