|Effect of eutrophication on phytoplankton productivity and growth in the Wadden Sea|
|Colijn, F.; van Beusekom, J.E.E. (2005). Effect of eutrophication on phytoplankton productivity and growth in the Wadden Sea, in: Wilson, J.G. (Ed.) (2005). The intertidal ecosystem: the value of Ireland’s shores. pp. 58-68|
|In: Wilson, J.G. (Ed.) (2005). The intertidal ecosystem: the value of Ireland’s shores. Royal Irish Academy: Dublin. ISBN 1-904890-09-1. 206 pp., more|
As in most estuarine areas and coastal seas the growth of phytoplankton in the Wadden Sea is regulated by underwater irradiance and nutrient concentrations. The Wadden Sea shows clear signs of eutrophication. It has been documented by the high nutrient concentrations due to the large inputs of nutrients in this area by several European continental rivers (Rhine, Ems, Weser and Elbe), by an increased phytoplankton primary production and by the observations of black spots, areas where anaerobic sediment horizons reach the sediment surface. Other indications of eutrophication are the mass occurrence of green algae (Ulva and Enteromorpha spp.) and the dominance of specific phytoplankton species like the bloom forming Prymnesiophyte Phaeocystis sp. In the stratified parts of the German Bight lowered oxygen values are regularly observed. To check the validity of the presumption that both irradiance and nutrients are important for the growth of phytoplankton, methods developed by Cloern (1999) were tested for different sites in the Wadden Sea and their effects on the growth of phytoplankton tested. In this review data are presented from another study where nitrogen was tested. Based on literature data on the Wadden Sea it can be shown that a clear increase in both phytoplankton and microphytobenthos primary production has occurred from the early-70s till the mid-80s. During the same period nutrient inputs increased several fold. It is unlikely that the underwater irradiance has changed substantially, so that most changes can be directly related to the increased nutrient inputs. The efforts to reduce the phosphate input from the European continent has been successful with regard to the objectives set during the second international Ministerial conference on the North Sea, where a 50% reduction of the anthropogenic load had been agreed upon. However, the productivity in the Wadden Sea does not seem to be affected proportionally. This is probably due to two factors: the less intensely studied role of irradiance, and the role of nitrogen as the main limiting nutrient.