Oxygen concentrations in European coastal and marine waters

Indicator Specification
Indicator codes: MAR 012
Created 29 Mar 2019 Published 15 Nov 2019 Last modified 27 Nov 2019
4 min read
The indicator illustrates the geographical distribution and trends in summer/autumn concentrations of oxygen in the near-bottom waters of the regional seas of Europe. It uses oxygen concentrations in the near-bottom layer during the period July-October. The following marine regions and subregions are covered, in line with the Marine Strategy Framework Directive (MSFD) (sub)regions: Baltic Sea, North-East Atlantic Ocean, Mediterranean Sea and Black Sea (see 'Regional seas surrounding Europe' map: Regional Seas ).

Assessment versions

Published (reviewed and quality assured)


Justification for indicator selection

Excessive nutrient flow into the sea (mostly from agricultural fertilisers) can lead to large phytoplankton blooms and subsequent increases in primary production (a process called eutrophication). When these organisms sink to the sea floor, oxygen is utilised in their decomposition. If mixing within the water column cannot supply enough oxygen to the sea floor, this can lead to a reduction in oxygen concentrations to levels that severely limit biological activity (hypoxia) and ultimately to complete oxygen depletion (anoxia) (Diaz, 2001; Diaz and Rosenberg, 2008).

Rising water temperatures affect a number of different biological and chemical processes in the marine environment. For example, as the temperature rises, oxygen becomes less soluble in water, resulting in lower oxygen concentrations; at the same time, the demand for oxygen for metabolism increases. Most organisms require oxygen for their metabolism. Therefore, lower oxygen concentrations in seawater affect the physiology, composition and abundance of species. Insufficient oxygen supply to organisms will eventually have knock-on effects on productivity, species interactions and community composition at the ecosystem level (Vaquer-Sunyer and Duarte, 2008).

Oxygen depletion can occur episodically (less than once per year), periodically (several times per year for short periods) and seasonally (each summer), and eventually it can become persistent.

The Baltic Sea and Black Sea have large areas of persistent oxygen depletion (Capet et al., 2013, 2016; Conley et al., 2007). Oxygen-depleted areas are an example of how one type of anthropogenic pressure (nutrient inputs that cause eutrophication) is exacerbated by climate change (increasing temperature) through multiple linkages with biology, from the cellular level to the community and ecosystem levels. For example, land-based nutrient enrichment can lead to a redistribution in the vertical distribution of primary production. Such increased nutrient input can increase primary production in the surface layer, where the oxygen produced can be exchanged with the atmosphere. The organic material produced will sink through the pycnocline (i.e. the ocean layer with a stable density gradient, which hinders vertical transport) using oxygen as it decomposes. At the same time, when primary production occurs below the pycnocline, the oxygen produced will stay in the bottom layer. As climate change influences stratification parameters, and consequently the depth of the pycnocline, it could influence light availability for primary production in the deeper layer, possibly decreasing oxygen production. In such a case, the interaction between climate change and eutrophication could have impacts on biodiversity, plankton communities and oxygen conditions. Oxygen depletion may also interact with other anthropogenic stressors, such as overfishing or the introduction of invasive species, and affect marine ecosystems and fisheries (Mee, 2006Kemp et al., 2009; Zhang et al., 2010).

The number of hypoxic areas increased between 1950 and 2000, probably as a result of enhanced nutrient inputs in the industrial age and the use of fertiliser in agriculture. Hypoxia occurs throughout the European seas, often due to a combination of eutrophication with specific hydrodynamical and meteorological conditions. Increased occurrences of reduced oxygen concentrations or hypoxic events have mostly been reported in the Black Sea, the Baltic Sea and the Greater North Sea (in Kattegat and Skagerrak). In the Mediterranean, the Adriatic Sea and Aegean Sea have areas that are at risk of becoming hypoxic (Klein et al., 2003Druon et al., 2004; Topcu et al., 2015Capet et al., 2016; Carstensen et al., 2016).

