Stockholm university

The Baltic Sea oxygen debt worse than ever

The Baltic Sea suffers from serious oxygen deficiency. Even large inflows of oxygen-rich seawater have in recent decades only provided short-term relief from oxygen deficiency in the deep water. In the latest issue of Ambio, a new method for quantifying the total oxygen debt from Öresund to Åland is presented. By using existing data dating back to the 1960s, the researchers show that we have gone from a supply of about 10 million tons of oxygen in the deep water to a net oxygen supply close to zero, with an oxygen debt of up to 4 million tons caused by accumulated hydrogen sulfide and ammonium. In the last 20 years, the oxygen debt has also spread to near-halocline waters west of Gotland.

The extent of so-called dead bottoms in the Baltic Sea is one of the largest in the world, with an area almost twice that of Denmark. Virtually all water below the halocline, i.e. deeper than 60–80 meters, is now deficient in oxygen and hard or impossible for animals to live in.

– The main cause of the lack of oxygen is eutrophication, says Carl Rolff, researcher at the Baltic Sea Centre and lead author of the article. The supply of nutrients from agriculture and sewage increased sharply until the 1980s. Since then, powerful measures from countries around the Baltic Sea have reduced the supply of the nutrients phosphorus and nitrogen by approximately half and a third, respectively. This has, however, not yet produced visible results in the form of reduced oxygen deficiency.

A two-edged sword

Carl Rolff, miljöanalytiker vid Östersjöcentrum

The oxygenation of the deep water in the Baltic Sea also depends strongly on large winter inflows of salty, oxygenated seawater from the North Sea.

– Such inflows are of great importance, but they have become increasingly rare since the 1980s. Nowadays they come about every ten years instead of every third to fifth year. These infrequent inflows are also a two-edged sword since while they improve the oxygen situation in the short term, they can worsen it in the longer term if too infrequent.

This is because inflows affect not only the oxygen content, but also the salinity of the water. This strengthens the density difference between deep and surface water, which prevents mixing of the water and thus oxygenation of the deep water. This effect is particularly noticeable when inflows are far between.

Important with sensitive measures of change

There are several ways to calculate and describe the oxygen deficiency in the Baltic Sea. Such descriptions are important for understanding its causes, following its development and setting targets for measures.

– One of the most common ways is to show the bottom area of the Baltic Sea affected by oxygen deficiency. This is an intuitive and important picture and shows, among other things, the conditions for bottom-dwelling animals. But since oxygen-poor water now reaches all the way up to the halocline, it is a measure less sensitive to changes, Carl Rolff points out. Neither does it show how large the accumulation of hydrogen sulphide and ammonium is. Hydrogen sulphide is toxic and, with ammonium, also constitutes an oxygen debt, as they will consume parts of the oxygen supplied with a new deep-water inflow.

Another indicator used by Helcom, the Baltic Sea countries' cooperation organization, is to calculate the average oxygen deficiency below the halocline.

– This does take the oxygen debt into account, but assumes that changes are equally important regardless of depth, says Carl Rolff. But oxygen in the upper layers of the deep water does not necessarily remove a deficiency in deeper layers. In addition, the geographical distribution of the oxygen debt has been largely overlooked as has the role of ammonium. Large winter inflows first improve conditions in the southern and eastern basins, but also push poorly oxygenated water up to near-halocline depths in the northern and western basins. Such a redistribution of the oxygen debt can have important consequences.

New and more sensitive method

In the new study, the researchers have studied the water under the halocline in the Baltic Sea Proper, i.e. the area between Åland and the Danish straits, where oxygen deficiency is greatest.

– We used all relevant measurements from environmental monitoring in Sweden and Finland, from the 1960s until now. There was a total of approximately 330,000 measurement values and six thousand profiles from the surface to the bottom, says Carl Rolff.

The researchers calculated how much oxygen is needed to oxidize both hydrogen sulfide and ammonium in the water in several sub-areas, at all depths and throughout the year and put this in relation to the existing amount of oxygen in the water.

– The oxygen debt is dynamic and can vary greatly in just a few years. We could also see that the layers and basins that contain the bulk of the oxygen debt can change rapidly. This is not well described by other measures of oxygen status.
Ammonium was also found to be more important than realized.

– In the past, people have focused on the hydrogen sulfide when calculating the oxygen debt. However, our calculations show that ammonium is also of great. In general, it contributes about 30 percent to the oxygen debt and in periods of severe oxygen deficiency up to half. Ammonium also appears to persist longer than hydrogen sulfide after oxygenating inflows.

Greater debt and new areas

The researchers show that the oxygen debt since the beginning of the 1970s has generally been below 200,000 tonnes, but has gradually increased since the 1980s, to over 3.5 million tonnes in 2019.

– The large inflow in 2014 only temporarily improved the situation as most of the supplied oxygen was consumed by the accumulated debt. Since then, the oxygen debt in the Baltic Sea Proper has reached its highest level to date. It is clear that isolated strong inflows are no longer sufficient to improve the oxygen situation beyond the short term, says Carl. What one can hope for is that more frequent inflows and effects of reduced nutrient loads can improve the situation.

Jakob Walve, researher at the Department of Ecology, Environment and Plant Sciences.

The oxygen debt was previously limited mainly to the large, deep basins east of Gotland, but has since the turn of the millennium also spread to the area northwest of Gotland. It has also moved higher up in the water column and much of the oxygen debt is now found in the water just below the halocline.

– For a few years there has now been poorly oxygenated, but very nutrient rich water near the Swedish east coast, says Jakob Walve, one of the authors who is also responsible for the regional environmental monitoring in the area. A lot of the water is probably exported to the Bothnian Sea, but it also easily enters the outer parts of the Stockholm archipelago and wells up along the eastern coast. This increases the risk of deteriorating conditions for bottom-living animals, and the nutrient richness of these waters counteract the measures taken to reduce eutrophication.

– A positive aspect of the oxygen-poor water being closer to the surface is that more of it can potentially be oxygenated when there is upwelling along the coast or strong winds, Jakob points out.

New targets and indicators are needed

When setting goals for the oxygen status in the Baltic Sea Proper, it is important to have reasonable expectations.

– If we set goals that are unlikely to be reached, even important improvements can be seen as failures, Carl Rolff points out. For the near future it seems very unlikely that we will regain oxygen concentrations high enough for benthic animals to live in the deeper depressions.

Instead, the researchers suggest monitoring the development of the oxygen debt and aim for the debt to disappear over longer periods. They suggest following the annual maximum oxygen debt and the annual minimum oxygen supply as a good description of oxygen status. These values can also be easily calculated for individual basins or several together.

– The oxygen debt is a sensitive variable. This new method should be a good way to understand and monitor the oxygen state and to follow the rapid changes that happen better than before. In addition, it only needs data that have long been routinely monitored, concludes Carl Rolff.

More about the study


Jakob Walve

Research project: Analyses of syntheses of long-term monitoring data


How oxygen deficiency in the Baltic Sea proper has spread and worsened: The role of ammonium and hydrogen sulphide.
Carl Rolff, Jakob Walve, Ulf Larsson & Ragnar Elmgren. Ambio 51, 2308–2324 (2022)