Baltic Sea Environment Fact Sheet 2015, Published on 29 February 2016
The Baltic Sea is a sensitive sea area. The region is characterised by its natural formation as an enclosed estuary with high freshwater input and restricted access to oceanic high saline water. The stratification and fjord-like conditions, in combination with eutrophication and other factors, form the basis for a problematic oxygen situation in the deep water.
Anoxic conditions are characterised by the total absence of oxygen. When all oxygen is consumed by microbial processes hydrogen sulphide (H2S) is formed, which is toxic for all higher marine life. Anoxic conditions lead to release of phosphate and silicate from the sediments to the water column, which, due to vertical mixing, can reach the surface layer and the photic zone. High concentrations of phosphate favour phytoplankton growth, especially cyanobacteria in the Baltic Sea during summer.
From the end of the 19th century till the 1990s
the oxygen situation in the Baltic Sea deep basins was characterized by varying
good and bad conditions. In 1999 there was a distinct regime shift and
thereafter bottom areas with completely anoxic conditions have increased and
are now constantly elevated to levels only observed occasionally before.
The inflows to the Baltic Sea that occurred in
recent years have only resulted in temporary improvement of oxygen conditions
in the southern Baltic. No inflows have been large enough to reach and improve
oxygen levels in the deep-water areas around Gotland, except for a few short lasting
The causes and consequences of the extreme oxygen conditions observed since
1999, is currently not fully understood. However, there are several possible
causes that may interact such as changing wind conditions, changes in the
frequency and nature of inflows, increased load of organic material to the deep
water, changes in stratification, vertical mixing and runoff.
Surface winter salinity (Figure 1) has remained fairly
constant in the Baltic Proper since 1990, around 7 psu from the Arkona Basin to
the Northern Baltic Proper. In the Eastern Gotland Basin there has been a
decrease in both surface and bottom water salinity since 2011. In the Gulf of
Bothnia salinity in the surface water has decreased slightly. Deep water
salinity levels in the Bornholm Basin continue to decrease from the 2004 peak.
In the south eastern Baltic Proper, Eastern Gotland Basin and Northern Baltic
Proper deep water salinity remains high, and are still higher than after the
inflows of 1992 – 1993, partly due to the inflow 2003. In both Eastern Gotland
Basin and the Northern Baltic Proper there has been a small decrease during the
last few years. During the winter 2013/14 salinity in SE Balic Proper increased
to the 2004 level. In the Western Gotland Basin there has been a continuous
increase since 1993. The difference between surface and deep water salinity, in
the main part of the Baltic Proper, is much greater than at the start of the
1990s, and this will hinder vertical mixing, which could otherwise have
oxygenated at least some of the sea floor. There have been some changes in the
strength of the stratification as well as in depth to the halocline. However it
is hard to find any significant trends in these parameters since the yearly variations
are very large.
Figure 1. Time
series of winter (November – March), surface (< 10 m; red) and deep-water
(blue) salinity in the Baltic, 1990 - 2014
The deep-water basins in the Baltic Proper suffer
severely from long-term oxygen depletion. Inflows from the North Sea are
currently the principle source of oxygen in the deep water. Between 1991 and
1993, at the end of a long stagnation period, a series of inflows finally
oxygenated the deep water, to the extent that hydrogen sulphide had almost
disappeared from the deep basins around Gotland. In the Bornholm Basin, levels
were above 2 ml/l throughout the water column. These inflows, however, also
strengthened the stratification of the deep basins, reducing vertical mixing,
and, despite the smaller 2002 – 03 inflows, hydrogen sulphide has returned, and
now affects more than 15% of the Baltic Proper including the Gulf of Riga and
the Gulf of Finland. In this area, around 30% of the bottom water has oxygen
levels below 2 ml/l. This has an impact on benthic and demersal fishes, such as
cod, which prey upon benthic animals.
In the Arkona Basin anoxia is a sparse phenomenon, while
in the Bornholm Basin it is a more seasonal feature. With the exception of the
inflow years 1992 - 93, the basin volume affected by levels below
1 ml/l has remained rather constant since the second half of the 1990s. The
offshore Gulf of Bothnia, including the Åland Sea does not suffer from low
For each of the basins, autumn (August, September and
October) oxygen profiles from 1990 – 2014 have been examined. Depths at which
the oxygen concentrations were found within certain limits (<0; 0 – 1; 1 –
2; 2 – 3; 3 – 4 ml/l) were calculated, and these values were interpreted in
terms of volume of water in each basin affected by reduced oxygen levels as a
percentage of the total basin volume. Results are presented as time series in
Figure 2. Bar charts showing autumn oxygen concentrations as a proportion of the volume of the deep basins. Low oxygen concentrations are not a problem in the Gulf of Bothnia. The effect of the large inflow in 1993, and also autumn 2002 – spring 2003 are apparent, particularly in the East Gotland Basin. The 2002 – 03 inflow only briefly benefitted the Northern Baltic Proper. No effect is apparent in the West Gotland Basin. Note: At present (mid dec. 2015) 2014 oxygen data from several stations has not been reported to ICES. An update of this figure will be available from end of January 2016 with updated reported oxygen data from 2014.
