The hydrographic-hydrochemical state of the Baltic Sea in 1994

The temperatures in the western and central Baltic Sea were determined by the relatively mild winter 1993/94 causing positive anomalies of 0.5-1 K in both the surface layer and the intermediate water. The summer 1994 belonging to the warmest in the last 60 years produced extreme high temperatures in the surface layer characterized by anomalies of 4-6 K in the whole Baltic Sea.

Inflow events in December 1993 and during 1994 did not reach the intensity of major Baltic inflows. They intensived, however, the effects of the major Baltic inflow in January 1993 and improved especially the oxygen conditions in central Baltic deep waters. Whereas the increase in the salinity was only moderate, the oxygen concentrations of 3-3.8 cm³/dm³ measured in the Gotland Deep below 170 m depth in May 1994 were the highest since the 1930ies. The deep water was free of hydrogen sulphide in the whole Baltic Sea in this month. This means, that the longest stagnation period beginning in 1977 terminated in 1994. The favourable oxygen conditions continued until the end of the year except in the Gdańsk Deep.

The winter concentrations of the nutrients observed in the surface layer of the central Baltic basins in 1994 correspond with the values measured in the period 1990-1993. High amounts of nitrate were transported into the Baltic Sea by the inflow events in December 1993 and March 1994. The input of phosphate was of lower significance in this connection. Intermediate phophate and silicate maxima and nitrate minima developed during the deep water renewal in the eastern Gotland Basin and preserved until November 1994.

The nitrate accumulation was measured at the permanent "Darss Sill" over a longer period in autumn 1994 using an autonomous autoanalyzer. This process was characterized by strong fluctuations and correlated with the salinity. Additionally, the distribution patterns of dissolved and particulate organic carbon and of dissolved free amino acids were studied in the western and central Baltic Sea.

The reproduction of the fish species herring, spratt and cod was favoured by the mild winter 1993/94 and by inflow events supplying central Baltic deep waters with oxygen and salt.