Indicator Assessment

Ocean oxygen content

Indicator Assessment
Prod-ID: IND-476-en
  Also known as: CLIM 054
Published 20 Dec 2016 Last modified 11 May 2021
12 min read
This is an old version, kept for reference only.

Go to latest version
This page was archived on 26 Jul 2019 with reason: Other (This indicator has been terminated. This indicator will continue here:
  • Dissolved oxygen in sea water affects the metabolism of species. Therefore, reductions in oxygen content (i.e. hypoxic or anoxic areas) can lead to changes in the distribution of species, including so called ‘dead zones’.
  • Globally, oxygen-depleted areas have expanded very rapidly in recent decades. The number of ‘dead zones’ has roughly doubled every decade since the 1960s and has increased from about 20 in the 1950s to about 400 in the 2000s.
  • Oxygen-depleted zones in the Baltic Sea have increased more than 10-fold, from 5 000 to 60 000 km2, since 1900, with most of the increase happening after 1950. The Baltic Sea now has the largest dead zone in the world. Oxygen depletion has also been observed in other European seas in recent decades.
  • The primary cause of oxygen depletion is nutrient input from agricultural fertilisers, causing eutrophication. The effects of eutrophication are exacerbated by climate change, in particular increases in sea temperature and in water-column stratification.
This indicator is discontinued. No more assessments will be produced.

Distribution of oxygen-depleted 'dead zones' in European seas

Note: Circles depict scientifically reported accounts of eutrophication-associated dead zones. The area covered by 'dead zones' is not presented, as such information is not generally available.

Data source:

Development of oxygen depletion in the Baltic Sea over time

Note: Spatial distribution of bottom hypoxia and anoxia in 1906 (left), 1955 (centre) and 2012 (right). Estimated bottom oxygen concentrations below 2 mg per litre are shown in red; black depicts the absence of oxygen. The spatial distribution represents means across all months.

Past trends

Dissolved oxygen in marine ecosystems has changed drastically in a very short period compared with other variables of the marine environment. While hypoxic zones occur naturally in some regions, increased nutrient loads from agricultural fertilisers have caused oxygen-depleted areas or even anoxic areas (so-called ‘dead zones’) to expand globally since the mid-20th century. The number of ‘dead zones’ globally has increased from about 20 in the 1950s to about 400 in the 2000s [i].

Extensive areas with oxygen depletion or even anoxic areas have been observed for decades in all European seas (Figure 1) [ii].

The largest area of human-induced hypoxia in Europe, and in fact globally, is found within the Baltic Sea (including the adjacent seas towards the North Sea). The Baltic Sea is a shallow basin with restricted inlets, meaning that the water has a high residence time, making this water body prone to hypoxia. Hypoxic areas in the Baltic Sea have increased more than 10-fold, from 5 000 to 60 000 km2, since 1900, with most of the increase happening after 1950 (Figure 2). This expansion is primarily linked to increased inputs of nutrients from the land, but increased respiration from higher temperatures during the last two decades (since the early 1990s) has contributed to worsening oxygen levels [iii].

Oxygen depletion initially affects benthic organisms (i.e. those living on or in the seabed). Benthic organisms carry out important ecosystem functions, such as bioturbation, bioirrigation and sediment nutrient cycling. Benthic organisms also play a crucial role in the marine food web, so reductions in benthic organisms can have large impacts to commercial fisheries. The loss of benthic macrofauna in the Baltic Sea (including adjacent seas) as a result of hypoxia has been estimated at 3 million tonnes or about 30 % of Baltic secondary production [iv].


Sea surface temperature is projected to continue to increase in the Baltic Sea and other European seas. As a result, oxygen-depleted areas will further expand, in particular in the Baltic Sea, unless nutrient intakes are reduced [v]. The combined effects of oxygen loss and ocean warming could force polewards movement and vertical contraction of metabolic viable habitats and species distribution ranges during this century [vi].

