Changes in the seasonal cycles of inorganic nutrients in the coastal zone of the southeastern North Sea from 1960 to 1997: effects of eutrophication and sensitivity to meteoclimatic factors.

A long-term analysis of seasonal cycles of inorganic nutrients by means of a seasonal index is presented for the German Bight and the southern Wadden Sea (SE North Sea). Multivariate analysis for the German Bight data series revealed dependence of ammonium and phosphate index time series on dissolved inorganic nitrogen concentrations and riverine nutrient loads. Both indices are assumed to reflect seasonal dynamics of remineralisation processes associated with increased supply of organic matter. Temporal analysis revealed breaks in nutrient dynamics in 1970/1972 and 1979/1980. After 1970/1972, an unprecedented increase in the summer concentrations of mineralisation endproducts with correspondingly low index values were observed, which further declined after 1979/1980. Further breaks for Wadden Sea data series were identified in 1985 and 1988/1989. The indicative value of the temporal breaks with respect to eutrophication is discussed against the background of changes in meteoclimatic factors and local environmental conditions. Collated information on eutrophication effects was consistent with the observed breaks. It is suggested that the break in 1970/1972 in the German Bight was the first sign of ecosystem response to eutrophication in the SE North Sea.