Eutrophication of Dutch coastal waters

Abstract

The concentrations of phosphorus and nitrogen compounds in the river Rhine have increased by a factor of about 7 since 1932; in recent years the rate of increase for P has been higher than for N. The concentration of reactive silicate, which is low in summer, has remained essentially constant. Because in the Southern Bight of the North Sea the contribution of nutrients from the deep oceanic reservoir is minor, these increases in riverborne nutrients have had a marked influence on nutrient values and nutrient ratios of the whole area, but especially so on the narrow strip of water along the Dutch Coast, which has an average salinity of 30%ₒ. These changes have caused silicon to be the first depleted nutrient element, limiting diatom blooms. It is shown that the spring bloom of Phaeocystis poucheti is occasionally able to consume all phosphorus, leaving some nitrogen. However, on the average the phosphorus and nitrogen left over by diatoms after all reactive silicate has been depleted is not consumed by other phytoplankton species. In the strip of 30%ₒ salinity water along the coast daily primary production may be up to 3500 mg carbon/m 2 , but yearly primary production is not very high. The vegetative season lasts only about 6 months, due to the influence of turbidity, and is characterized by a succession of periods with high and low productivity. This instability in the phytoplankton regime sometimes causes very high phytoplankton crops and at other times advection and diffusion of unused nutrient loads from the rivers to other parts of the North Sea.