[i] Robert J. Diaz and Rutger Rosenberg, ‘Spreading Dead Zones and Consequences for Marine Ecosystems’,Science 321, no. 5891 (15 August 2008): 926–29, doi:10.1126/science.1156401.

[ii] Diaz and Rosenberg, ‘Spreading Dead Zones and Consequences for Marine Ecosystems’; HELCOM, ‘Eutrophication in the Baltic Sea — An Integrated Thematic Assessment of the Effects of Nutrient Enrichment and Eutrophication in the Baltic Sea Region: Executive Summary’, Baltic Sea Environment Proceedings (Helsinki: Helsinki Commission, 2009); UNEP/MAP, ‘State of the Mediterranean Marine and Coastal Environment’ (Athens: UNEP/MAP — Barcelona Convention, 2013); J. Friedrich et al., ‘Investigating Hypoxia in Aquatic Environments: Diverse Approaches to Addressing a Complex Phenomenon’,Biogeosciences 11, no. 4 (27 February 2014): 1215–59, doi:10.5194/bg-11-1215-2014; Tamara Djakovac et al., ‘Mechanisms of Hypoxia Frequency Changes in the Northern Adriatic Sea during the Period 1972–2012’,Journal of Marine Systems 141 (January 2015): 179–89, doi:10.1016/j.jmarsys.2014.08.001; H.D. Topcu and U.H. Brockmann, ‘Seasonal Oxygen Depletion in the North Sea, a Review’,Marine Pollution Bulletin, July 2015, doi:10.1016/j.marpolbul.2015.06.021.

[iii] Jacob Carstensen et al., ‘Deoxygenation of the Baltic Sea during the Last Century’,Proceedings of the National Academy of Sciences 111, no. 15 (15 April 2014): 5628–33, doi:10.1073/pnas.1323156111; M. Pyhälä et al., ‘Oxygen Debt — HELCOM Core Indicator’, 2014,

[iv] K Karlson, R Rosenberg, and E Bonsdorff, ‘Temporal and Spatial Large-Scale Effects of Eutrophication and Oxygen Deficiency on Benthic Fauna in Scandinavia and Baltic Waters — A Review’,Oceanography and Marine Biology: An Annual Review 40 (2002): 427–89; Diaz and Rosenberg, ‘Spreading Dead Zones and Consequences for Marine Ecosystems’.

[v] Angela M. Caballero-Alfonso, Jacob Carstensen, and Daniel J. Conley, ‘Biogeochemical and Environmental Drivers of Coastal Hypoxia’,Journal of Marine Systems, Biogeochemistry-ecosystem interaction on changing continental margins in the Anthropocene, 141 (January 2015): 190–99, doi:10.1016/j.jmarsys.2014.04.008.

[vi] C. Deutsch et al., ‘Climate Change Tightens a Metabolic Constraint on Marine Habitats’,Science 348, no. 6239 (5 June 2015): 1132–35, doi:10.1126/science.aaa1605.

Supporting information

Indicator definition

  • Distribution of oxygen-depleted ‘dead zones’ in European seas
  • Development of oxygen depletion in the Baltic Sea over time


  • Location (dimensionless)
  • Estimated oxygen bottom concentrations (mg/l)


Policy context and targets

Context description

In April 2013, the European Commission (EC) presented the EU Adaptation Strategy Package. This package consists of the EU Strategy on adaptation to climate change (COM/2013/216 final) and a number of supporting documents. The overall aim of the EU Adaptation Strategy is to contribute to a more climate-resilient Europe.

One of the objectives of the EU Adaptation Strategy is Better informed decision-making, which will be achieved by bridging the knowledge gap and further developing the European climate adaptation platform (Climate-ADAPT) as the ‘one-stop shop’ for adaptation information in Europe. Climate-ADAPT has been developed jointly by the EC and the EEA to share knowledge on (1) observed and projected climate change and its impacts on environmental and social systems and on human health, (2) relevant research, (3) EU, transnational, national and subnational adaptation strategies and plans, and (4) adaptation case studies.

