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The hydrographic-hydrochemical state of the Baltic Sea in 1995

The mild winter 1994/95 and the warm summer 1995, the latter ranging at the second place of the observations in Warnemünde since 1946, caused relatively great positive anomalies of the temperatures in both the surface layer of the entire Baltic and the intermediate water of the central Baltic Sea.

The inflow into the Baltic Sea diminished considerably in 1995. The meteorilogical conditions gave rise to neither major Baltic inflows nor greater inflow events of saline oxygen rich water across the sills. Thus, a new stagnation period began in central Baltic deep waters causing anoxic conditions in the Bornholm and Gdańsk Basins in August 1995.

The winter concentrations of inorganic nutrients are characterized by a decreasing tendency in the surface layer of the Baltic Proper in recent years. The reduction is more pronounced in the phosphate concentrations than in the nitrate concentrations. The decrease of the nutrient winter concentrations is discussed in connection with the strongly reduced fertilizer consumption in the cathcment area of the Baltic Sea since the late 1980s. In central Baltic deep waters, the distribution of the inorganic phosphorus and nitrogen compounds was dominated by the redox conditions.

Investigations on organic phosphorus compounds indicated a pool in the euphotic layer of the Baltic Sea increasing in seasons with biological productivity. Phosphate is the dominating phosphorus compound in Baltic deep waters, especially in the near-bottom layer.

The share of particulate organic carbon (POC) related to total organic carbon is about 10 % and thus relatively low. The POC concentrations show clear seasonal and regional differences in the Baltic Sea depending on the phytoplankton development and allochtonic inputs. The corresponding differences of dissolved organic carbon (DOC) are relatively low.

The begin of a new stagnation period causing the formation of hydrogen sulphide in the deep water of the Bornholm and Gdańsk Basins deteriorated the environmental conditions of Baltic cod in 1995. On the contrary, the mild winter 1994/95 generated favourable spawning conditions for herring and spratt.

Complete report in:
Meereswiss. Ber. 16 (1996)
Nehring, Dietwart; Matthäus, Wolfgang; Lass, Hans Ulrich; Nausch, Günther; Nagel, Klaus:
Hydrographisch-chemische Zustandseinschätzung der Ostsee 1995

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