Figure 3 shows the regional distribution of the bottom
areas where oxygen concentrations are below the critical level of 2 ml/l. Since
the inflows of 1992 – 93, there has been no significant ventilation of the deep
water in the Baltic Proper. The oxygen has been consumed across an increasing
area. Hydrogen sulphide exists in a large area of the East Gotland Basin below
about 125 metres, and below 70 to 90 metres in the West Gotland Basin and
Northern Baltic Proper. The deep anoxic water even extends into the Gulf of
Finland, although the volume of water affected (Figure 2) is not great. This
deep, more saline, water does not make it over the sill and into the Gulf of
Bothnia. As a result, despite its depth, the stratification is weak and the
Åland Sea remains well oxygenated, even during autumn.
Figure 3. Estimates of the extent of hypoxic (oxygen
content less than 2 ml/l) and anoxic (oxygen content nil; often with presence
of hydrogen sulphide) in autumn 2011 – 2014. There has been a steady increase
in the area affected by hydrogen sulphide in the East and West Gotland Basins,
the Northern Baltic Proper and the outer Gulf of Finland.
For more information on oxygen depletion in the deep
waters of the Baltic Sea, see Hansson and Andersson 2014.
temperature and oxygen are physical background parameters, constraining
bio-diversity, fish recruitment and water quality in a semi-enclosed water body
such as the Baltic Sea. For example, cod eggs and larvae are dependent on water
with salinity and oxygen levels above 11 psu and 2 ml/l, respectively.
surface waters are strongly influenced by run-off of freshwater from land.
Changes in run-off alter the surface salinity while inflows through the Belt
Sea and Öresund control the salinity of the deep waters. The density difference
between the upper and lower layers inhibits mixing between surface and deep
water, thus preventing the oxygenated surface water penetrating to depths, as
well as hindering the transfer of phosphorus (which is abundant in the deep
water) to the surface waters. The strength of the stratification is indicated
by the salinity difference between the surface and deep water. Figure 1 indicates
the difference between surface and deep water winter salinity.
depletion is widely used as an indicator for the indirect effects of nutrient
enrichment. While oxygen levels above 4.5 ml/l are considered to cause no
problems for macroscopic animals, levels below this cause increasing stress to
oxygen levels are experienced at the end of summer, between August and October,
when detritus from biological activity in the surface water has sunk, and is
decomposed by bacteria. This process consumes oxygen. When oxygen
concentrations fall below 1 ml/l, bacteria start to use anaerobic processes,
producing hydrogen sulphide. Hydrogen sulphide is toxic, and its concentration
is described in terms of negative oxygen. In the south-western Baltic Proper,
Danish Straits and Kattegat oxygen depletion is a seasonal phenomenon which
occurs during autumn. The deep-water basins in the inner Baltic Proper suffer, however,
severely from long-term oxygen depletion.
levels are used as an indicator of eutrophication by both HELCOM and OSPAR. It
is listed as a core variable of the HELCOM COMBINE programme. Oxygen is transported
to the deep waters of the Baltic by the saline inflows that come through the Sound
and Belt Sea. Hydrographic measurements (temperature and salinity) make it
possible to trace these inflows, and other water movements within the Baltic.
The vertical stratification, which is governed by the temperature and salinity,
inhibits the vertical exchange of heat, salt, nutrients and oxygen, and
describes the separation between `surface´ and `deep´ waters.
has made use of HELCOM data provided by the Baltic marine institutions through
Hansson M. and L. Andersson, 2013, Oxygen Survey in
the Baltic Sea 2013 -Extent of anoxia and hypoxia, 1960 - 2013, SMHI Report No.
49, 2013. Available online at http://www.smhi.se/polopoly_fs/1.35317!Oxygen_timeseries_1960_2013_final.pdf
Hansson M. and L. Andersson, 2013, Oxygen Survey in
the Baltic Sea 2014 -Extent of anoxia and hypoxia, 1960 – 2014. (will be
published in Jan 2015)
For reference purposes, please cite this Baltic Sea environment fact sheet as follows:
[Author's name(s)], [Year]. [Baltic Sea environment fact sheet title]. HELCOM Baltic Sea Environment Fact Sheets. Online. [Date Viewed], http://www.helcom.fi/baltic-sea-trends/environment-fact-sheets/.
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