Is phytoplankton growth in the Wadden Sea light or nitrogen limited?

The growth of phytoplankton in the Wadden Sea, a shallow tidal sea bordering the North Sea, is strongly regulated by nutrients and it-radiance. The similarity and unity of this sea is documented on the basis of the annual phytoplankton biomass cycles at different sites. Due to the large inputs of nutrients in this area by several rivers (Rhine, Ems, Weser, Elbe) the Wadden Sea shows signs of eutrophication. The strong preference for eutrophication-related studies in this area can only be explained by the impact of policy-makers through funding of eutrophication-related studies and their will to manage the inputs of nutrients to the Wadden Sea. This strong preference has caused the relatively little interest for studies on the regulatory mechanisms of underwater irradiance on phytoplankton growth in this turbid area. Procedures developed by Cloem [Aquat. Ecol. 33 (1999)] were used to compare nutrient and light limitation at different sites of the Wadden Sea. The analysis showed that in many cases both spatial and temporal light limitation far exceed effects of nutrient limitation. During the 1990s the dominant influence of high DIN concentrations implies that underwater irradiance by far exceeds effects of nutrients on the production of phytoplankton biomass. Results of this analysis also show how narrowly focussed research may develop when policy-makers direct research priorities. (C) 2003 Elsevier Science B.V. All rights reserved.