A method for estimating primary production from chlorophyll concentrations with results showing trends in the Irish Sea and the Dutch coastal zone

Abstract

Analyses of long-term trends in physical, chemical, and biological parameters at selected locations along the residual current represent one of the objectives of the North-West European Shelf Programme (NOWESP). Within this framework, trends in chlorophyll concentrations at two of these localities, the Irish Sea and the Dutch coastal zone, were analysed. The results show an increase in the Irish Sea from the end of the 1960s onwards. Chlorophyll concentrations increased from the early 1980s at two stations in the Dutch coastal zone (Marsdiep and 6 km oVshore of Goeree), followed by a decrease during the end of the 1980s and 1990s to values comparable with those of the 1970s. In both areas, the increase in chlorophyll is almost exclusively due to higher summer values. Estimates of gross annual primary production calculated from the relation between chlorophyll and primary production measurements indicate a 50 to 100% increase in the Irish Sea over the last three decades. Estimated mean annual production between 1976 and 1992 at Goeree 6 was 375 gC m 2 . Marsdiep estimates are influenced by an increase of suspended particulate matter (SPM) during the 1980s followed by a marked reduction during the past 5 years. These changes in SPM may mask eVects of changes in nutrient input. ? 1996 International Council for the Exploration of the Sea