Oxygen concentrations in Europe's coastal and marine waters

The occurrence of reduced oxygen concentrations in near-seafloor waters is increasing, largely due to a combination of natural causes and human-induced pressures including excess nutrient inputs and climate change. Over 25% of assessed areas reveal reduced concentrations (<6mg/l), falling below the threshold needed to support marine life with minimal stress. The semi-enclosed Baltic and Black seas are the most affected areas in Europe. There, oxygen-depleted zones are caused by restricted vertical mixing of water layers, intensified by eutrophication and ocean warming. Areas in the Mediterranean Sea are also particularly affected.

Excessive nutrient inputs into marine ecosystems from agriculture run-off, aquaculture effluents, wastewater and industrial discharges lead to harmful algal blooms. As these blooms die and decompose, they consume oxygen. This can be further aggravated by ocean warming, which:

decreases the solubility/content of oxygen in seawater;

increases the metabolic oxygen demand of most marine organisms;

intensifies stratification of the water column, reducing oxygen exchange with deeper waters.

Oxygen depletion can severely impact marine life and disrupt ecosystems, leading to significant environmental and socio-economic consequences, including loss of biodiversity and the decline or displacement of fisheries resources.

Reduced oxygen concentrations serve as an indicator of the indirect effects of nutrient enrichment and, consequently, eutrophication. It is key for assessing progress towards improved water quality in line with EU policy objectives, such as the Water Framework Directive and Marine Strategy Framework Directive. These directives aim to achieve ‘good ecological status’ and ‘good environmental status’ of Europe’s waters, respectively. The European Green Deal supports this by introducing ambitious targets for reducing nutrient use in agriculture and losses into the environment, outlined in key policies including the EU Biodiversity Strategy 2030, Farm-to-Fork Strategy and Zero Pollution Action Plan.

Oxygen concentrations are monitored from July to October, the period most likely to experience oxygen depletion due to higher water temperatures. Results show that large parts of the Baltic and Black seas suffer from reduced oxygen levels. During 2011-2022, 28% of the assessed spatial area in the Baltic Sea and 44% in the Black Sea recorded concentrations below 4mg/l (Figure 1), reaching near critical conditions. These areas typically occur in deeper, denser water layers where the inflow or downward movement of oxygen is irregular or limited.

In the Mediterranean Sea, oxygen depleted areas are more localised, occurring near the Balearic Islands, in the Gulf of Taranto on Italy's Ionian coast and south of Cyprus. Almost 18% of the assessed area in this region experiences reduced concentrations (<6mg/l). While the North-East Atlantic region experiences localised and short periods of oxygen deficiency, most of the assessed areas show concentrations above the 6mg/l threshold value.

Trends in oxygen concentrations in the near-bottom waters of the North-East Atlantic Ocean and Baltic Sea regions (1989-2021)

Analysing trends is crucial for understanding whether the state of Europe's seas is improving or declining. Figure 2 shows stations with average oxygen concentrations in three classes: (1) below 4mg/l (includes <2mg/l class); (2) between 4 and 6mg/l; and (3) above 6mg/l.

Trends analysed here are for the North-East Atlantic Ocean and Baltic Sea. Other regions lack sufficient time series data (>5 years). Results reveal no significant change in 80% of the 270 stations assessed, while 15.6% deteriorate (i.e., decreasing trend) (Figure 2).

Stations in the Baltic Sea and some Danish fjords facing severe oxygen depletion (<4mg/l), see conditions worsening in 27% of the cases and some improvement in 5%. In the Baltic Sea, stations with oxygen levels above 6mg/l show 13% worsening and 5% improving. In the North-East Atlantic Ocean, 9% of such are deteriorating and 8% improving while stations with low oxygen concentrations (<6mg/l) see 17% deteriorating and 13% improving.

Concerted action against nutrient pollution is needed to combat oxygen deficiency, alongside efforts to mitigate ocean warming. More comprehensive data collection and monitoring are essential to better understand the impacts of these measures and the broader effects of climate change across all regional seas.

Supporting information

Definition

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.

Methodology

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 25-percentile of observations from the years 2011-2022 is calculated. Results are aggregated at the level of 100x100km grid cells. For each marine region, the 25-percentile of oxygen concentrations in a grid cell is used to classify the grid cells by oxygen concentration classes.

