Oxygen concentrations in Europe's coastal and marine waters
Reduced oxygen levels in near-seafloor waters are becoming increasingly common, driven by natural factors and human-induced pressures, such as excess nutrient inputs and climate change. Over 16% of assessed areas show concentrations below 6mg/l, the threshold generally considered necessary to support marine life with minimal stress. The Baltic and Black seas are the most affected regions, with a third of assessed areas below this level. Oxygen depletion can severely impact marine ecosystem functioning, leading to significant environmental and socio-economic consequences.
Figure 1. Occurrence of reduced oxygen concentrations in Europe's coastal and marine waters (average for the years 2013-2024)
Please select a resource that has a preview image available.
Excessive nutrient inputs from agriculture runoff, wastewater and industrial discharges can trigger harmful algal blooms. When these blooms die and decompose, they consume large amounts of oxygen, reducing its availability in seawater. Ocean warming further exacerbates this by reducing oxygen solubility, increasing the metabolic oxygen demand of marine organisms and strengthening water-column stratification, which limits oxygen exchange with deeper layers.
Ocean deoxygenation, together with warming and ocean acidification, is recognised as one of the three major global marine stressors, collectively referred to as the ‘deadly trio'. These stressors can severely affect marine biodiversity, disrupt food-webs and alter the abundance and distribution of commercially important fish stocks.
Reduced oxygen concentrations indicate the indirect effects of nutrient enrichment and, consequently, eutrophication. Monitoring oxygen levels is essential for assessing progress towards improved water quality in line with EU policy objectives. The Water Framework and the Marine Strategy Framework directives aim to achieve ‘good ecological status’ and ‘good environmental status’ of Europe’s waters, respectively. The European Green Deal supports these goals through targets to reduce nutrient use and losses to the environment, as outlined in the EU Biodiversity Strategy for 2030, the Farm-to-Fork Strategy and the Zero Pollution Action Plan.
Oxygen concentrations measured between July and October were used for this assessment, as this period is most prone to low oxygen levels due to higher water temperatures. Results show that large parts of the Baltic and Black seas experience severeoxygen depletion. During 2013-2024, almost 14% of the assessed area in the Baltic Sea and 23% in the Black Sea recorded near critical conditions, with concentrations below 4mg/l (Figure 1). These low-oxygen areas typically occur in deeper, denser water masses where oxygen inflow or downward mixing is limited.
In the Mediterranean Sea, reduced oxygen levels are generally more localised, occurring mainly in coastal zones. Around 19% of the assessed area showed concentrations below 6mg/l. In contrast, the North-East Atlantic experiences only localised and short-term oxygen deficiencies, with 93% of the area assessed maintaining concentrations above the 6mg/l threshold.
Figure 2. Trends in oxygen (O₂) concentrations in the near-bottom layer by concentration group (2000-2024)
Figure 2 shows trends for monitoring stations in the Baltic Sea and North-East Atlantic, grouped into three classes based on average oxygen concentrations: (1) below 4mg/l (includes <2mg/l class); (2) between 4 and 6mg/l; and (3) above 6mg/l. Trends were not possible for other regional seas because available monitoring data were insufficient for robust analysis.
Between 2000 and 2024, 90% of the 1,611 monitored stations showed no significant change in oxygen concentrations, while 9% showed declining trends, indicating worsening conditions.
Among stations with severe oxygen depletion (below 4mg/l) in the Baltic Sea and some Danish fjords, conditions worsened at 7% of sites, while 3% improved. In the North-East Atlantic, stations with concentrations below 6mg/l showed no significant changes, except for one improving location. Among stations with concentrations above 6mg/l, 7.2% of Baltic Sea sites deteriorated and 1.3% improved, while in the North-East Atlantic 14.7% deteriorated and less than 1% improved.
These findings highlight the need for concerted action to reduce nutrient pollution alongside efforts to mitigate ocean warming. Strengthening monitoring across all regional seas is crucial to assess measures and understand the broader impacts of climate change.
Supporting information
This indicator displays the geographical distribution and trends in summer-autumn oxygen concentrations, measured in milligrams per litre (mg/l), in near-bottom waters of Europe’s regional seas.
Threshold values (TVs) for dissolved oxygen concentrations in coastal waters are set under the Water Framework Directive (WFD). The Marine Strategy Framework Directive (MSFD) aligns its TVs for coastal waters with those set under the WFD and extends these beyond coastal waters to ensure consistency. Member States establish TVs through (sub)regional cooperation.
Data on oxygen concentrations during the summer-autumn months (July-October) are used as this period has the highest probability of oxygen depletion due to higher water temperatures.
For each monitoring site, the mean of the 5-percentile of observations for the years 2000-2024 was used. Results are aggregated at the level of 100x100km grid cells. For each marine region, the 5-percentile of oxygen concentrations in a grid cell is used to classify the grid cells by the oxygen concentration classes: O2 < 2mg/l, 2 ≤ O2 < 4mg/l, 4 ≥ O2 ≤ 6mg/l, and > 6mg/l.
The main sources of data include:
the International Council for the Exploration of the Sea (ICES);
the European Marine Observation and Data Network (EMODnet) data sets; and
Data maintained by ICES are collected through the Eionet Central Data Repository (Eionet CDR) from the marine conventions and represent a sub-set of national data compiled to provide comparable indicators of the condition and impacts on transitional, coastal and marine waters (TCM data) across Europe. When data from both ICES and EMODnet are available for the same station (defined by position and time), ICES data are used.
The procedures of data extraction, data selection and aggregation, trend analysis and the plotting of results are carried out using the R programming language.
Stations are geographically defined by their longitude and latitude in decimal degrees. All geographical positions in the data are assigned to marine (sub)regions based on their coordinates.