This indicator is part of a set of indicators that focus on the state of and pressures acting on Europe's seas. In terms of pressures, this indicator is linked to the marine indicators MAR005/CSI021 'Nutrients in transitional, coastal and marine waters and MAR006/CSI023 'Chlorophyll in transitional, coastal and marine waters. Reduced oxygen concentrations are used as an indicator of the indirect effects of eutrophication.

Scientific references

Indicator definition

The indicator illustrates the geographical distribution and trends in summer/autumn concentrations of oxygen in the near-bottom waters of the regional seas of Europe. It uses oxygen concentrations in the near-bottom layer during the period July-October.

The following marine regions and subregions are covered, in line with the Marine Strategy Framework Directive (MSFD) (sub)regions: Baltic Sea, North-East Atlantic Ocean, Mediterranean Sea and Black Sea (see 'Regional seas surrounding Europe' map: Regional Seas).


The concentration of oxygen is expressed as mg/l in the bottom 10 m of the water column during summer/autumn.

Policy context and targets

Context description

With respect to eutrophication, there are a number of EU directives specifically aimed at reducing the loads and impacts of nutrients, such as the Nitrates Directive (91/676/EEC), the Urban Waste Water Treatment Directive (91/271/EEC) and the Integrated Pollution Prevention and Control Directive (96/61/EEC).

The Water Framework Directive (WFD; 2000/60/EC) requires the achievement of good ecological status or the good ecological potential of transitional and coastal waters across the EU and the MSDF (2008/56/EC) requires the achievement or maintenance of good environmental status in European sea basins by the year 2020. The WFD mentions dissolved oxygen concentrations as one of the physico-chemical parameters for measuring ecological status. In the MSFD, dissolved oxygen concentration in the bottom of the water column is one of the primary criteria (D5C5) for Descriptor 5 human-induced eutrophication: 'The concentration of dissolved oxygen is not reduced, due to nutrient enrichment, to levels that indicate adverse effects on benthic habitats (including on associated biota and mobile species) or other eutrophication effects.'

With respect to climate change, the European Commission has developed the EU Adaptation Strategy with the overall aim of contributing to a more climate-resilient Europe. One of the objectives is better-informed decision-making, e.g. by sharing 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.


Under the WFD, targets for dissolved oxygen concentrations in coastal waters have been set by Member States. Under the MSFD, threshold values for coastal waters are set in accordance with the WFD, and beyond coastal waters threshold values must be consistent with those under the WFD. Member States establish those values through (sub)regional cooperation.

Related policy documents

Key policy question

Is the occurrence of oxygen depletion in European coastal and marine waters decreasing?


Methodology for indicator calculation

The two main sources of data for this indicator are the International Council for the Exploration of the Sea (ICES) and the European Marine Observation and Data Network (EMODnet) data sets.

Data kept by the ICES are collected through the Eionet Central Data Repository (Eionet CDR) from the marine conventions and are therefore sub-samples of national data assembled for the purpose of providing comparable indicators of the state of and impacts on transitional, coastal and marine waters (TCM data) on a Europe-wide scale. In addition, data supplied by EMODnet are combined with ICES data. The latest EMODnet data set submission (by 22 June 2018) was used.

In cases in which both ICES and EMODnet data were available for the same station (defined by position and time), ICES data were used.

Concentrations are presented on maps showing means of the 25th percentile per station and year for the most recent 5-year period (2013-2017).

Consistent time series of the concentrations (mean of the 25-quartile) per station and year are used as the basis for the assessment of developments over time. The trend analyses are based on time series from 1990 onwards. Stations with data from at least 5 or more years in the period since 1990 were selected for trend calculations.

The primary aggregation involves:

  • identifying (clusters of) stations and assigning them to marine (sub)regions;
  • creating statistical estimates for each combination of station and year.

The procedures of data extraction, data selection and aggregation, trend analysis and the plotting of results are carried out in R.


Geographical classification: sea region, coastal or offshore and station

All geographical positions defined in the data are assigned to marine (sub)region by coordinates.

