Ecosystem Functioning in the German Bight under Continental Nutrient Inputs by Rivers

Abstract

The functioning of the German Bight ecosystem is determined largely by nutrient fluxes in and out of the system, namely by the advection of nutrients from the central and southern North Sea, including the influence of the Rhine River; by nutrient inputs through direct continental river runoff into the German Bight (Elbe, Weser, and Ems rivers); and by atmospheric nutrient inputs originating from land. The nutrient situation in the German Bight and the entire North Sea is assessed by estimating these fluxes from available nutrient data. The advective inflowes are based also on simulated water transports. The circulation system in the North Sea is divided into a northern and a southern cell, with only little net water exchange. The nutrient inflow into the southern North Sea from the north is also small, with no effect on the continental coastal areas. For the entire North Sea, the total input of phosphorus increased by 7.7% an nitrogen by about 11.4% from 1950 to 1980. The percentage of Atlantic input of phosphorus into the entire North Sea decreased from 91% to 85%, while river input increased from 2% to 13%. In the continental coastal strip the total inputs increased by 80%. The share of river input increased to 52%, both for phosphorus (1950: 14%) and nitrogen (1950: 20%). Of the winter nutrient content of the upper 30 m of the entire North Sea 33.5% of phosphate and 16.1% of nitrate are taken up by algae until summer. About 50% of total new production is generated in the coastal areas, with 32.8% of the volume and 34.4% of the area of the North Sea. The ratio of new to regenerated production ranges from 2.8 to 12, depending on the method of derivation. In the German Bight, phosphate and nitrate concentrations increased during the last four decades. At Helgoland the five-year-medians of phosphate and nitrate increased by a factor of 1.7 and 2.5, respectively. As the nutrient inputs by river discharges are only slightly larger than advective contributions, the nutrient concentrations rose comparatively slowly. Diatoms stagnated, while flagellates increased 10-fold. Common winter values in the early 1980s resemble those during summer blooms in the early 1960’s. The German Bight ecosystem has changed drastically on all time scales under the anthropogenic nutrient inputs during the last 40 years; the plankton system is no longer in an annual quasiperiodic state.