Swedish Agency for Marine and Water Management

The major theme of this paper is the trend towards stagnation of the deepwater basins of the Baltic proper.

Samples of Baltic water were taken from established stations periodically from the beginning of the century and these measurements constitute the data for this study.

Certain peculiarities of the Baltic’s topography are shown to produce two water layers which are separated by a permanent halocline which impedes the mixing of the water layers.

Data from measurements at various Baltic stations illustrate a salinity increase in the Baltic in the present century. This salinity increase is considered to be the most significant factor contributing to the oxygen deficit of the Baltic’s deep water. It is shown that the salinity increase raises the halocline’s stability. The existence of a halocline implies that very little oxygen from the aerated surface water can penetrate through the halocline to the deep water. Oxygen in the deep water is exhausted through oxidation processes. Increased salinity is shown to make the halocline even less permeable to the mixing down of oxygen from the surface.

An analysis of phosphorus data in Baltic sea water for the present century shows a phosphate increase in the deep and surface waters. This increase is shown to have increased primary production and through this to have increased oxygen consumption in the deep water because greater amounts of decaying matter sink down into the deep water.

It is shown that once a body of water becomes stagnant the condition tends to perpetuate itself and worsen. This is because inflowing water of a lower density than the stagnant water passes over the stagnant water and because when new water does penetrate, its oxygen quickly reacts with the H2S present in the stagnant water and is consumed.The reasons for the increace in salinity and phosphorus (the causes of the stagnation development) are discussed. The salinity increase is influenced by meteorological factors and the reasons why it occurred are: the decreased runoff to the Baltic from the rivers which discharge into it, the decreased water exchange through the Danish sounds and the increased intensity of the bottom current in the Belts and Kattegat.

The reasons for the increase in phosphorus are that the increased H2S in the Baltic creates reducing conditions which cause phosphorus to be released from the bottom sediments and from inorganic particulate matter and that there is a great amount of phosphorusin the sewage water which is emptied into the Baltic.

The implication of the findings is that if there is a continuation of the trends toward increased salinity and phosphorus concentration in the Baltic, the sea’s deep water shall soon become devoid of organic life.