Stations are defined geographically by position, given as longitude and latitude in decimal degrees; however, the data reported do not contain reliable and consistent station identifiers. The coordinates reported for what is intended to be the same station may vary between visits, because the exact position achieved is recorded, not the target position. Identifying stations strictly by position may fragment time series too much, as the position of the same station may vary slightly over time.

To improve aggregation into time series, data are aggregated into squares with sides of approximately 1.375 km for coastal stations within 20 km from the coastline and approximately 5.5 km for open-water stations more than 20 km from the coastline. This procedure however does not completely prevent the erroneous aggregation of data belonging to stations close to each other or the erroneous breakup of time series into fragments due to small shifts in position, but does reduce the problem considerably.

The following marine regions and subregions are covered, in line with the MSFD (sub)regions:

  • Baltic Sea (none);
  • North-East Atlantic Ocean (Greater North Sea, Celtic Seas, Bay of Biscay and the Iberian coast, Macaronesian biogeographic region);
  • Mediterranean Sea (Western Mediterranean Sea, Adriatic Sea, Ionian Sea and Central Mediterranean, Aegean-Levantine Sea);
  • Black Sea (none).


Statistical aggregation per station and per year

The aggregation includes:

  • selecting the season (months July-October);
  • selecting the sample depth (0-20 m above the sea floor when the depth of the sea floor is less than 100 m; 0-50 m above the sea floor when the depth of the sea floor is more than 100 m);
  • selecting data per station and year within the lower quartile (≤ 25th percentile);
  • calculating the mean of the data selected per station and year.


Trend analysis

Trend analysis was carried out for each station in regions for which there were at least some data from the past 6-year period (2013 or later) and data for 5 or more years from the period since 1990. Trends were detected in each time series using the non-parametric Mann-Kendall trend test.

The Mann-Kendall method is a non-parametric test suggested by Mann (1945) and has been extensively used to analyse environmental time series (Helsel and Hirsch, 2002; Hipel and McLeod, 2005). The Mann-Kendall method tests for monotonic trends in a time series, y(x), which in this analysis is oxygen concentration (y) as a function of year (x). The test is based on Kendall's rank correlation, which measures the strength of monotonic association between the vectors x and y. In the case of no ties in the x and y variables, Kendall's rank correlation coefficient, tau, may be expressed as tau = S/D, where S = sum {i<j} (sign(x[j]-x[i]) * sign(y[j]-y[i])) and D = n(n - 1)/2, S being the score and D being the denominator; D is the maximum possible value of S. The p-value of tau is computed by an algorithm given by Best and Gipps (1974). 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 size of the change.

The Mann-Kendall test is a robust and accepted approach. Because of 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.

Methodology for gap filling

Not applicable

Methodology references

Data specifications

EEA data references

External data references

Data sources in latest figures


Methodology uncertainty

Only data for the lower 20 m or 50 m of the water column are selected. Not all available data are associated with reliable attributes on sampling depth and bathymetry.

In shallow water, the selection of the lower 20 m of the water column may not be optimal. However, low oxygen concentrations generally occur in deeper waters, as vertical mixing will diminish the occurrence of oxygen depletion in shallow waters.

Data sets uncertainty

Data for this assessment are still scarce considering the large spatial and temporal variations inherent in European transitional, coastal and marine waters. Long stretches of European coastal and marine waters are not covered by the analysis because of this lack of data. Trend analyses are consistent only for parts of the Greater North Sea, the Baltic Sea, the Celtic Seas, the Adriatic Sea and the north-west shelf of the Black Sea.

Some of the stations are also in transitional water bodies

Rationale uncertainty

Not applicable

Further work

Short term work

Work specified here requires to be completed within 1 year from now.

Long term work

Work specified here will require more than 1 year (from now) to be completed.

General metadata

Responsibility and ownership

EEA Contact Info

Monika Peterlin


European Environment Agency (EEA)


Indicator code
MAR 012
Version id: 2

Frequency of updates

Updates are scheduled every 3 years


DPSIR: Impact
Typology: Efficiency indicator (Type C - Are we improving?)

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