THE HUMAN IMPACT IN THE GERMAN BIGHT - EUTROPHICATION DURING 3 DECADES (1962-1991)

The human impact in the German Bight, in the form of anthropogenic eut rophication, has been documented by a 30-year time-series measurement near the island of Helgoland. Since 1962, the Biologische Anstalt Helg oland has measured inorganic nutrients and phytoplankton abundance fro m daily samples at Helgoland Roads, a position 60 km off the main sour ce of eutrophication, the River Elbe. Since the early 'sixties, phosph ate concentrations rose for about a decade, levelling off to about twi ce the former concentrations for another decade, and then decreasing ( since 1982) as a result of phosphate-reducing measures. Nitrate concen trations, however, have only increased since 1980/81, following Elbe r iver flood events. In 1987, three times the former concentrations were reached. A decrease has been observed only since 1991. This different development of phosphorus and nitrogen eutrophication led to a shift of inorganic N/P-ratios in the German Bight. The phosphate increase wa s more pronounced in the late summer ''regeneration made'' conditions, the nitrate increase in the winter months. The eutrophication is not restricted to the inner German Bight and coastal waters of a salinity of < 33, but has also occurred in more saline waters at S > 33 psu (pr actical salinity unit), as characteristic for the outer German Bight. In this more saline water, phosphate and nitrate maximum levels occurr ed three years later, compared with the average Helgoland data, which are more representative of the inner German Bight. It is suggested tha t suspended particulate organic matter, as a long-distance carrier of nutrients, might have caused this delayed eutrophication in the outer German Bight waters. While the human impact is obvious as to nutrient concentrations, it is less obvious in phytoplankton stock enhancement. A general increase in phytoplankton biomass (about 3-4 times) was fou nd, but this was mainly due to unidentified nanoflagellates of unknown trophic state, and subject to methodological errors. The causal relat ionships of phytoplankton stocks and eutrophication are not clearly un derstood, as natural variability is large and hydrographical factors p ossibly dominate. Additional nutrient input by Elbe river floods did n ot always result in elevated phytoplankton stocks near Helgoland, whil e extended periods of vertical density stratification of the German Bi ght water caused large plankton blooms.