Baltic Sea deep waters

Due to the restricted water exchange with the North Sea and the permanent haline stratification in the central Baltic Sea, stagnation periods occur regularly in the deep basins. Stagnation periods are characterized by the depletion of nitrate, increasing phosphate and ammonium concentrations and decreasing salinity and oxygen content in the deep water, sometimes culminating in the formation of considerable hydrogen sulphide concentrations. A renewing of the deep water can only take place by extreme inflow events into the Baltic Sea - termed major Baltic inflows - which transport salt and oxygen rich water into the deep basins.

During the first three quarters of the last century these major inflows were recorded more or less regularly. Since the mid-1970s, their frequency and intensity have decreased.The longest stagnation period ever recorded lasted from the beginning of 1983 until the end of 1992 and was terminated by the very strong single inflow event in January 1993 and subsequent smaller inflows in winter 1993/1994. For the first time since 1977 the deep water in the whole Baltic Sea was oxic whereby oxygen reached highest concentrations in the Gotland Deep area since 1930s.

As can be seen in the following graphs a new stagnation period had started in 1995 which continues until now.

The graphs show the maximal areas of oxygen deficiency (<2 cm³/dm³) and hydrogen sulphide in the near-bottom layer of the Baltic Sea in the last years. Histograms show the maximum oxygen resp. hydrogen sulphide concentrations in this layer for selected stations. Additionally, distribution patterns from special monitoring cruises (February, March/April, May, July/August, October/November) from 1996 onwards are shown.

* No H2S map available due to lacking data from the central Baltic Sea

Dr. Rainer Feistel