Towards a natural Wadden Sea?

Since 1990 the input of phosphorus into Dutch coastal waters and the Wadden Sea has been strongly reduced but, primary production in these waters stayed high. Shortly after 2000, however, the years with a persistently high primary production under low P-discharge of the Rhine seem to have come to an end. In order to investigate whether nutrient shortage is the reason for the present small spring bloom magnitude and duration, bacterial bioassays were conducted in the winter and early spring of 2003 and 2004. We used weekly high tide samples from the Marsdiep Station to determine the natural abundances of bacteria and algae (diatoms and Phaeocystis globosa) and, to measure the effect of additions of C, P and N on bacterial growth-rate. In both years, only Phaeocystis showed a spring bloom at the end of March, which was accompanied by an outburst of bacterioplankton. The bioassays showed that, until the start of the Phaeocystis bloom, bacteria were carbon limited. During the exponential phase of the bloom, the bacteria were not limited by carbon nor by nutrients. In both years the Phaeocystis bloom stopped already in April. At the same time the bacterial bioassay pointed to severe P-limitation. In the Marsdiep, the PO4-concentration dropped to only 0.048 µM and we conclude that the early termination of the spring bloom of Phaeocystis was due to P-depletion. Possible reasons why it took more than 10 yr before the spring phytoplankton bloom in Dutch coastal waters showed any response to the reduction in the P-discharge of the Rhine and possible consequences for the Management of the Wadden Sea are discussed.