The only oxygen supply in deep water is either the addition of new water or vertical mixing. In areas with poor water turnover the oxygen in the deep water can become completely used up. When dissolved oxygen levels sink to 2 ml/l many species leave the area, and if levels continue to drop then more and more species experience problems.
When the oxygen is used up, the organic material continues to break down, as bacteria use the sulphates as an oxygen source, producing hydrogen sulphide. Hydrogen sulphide is poisonous to all higher organisms, and high levels lead to dead zones on the sea floor.
Instead of stating the amount of hydrogen sulphide present, the term "negative oxygen" is used, corresponding to the amount of oxygen needed to oxidise the hydrogen sulphide.
Oxygen deficiency in the Baltic – nothing new
Oxygen deficiency and hydrogen sulphide occur in water masses that have been isolated from the surface for a long time. In some of the fjords on the west coast, in southern Kattegat and the Baltic, vertical water exchange can be difficult due to an almost permanent barrier between waters of different salinity. This is called the halocline.
The deep water in these areas is hardly ever exchanged and can be still for long periods of time. These long stagnation periods begin with a significant water inflow. The bottom water is replaced by saltier, (ideally) oxygen-rich water origination from the North Sea and the Skagerrak. The interface between the overlying, fresher water and the denser salt water at the top of this inhibits vertical mixing, perhaps for a long period of time. The oxygen in the inflow is quickly consumed, but the stratification takes longer to break down. The result is that the saline water quickly becomes oxygen deficient, while fresh oxygen is unable to be mixed down from above.
Following an inflow, the oxygen levels in the deep water depend not only on how much oxygen the water contained as it flowed in, but also on how much it was diluted as it travelled through the Baltic. If the inflow water is heavier, the old water is forced up and mixes with the layers above. This uses up oxygen as the hydrogen sulphide oxidises.
Periods of oxygen deficiency are not a new phenomenon in the Baltic. Evidence of a lack of oxygen has been found in sediment from as far back to the 17th century.
However, there is a difference in that the extent of oxygen-free bottom areas has significantly increased. More recently, low oxygen levels and high concentrations of hydrogen sulphide have been measured closer to the surface than previously.