Flux de nutriments entre la Manche et la Mer du Nord. Situation actuelle et évolution depuis dix ans

As part of the MAST I-FluxManche initiative, a monthly (September 1990 to November 1991) sampling programme has been carried out in the Dover Strait. Samples were taken (weather permitting) at two depths at each of six standard stations between Folkestone and Cap Gris-Nez. Data from all surveys, in conjunction with estimates of water movement derived from hydrodynamic numerical model, have been used to calculate fluxes of nutrients between the Channel and the North Sea. Results demonstrate that although nutrient concentrations are lower in the central channel-waters than in the coastal zone, the dissolved nutrient flux to the North Sea is mom important for stations of the central part of the Dover Strait as water. Current concern about possible increases in nitrogen and phosphate discharges to seawater, apparently confirmed for the southern North Sea by Dutch workers, prompted a comparison of the present data set with values obtained by sampling from car-ferries during the period 1979-81. That long time scale investigation does not show more important increase than interannual variability can do (+ 0,85 mumol/l N-NO2 + NO3, + 0,1 mumol/l P-PO4, + 1,1 mumol/l Si from 1979-1980 to 1990-1991).