The hydrographic-hydrochemical state of the Baltic Sea in 1993

The temperatures in the surface layer and in the intermediate water of the Baltic sub-regions under investigation were dominated by the mild winter 1992/93, the warm spring and the cool summer 1993.

A major Baltic inflow of moderate intensity was observed in January 1993 transporting water of higher salinity into the central Baltic basins. This event was characterized by some particularities such as the short duration of the inflow lifting the sea level of the Baltic up to 70 cm above the mean value within only three weeks and the following considerable back flow of salt from the Arkona Basin into the Belt Sea. Additionally, the oxygen containing water with the higher salinity proceeded very quickly to the central Baltic basins influencing the bottom water of the Gotland Deep as early as the beginning of April. This influx interrupted the longest stagnation period observed so far lasting there for about 16 years.

Short-term fluctuations between oxic and anoxic conditions were observed during the deep-water renewal in the Gotland Deep favoured by small vertical salinity gradients and by pulsations of the inflow through the Stolpe Channel. An intermediate layer containing hydrogen sulphide was preserved during most of the time. Only in the beginning of July 1993, oxygen was measured in the whole water column. The concentrations of the inorganic phosphorus and nitrogen compounds closely correlated with the distribution of oxygen and hydrogen sulphide.

During the first half of 1993, the oxygen conditions were the most favourable in central Baltic deep waters since the late 1970ies. Hydrogen sulphide was only present in the Fårö Deep. Under stagnant conditions in the second part of 1993, oxygen conditions strongly deteriorated in the deep water of the Bornholm and Gdańsk Basins, whereas hydrogen sulphide again spread in the bottom water of the Gotland Deep.

In 1993, the winter concentrations of phosphate and nitrate met the positive overall trends observed in the surface layer of the Baltic proper in recent decades. The long lasting nutrient impoverishment in the euphotic layer of the eastern Gotland Sea also including extremely low silicate concentrations must be emphasized as particularity in November 1993.