Inorganic Nutrient Reservoirs in the Deep Basins

The balance of phosphate in the mixed surface layer is determined through uptake processes by autotrophic organisms, mineralization of organic matter, and vertical exchange. The downward transport takes place mainly in particulate form by sedimentation of organic matter. The upward transport is mainly passive and connected with hydrodynamic processes. Also upwelling processes can play an important role. 

FIGURE 12.4 Mean phosphate concentrations in the mixed winter surface layer (0-10 m) pooling five stations in the Eastern Gotland Basin from 1958 to date (1962, 1966, and 1979 no data). 

Sea (HELCOM, 2002). Since the end of the 1970s winter nutrient concentrations fluctuate on a high level (cf. Fig. 12.3) suggesting strong interannual variations in vertical mixing. 

The upward flux of nutrients across the halocline is accompanied by the transport of salt. In correspondence with that, phosphate as well as nitrate are closely correlated with salinity or density in the winter surface layer (Nehring, 1981, 1982b, 1984a). Thus, the intensity of the upward transport of deepwater seems to be responsible for periods with increasing and decreasing nutrient concentrations lasting for several years in the winter surface layer in recent decades. 

NUTRIENT CONCENTRATIONS, TRENDS AND THEIR RELATION TO EUTROPHICATION 

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