[Risk of anthropogenic nitrogen and phosphorus entry into the North Sea ecosystem].

The situation of inorganic nutrients (nitrate, ammonia, phosphate, and silicate) in the German Bight was studied during several cruises and found to be very different in early summer of different years. Because of the many permanently changing processes which govern nutrient concentrations, an exact interpretation of the results is difficult. Impact on nutrient situation is given mainly by input from land runoff and rivers, by import from other sea regions, by atmospheric input, by horizontal and vertical mixing of imported substances, by incorporation into living organisms, as well as by the release from biological material. In spite of the most complex nutrient situation the presented data demonstrate the anthropogenic burden on the German Bight in a high degree. In early summer nearly all nitrate and phosphate would be used up by phytoplankton under natural conditions. But nutrients are supplied continuously by river input. On their way into the German Bight they are distributed within the area of the plume of riverine water coming from the rivers Elbe and Weser. Here they are taken up by phytoplankton as long as phosphate is available. This results in water bodies with a high surplus of nitrogen and little growth of phytoplankton, since phosphate is missing then. In many parts of the German Bight phosphate concentrations are below the detection limit in early summer, in spite of the high anthropogenic input. In the area of the fresh water plume the ratio of nitrogen containing nutrients to phosphate can reach a maximum of 3000:1 instead of 15:1, which would be found under natural conditions. The drastic shifting of nutrient ratios is man made and supposed to result in an increase of toxic algal blooms and in a change of the natural phytoplankton species composition of the German Bight. In consequence significant changes of the existing ecosystem are to be suspected. At areas where water bodies rich in nitrate and poor in phosphate get mixed with phosphate containing water bodies, rapid growth of algal blooms is possible. Onset and localities of such events are not predictable, as long as the meteorological and hydrographic conditions as well as the chemical situation are not known exactly. But this would require permanent and high activities of several research vessels in summer half-year. Nevertheless it would hardly be predictable which of the many different phytoplankton species would bloom. Onset, localities and intensities of consequential damages on exceptional algal blooms as accumulation of foam on beaches are not predictable, too.(ABSTRACT TRUNCATED AT 400 WORDS)