The primary aggregation process involves:
identifying (clusters of) stations and assigning them to marine (sub)regions;
creating statistical estimates for each combination of station and year.
Trend analysis is conducted for each station in regions for which there were at least data for five or more years in the period 2000-2024. Trends are detected using the non-parametric Mann-Kendall trend test. The tests reported here are two-sided (testing for both positive and negative trends). Data series with p-values of <0.05 are reported as significantly positive or negative. The test analyses only the direction and significance of the change, not the magnitude of the change. It assesses whether there is a monotonic upward or downward trend over time without assuming any specific distribution for the data. This makes it useful for environmental data analysis, such as detecting trends in oxygen concentrations.
Analysing oxygen concentrations and their changes over time is key to assessing progress towards improved marine and coastal water quality in line with EU policy objectives, such as those under the Marine Strategy Framework Directive (MSFD) , and the Water Framework Directive (WFD). The WFD mandates the achievement of good ecological status or the good ecological potential of transitional and coastal waters across the EU, and specifically identifies dissolved oxygen concentrations as one of the physio-chemical parameters for assessing ecological status. The MSFD requires the achievement or maintenance of good environmental status in EU sea basins and designates dissolved oxygen concentration in the bottom of the water column as one of the primary criteria (D5C5) for Descriptor 5 human-induced eutrophication.
EU policies and legislation also support the implementation of the Regional Seas Conventions and Action Plans (RSCAPs) — the Oslo Paris Convention (OSPAR), the Helsinki Convention (HELCOM), the Barcelona Convention (UNEP-MAP) and the Bucharest Convention, which also outline measures that aim to reduce the loads and impacts of nutrients.
Excessive nutrient flow into the sea, primarily from agricultural fertilizers, can trigger large phytoplankton blooms, increasing primary production—a process known as eutrophication. When these organisms die and sink to the seafloor, oxygen is consumed during decomposition. If the water column cannot mix adequately to replenish the supply of oxygen at the seafloor, this can lead to significantly reduced oxygen levels that limit biological activity (hypoxia) and may even result in complete oxygen depletion (anoxia).
Oxygen-depleted areas demonstrate how one type of anthropogenic pressure (eutrophication) is intensified by climate change effects, such as rising water temperature. Increased water temperatures affect various biological and chemical processes in the marine environment. For example, warmer water decreases oxygen solubility, reducing oxygen concentrations, while simultaneously increasing organisms' metabolic demand for oxygen. Most marine organisms rely on oxygen for metabolism, so lower oxygen levels can adversely affect their physiology, species composition and abundance. Insufficient oxygen supply can lead to broader ecological and economic impacts, affecting productivity, species interactions and community composition at the ecosystem level.
Recent discussions in OSPAR have suggested that the 25th percentile, used previously in this assessment, is not precautionary enough and the 5th percentile should be used, and is now used in this update. In HELCOM, minimum concentrations are assessed.
Additionally, OSPAR and HELCOM use a sample depth of 10m above the seafloor. However, due to the uncertainty in sample depth data, 20m was used in this assessment. These uncertainties will be further explored in future updates of the indicator.
Geographical comparability
Data for this assessment are still limited considering the large spatial and temporal variations inherent in transitional, coastal and marine waters surrounding Europe. This lack of data means that long stretches of coastal and marine waters remain uncovered by the analysis. Most of the available time series data are concentrated in the Greater North and Baltic seas, particularly in the Kattegat, and the Dutch, German and Danish parts of the North Sea, as well as the central and western parts of the Baltic Sea. In the other regions, longer time series data are limited.
For the analysis, only data for the lower 20m or 50m of the water column are considered. However, not all available data have reliable attributes on sampling depth and bathymetry. In shallower waters, selecting data from the lower 20m of the water column may not be optimal as vertical mixing tends to reduce oxygen depletion.
Stations are defined geographically based on longitude and latitude in decimal degrees; however, the datasets do not always contain reliable and consistent station identifiers. Coordinates for supposed identical stations may vary between visits due to actual positioning versus target positioning, which can fragment time series. To improve data aggregation into time series, data are grouped into squares of approximately 1.375km for stations within 20km of the coastline and 5.5km for open-water stations beyond 20km. Although this method reduces errors in data aggregation and fragmentation of time series due to minor positional shifts, assessing broader assessment areas might be more effective than the current station-based approach.
Comparability over time
The Mann-Kendall test is a robust and accepted approach, however, due to the multiple trend analyses, with a probability level of p <0.05, approximately 5% of the tests conducted will appear significant (identify a trend) even if in fact there is no trend.
To better understand trends in dissolved oxygen in relation to climate change, more data are required than currently available. This would require expanding the monitoring network to all regions to ensure long-term changes are captured.
Baltic Sea, Bay of Biscay, Black Sea, Celtic Seas, Central Mediterranean Sea, Eastern Mediterranean Sea, Greater North Sea, Iberian Coast, Marine Macaronesian/Atlantic ocean, Mediterranean Sea, North-East Atlantic Ocean, Western Mediterranean Sea
Efficiency indicator (Type C - Are we improving?)SDG14: Life below water
Milligrams per litre (mg/l)
Once a year
References and footnotes
EU, 2008, Directive 2008/56/EC of the European Parliament and of the Council of 17 June 2008 establishing a framework for community action in the field of marine environmental policy (Marine Strategy Framework Directive), OJ L 164, 25.6.2008, p. 19-40.
EU, 2017, Commission Decision (EU) 2017/848 of 17 May 2017 laying down criteria and methodological standards on good environmental status of marine waters and specifications and standardised methods for monitoring and assessment, and repealing Decision 2010/477/EU, OJ L 125, 18.5.2017, p. 43-74.