Further objectives include Promoting adaptation in key vulnerablesectors through climate-proofing EU sector policies and Promoting action by Member States. Most EU Member States have already adopted national adaptation strategies and many have also prepared action plans on climate change adaptation. The EC also supports adaptation in cities through the Covenant of Mayors for Climate and Energy initiative.

In September 2016, the EC presented an indicative roadmap for the evaluation of the EU Adaptation Strategy by 2018.

In November 2013, the European Parliament and the European Council adopted the 7th EU Environment Action Programme (7th EAP) to 2020, ‘Living well, within the limits of our planet’. The 7th EAP is intended to help guide EU action on environment and climate change up to and beyond 2020. It highlights that ‘Action to mitigate and adapt to climate change will increase the resilience of the Union’s economy and society, while stimulating innovation and protecting the Union’s natural resources.’ Consequently, several priority objectives of the 7th EAP refer to climate change adaptation.


No targets have been specified.

Related policy documents

  • 7th Environment Action Programme
    DECISION No 1386/2013/EU OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 20 November 2013 on a General Union Environment Action Programme to 2020 ‘Living well, within the limits of our planet’. In November 2013, the European Parliament and the European Council adopted the 7 th EU Environment Action Programme to 2020 ‘Living well, within the limits of our planet’. This programme is intended to help guide EU action on the environment and climate change up to and beyond 2020 based on the following vision: ‘In 2050, we live well, within the planet’s ecological limits. Our prosperity and healthy environment stem from an innovative, circular economy where nothing is wasted and where natural resources are managed sustainably, and biodiversity is protected, valued and restored in ways that enhance our society’s resilience. Our low-carbon growth has long been decoupled from resource use, setting the pace for a safe and sustainable global society.’
  • Climate-ADAPT: Adaptation in EU policy sectors
    Overview of EU sector policies in which mainstreaming of adaptation to climate change is ongoing or explored
  • Climate-ADAPT: Country profiles
    Overview of activities of EEA member countries in preparing, developing and implementing adaptation strategies
  • DG CLIMA: Adaptation to climate change
    Adaptation means anticipating the adverse effects of climate change and taking appropriate action to prevent or minimise the damage they can cause, or taking advantage of opportunities that may arise. It has been shown that well planned, early adaptation action saves money and lives in the future. This web portal provides information on all adaptation activities of the European Commission.
  • EU Adaptation Strategy Package
    In April 2013, the European Commission adopted an EU strategy on adaptation to climate change, which has been welcomed by the EU Member States. The strategy aims to make Europe more climate-resilient. By taking a coherent approach and providing for improved coordination, it enhances the preparedness and capacity of all governance levels to respond to the impacts of climate change.


Methodology for indicator calculation

Scientifically reported accounts of eutrophication-associated dead zones have been used to produce an overview of the distribution of oxygen-depleted ‘dead zones’ in European seas.

Simulations using observed dissolved oxygen concentrations as input have been carried out to estimate oxygen bottom concentrations and ‘dead zone’ areas.

Methodology for gap filling

Not applicable

Methodology references



Methodology uncertainty

See under "Methodology".

Data sets uncertainty

In general, changes related to the physical and chemical marine environment are better documented than biological changes. Observations of dissolved oxygen concentrations began around 1900. Since the 1960s, more regularly spaced measurements are undertaken and a more consistent data base for the assessment of the spatial extent of ‘dead zones’ is available.

Rationale uncertainty

No uncertainty has been specified

Data sources

Other info

DPSIR: State
Typology: Descriptive indicator (Type A - What is happening to the environment and to humans?)
Indicator codes
  • CLIM 054
Frequency of updates
This indicator is discontinued. No more assessments will be produced.
EEA Contact Info


Geographic coverage

Temporal coverage