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

Phytoplankton primary production in the Marsdiep estimated regularly i n 1991 and 1992 showed values as high as measured during the peak year s in the mid-1980s. Lowering of P-inputs from the Rhine since 1981 cau sed an almost simultaneous but less marked decrease in phosphate conce ntrations in the Marsdiep, resulting in winter concentrations during 1 991 and 1992 that were ca 40% lower than those during the peak in the early 1980s. Apparently, the P-concentration reaches values limiting p rimary production only for a short period during the phytoplankton spr ing 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.