The hydrographic-hydrochemical state of the Baltic Sea in 1998

The winter of 1997/1998 was mild in the northern Baltic Sea and even very mild in the southern Baltic, among the warmest of this century. The summer and autumn were relatively cool in northern Europe.

The mild winter led to sea surface temperatures that were by 1 - 1.5 K above the long-term mean during winter and early spring. This resulted in relatively warm water being trapped in the summer intermediate water layer of the central Baltic Sea. The surface temperatures in the sumemr lay close to the long-term mean due to the cool summer. The deep water temperatures of the central Baltic Sea were anomalously high, 0.5 - 1.5 K above the mean for the period 1950 - 1998.

The meteorological conditions in 1998 gave rise to only several small inflows which had no significant effects in the central Baltic in 1998. As a result temperature and salinity in the deep water of the Gotland Deep decreased during the year.

The inflow of saline and oxygen-rich, but unusually warm water into the Baltic Sea in autumn 1997 temporarily interrupted the anoxic conditions in the eastern Gotland Basin between January and April 1998. From May onwards hydrogen sulphide was again measured in the Bornholm Basin and, beginning in June, in the eastern Gotland Basin. The thickness of the anoxic deep layer increased in 1998 and spread southwards. Oxygen depletion also continued in the deep water of the western Gotland Basin resulting in the lowest annual mean in oxygen concentration since the mid-eighties. This development indicates continuation of the stagnation that started in 1995 in the central Baltic deep water.

In the winter of 1997/1998, the decrease in winter concentrations of nitrate appears to have continued whereas the phosphate concentrations were comparable to 1996/1997 levels.

In 1998, the concentration of organic nutrients lay in the range measured over the last years. The time series of dissolved organic carbon and nitrogen compounds indicated a gradient with increasing concentrations from the western to the central Baltic.

The exceptional surface temperatures in summer 1997 combined with inflow events in autumn caused an unusual increase in the deep water temperatures of the central Baltic Sea in 1998. During the century, such unusual high temperatures were only measured in 1977. Data from long-term measurements at representative stations were studied to identify periods of extreme temperatures and marked temperatures trends in the central Baltic deep water between 1950 and 1998.

In the entire Gotland Basin, the highest temperatures were observed in the late 1970s/early 1980s, and in the eastern Gotland Basin additionally in 1998. Low temperatures were measured before 1935, at the end of the 1980s/beginning of the 1990s and during mid-nineties.

Significant trends in the deep water temperature regime occured during the first half of the present century with a mean increase of 0.8 to 1.3 °C, and from the mid-seventies to the early 1990s when a drastic downward trend of 1.3 - 1.6 °C was observed due to the absence of major Baltic inflows.

Dr. Wolfgang Matthäus