The effects of reduced nutrient loadings in dutch coastal waters

Theo Prins (co-authors L.P.M.J. Wetsteyn, J.C. Kromkamp, V. Escaravage)
National Institute for Coastal and Marine Management / RIKZ
PO Box 8039
4330 EA Middelburg
The Netherlands
Email: t.prins@rikz.rws.minvenw.nl

Anthropogenic eutrophication is considered to be the cause of elevated phytoplankton concentrations, harmful algal blooms and, possibly, increased secondary production in Dutch coastal waters. The OSPAR policy, aiming at a 50% reduction of riverine N and P loadings, has been implemented successfully for phosphate, while nitrogen levels show only a slight decrease. Future reductions in nitrogen loading to Dutch coastal waters are expected, however.

In order to be able to predict the changes in phytoplankton as a consequence of reduced nutrient loadings, several approaches have been taken. Mesocosm experiments, mimicking conditions in the Dutch coastal zone, were carried out to study the responses in phytoplankton biomass and production to various nutrient loading scenarios. In a recent analysis, the Oosterschelde estuary (SW Netherlands) was used as a case study. This system has experienced a drastic reduction in freshwater input in the 1980's, as a consequence of engineering works, and provides an example of a system with a major reduction in nutrient loadings. As this estuary is important as a shellfish culture site, it has been argued that this reduction in nutrient loading has affected the carrying capacity for aquaculture. The developments in nutrient levels, phytoplankton concentrations and production, and the significance for shellfish culture will be discussed in relation to the decreased nutrient loadings.