PERSISTING HIGH-LEVELS OF PRIMARY PRODUCTION AT DECLINING PHOSPHATE CONCENTRATIONS IN THE DUTCH COASTAL AREA (MARSDIEP)

Phytoplankton primary production in the Marsdiep estimated regularly in 1991 and 1992 showed values as high as measured during the peak years in the mid-1980s. Lowering of P-inputs from the Rhine since 1981 caused an almost simultaneous but less marked decrease in phosphate concentrations in the Marsdiep, resulting in winter concentrations during 1991 and 1992 that were ca 40% lower than those during the peak in the early 1980s. Apparently, the P-concentration reaches values limiting primary production only for a short period during the phytoplankton spring bloom. It is concluded that the decline of the P-load of the river Rhine does not yet reduce the productivity in the Marsdiep area.