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
WISE SoE – Water Quality (WISE-6).

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.

Geographical classification: sea region, coastal or offshore and station

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.

Statistical aggregation per station and per year includes:

selecting the season (months July-October);
selecting the sample depth (0-20m above the sea floor for depths less than 100m; 0-50m above the sea floor for depths greater than 100m);
selecting data for each station and year that fall within the lower quartile (≤ 25th percentile);
calculating the mean of the data selected per station and year.

Trend analysis

Trend analysis is conducted for each station in regions that have data from at least the past six-year period and data spanning five or more years since 1989. Trends in each time series are identified using the non-parametric Mann-Kendall trend test, which is two-sided (testing for both positive and negative trends). Data series with p-values less than 0.05 are considered as statistically significant, indicating either a positive or negative. It is important to note that this test only determines the direction and significance of trends, not the magnitude of the change.

Methodology for gap filling

Not applicable

Policy/environmental relevance

The analysis of 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.

Other EU directives are also related to the control of eutrophication by aiming to reduce the loads and impacts of the nutrients, and include: the Urban Waste Water Treatment Directive aimed at reducing pollution from sewage treatment works and certain industries; the Nitrates Directive aimed at reducing nitrate pollution from agricultural sources and the Integrated Pollution Prevention and Control Directive aimed at controlling and preventing the pollution of water from industry. In addition, the EU Biodiversity Strategy 2030, Farm to Fork Strategy and Zero Pollution Action Plan are main policies under the EU Green Deal setting ambitious targets for reducing nutrients from agriculture.

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.

Reduced oxygen concentrations serve as an indicator of the indirect effects of nutrient enrichment and, consequently, eutrophication. It is part of a set of indicators assessing the state and pressures on Europe's seas. It is closely linked to the other two EEA marine indicators on eutrophication: 'Nutrients in transitional, coastal and marine waters’ and 'Chlorophyll in transitional, coastal and marine waters’.

Oxygen depletion can also interact with other anthropogenic stressors, such as climate change, overfishing and the introduction of invasive alien species, further impacting marine ecosystems and fisheries. Thus, it is also associated with indicators of: Ocean acidification, Marine non-indigenous species in Europe's seas, Status of marine fish and shellfish stocks in European seas and Changes in fish distribution in European seas.

Accuracy and uncertainties

Methodology uncertainty

Recent discussions in OSPAR suggest that the 25th percentile is not precautionary enough and the 5th percentile should be used instead. Additionally, OSPAR and HELCOM use a sample depth of 10m above the seafloor, considering that 20m is not adequate. These and other developments will be further tested in future updates of this 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. Long stretches of coastal and marine waters continue to be not covered by the analysis because of this lack of data. The majority 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

In terms of data comparability over time, while the Mann-Kendall test is a robust and widely accepted method for trend analysis, the use of a p-value threshold of <0.05 means that roughly 5% of the results might falsely indicate significant trends due to the volume of tests conducted.

To accurately track trends in dissolved oxygen related to climate change, more comprehensive data collection is essential. Expanding the monitoring network across all regions is crucial to ensure that long-term changes are thoroughly captured and understood.

Data sources and providers

Metadata

DPSIRTopicsTagsTemporal coverageGeographic coverage

TypologyUN SDGsUnit of measureFrequency of dissemination

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.

^↵
Díaz, R., 2001, 'Overview of hypoxia around the world.', Journal of environmental quality.

^↵
Diaz, R. J. and Rosenberg, R., 2008, 'Spreading Dead Zones and Consequences for Marine Ecosystems', (https://www.science.org/doi/10.1126/science.1156401?cookieSet=1) accessed July 20, 2022.

^↵
Vaquer-Sunyer, R. and Duarte, C. M., 2008, 'Thresholds of hypoxia for marine biodiversity.', Proceedings of the National Academy of Sciences.

^↵

Oxygen concentrations in Europe's coastal and marine waters

Figure 1. Occurrence of reduced oxygen concentrations in Europe's coastal and marine waters (average for the years 2011-2022)

Figure 2. Trends in oxygen concentrations in the near-bottom waters of the North-East Atlantic Ocean and Baltic Sea regions (1989-2021)

Supporting information

Metadata

References and footnotes