Oxygen concentrations in coastal and marine waters surrounding Europe
The occurrence of reduced oxygen concentrations in bottom waters of coastal and marine waters has increased both in spatial extent and duration in the last decades, owing mainly to a combination of natural causes and anthropogenic pressures such as eutrophication. Results indicate that conditions have deteriorated in 9% of the cases, improved in 3%, while for the majority (88%) no trend could be established. The Baltic and Black seas suffer the most from oxygen depletion, caused by stratification but which has intensified due to eutrophication and ocean warming. To better understand the effects of climate change, more robust datasets and monitoring networks are required to ensure that long-term changes are captured.
Anthropogenic nutrient enrichment and rising water temperatures can lead to reduced oxygen concentrations. This can negatively impact marine life, affecting biogeochemical processes and altering ecosystems, leading to both environmental and socio-economic impacts, such as decreased biodiversity, expanding algal blooms, displacement and decline of fish stocks. Physical processes can influence the extent, duration and frequency of hypoxia through mixing of the water column and supply of oxygen to deeper waters. As vertical mixing is largely determined by the level of stratification of the water column, areas with permanent or seasonal stratification have the highest risk of oxygen depletion.
The analysis of oxygen concentrations and their change over time is key to assessing progress towards better marine and coastal water quality in line with EU policy objectives. The Water Framework Directive and the Marine Water Framework Directive aim to achieve ‘good ecological status’ and ‘good environmental status’ of Europe’s waters respectively. Reduced oxygen concentrations is one of the indicators used to measure the indirect effects of nutrient enrichment and, consequently, eutrophication (chlorophyll-a concentrations in surface waters is another). The EU biodiversity strategy 2030 and Farm-to-Fork are central policies under the European Green Deal setting ambitious targets for reducing the use of nutrients in agriculture.
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. Results show that parts of the Baltic and Black seas suffer from oxygen depletion, with large areas exhibiting reduced oxygen concentrations (i.e., <6mg/l). For the period 2009-2019, 36% of the relative spatial area assessed in the Baltic Sea and 37.5% in the Black Sea showed dissolved oxygen (DO) concentrations less than 6mg/l. These areas mainly occur in the deeper water layers beneath the pycnocline. In the Mediterranean Sea, observations of oxygen depletion are quite localised, occurring mainly near the Balearic Islands and south of Cyprus. Almost 21% of the relative spatial area assessed in this region experienced reduced DO concentrations. The North-East Atlantic experiences some localised and short time periods of oxygen deficiency, with most observations occurring in the Greater North Sea (Figure 1). In this region, a combination of physical drivers and biological processes (organic matter supply through primary production) can lead to hypoxia.
In addition to the observation of reduced oxygen concentrations, knowledge on trends is necessary to understand if the state of Europe’s seas is improving. Figure 2 shows trends for stations with average oxygen concentrations by three concentration classes: (1) below 4mg/l (includes <2mg/l class); (2) between 4 and 6mg/l and (3) above 6mg/l. Only a limited number of longer time series (>5 years) with measurements at a specific site is available, most of these are for the Greater North Sea and the Baltic Sea. In the other seas, time-series data are limited.
Time series for stations with oxygen concentrations below 4mg/l were mainly available for the Baltic Sea, including a few Danish fjords. For these, a decrease in oxygen concentrations and thus a further deterioration of environmental conditions was observed in 20% of the cases, whereas only a few improvements were observed (<1%). In all other cases, there was no significant trend.
Most stations with concentrations between 4-6mg/l were also found in the Baltic Sea and several Danish fjords. For these stations, 8% showed a deterioration, 3% an improvement and the remaining 89% showed no trend. For the North Sea similar percentages were found.
For stations with concentrations above 6mg/l, 9% of the stations in the Baltic Sea observed worsening conditions while 3% showed an improvement. For the Greater North Sea subregion, 6% showed a deterioration and 3% an improvement.
For the assessed stations in all marine regions combined, 9% showed a deteriorating trend, 3% an improving trend and for the remaining 88% no trend could be established.
Supporting information
This indicator illustrates the geographical distribution and trends in summer-autumn concentrations of oxygen in the near-bottom waters of Europe’s regional seas. Reduced oxygen concentrations are used as an indicator of the indirect effects of nutrient enrichment and, consequently, eutrophication.
Under the Water Framework Directive (WFD), targets for dissolved oxygen concentrations in coastal waters have been set by Member States. Under the Marine Strategy Framework Directive, 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.
Data on oxygen concentrations during the summer-autumn months (July-October) were 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 during the years 2009-2019 was used. Results were aggregated at the level of 100x100km grid cells. For each marine region, the 25-percentile of oxygen concentrations in a grid cell was used to classify the grid cells by oxygen concentration classes. The procedures of data extraction, data selection and aggregation, trend analysis and the plotting of results are carried out in R.
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 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. Data supplied by EMODnet are combined with ICES data. In cases where both ICES and EMODnet data are available for the same station (defined by position and time), ICES data were used.
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.
Stations are defined geographically by position, given as longitude and latitude in decimal degrees. All geographical positions defined in the data are assigned to marine (sub)region by coordinates.
Statistical aggregation per station and per year
The aggregation includes:
· selecting the season (months July-October);
· selecting the sample depth (0-20m above the sea floor when the depth of the sea floor is less than 100m; 0-50m above the sea floor when the depth of the sea floor is more than 100m);
· 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 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.
Methodology for gap filling
Not applicable
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).
Oxygen-depleted areas are an example of how one type of anthropogenic pressure (eutrophication) is exacerbated by climate change (increasing temperature). 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. Oxygen depletion may also interact with other anthropogenic stressors, such as overfishing or the introduction of invasive species, and affect marine ecosystems and fisheries.
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, as is the case for large areas in the Baltic and Black seas. The Baltic Sea is the largest anthropogenically induced hypoxic area in the world and hypoxia has significantly increased over the last 100 years The Black Sea has the largest volume worldwide of naturally permanent anoxic waters, where waters below 100-150m (ca. 87% of the volume) are deprived of oxygen.
The main aim of the marine indicators set is to support and evaluate efficiency of the MSFD and the UN Agenda 2030 (SDG 14), as well as the Maritime Spatial Planning Directive (MSPD) and other EU and international polices. The objective is to illustrate long-term trends where possible.
The WFD requires the achievement of good ecological status or the good ecological potential of transitional and coastal waters across the EU and the MSFD 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 physio-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.
Other EU directives are also related to the control of eutrophication by aiming to reduce the loads and impacts of the nutrients. These 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 and Zero Pollution Action Plan are main policies under the EU Green Deal setting ambitious targets for reducing nutrients from agriculture. Also, under the EGD, and with respect to climate change, the European Commission adopted the new EU Adaptation Strategy and a Climate Law (EU, 2021c) with the overall aim of contributing to a more climate-resilient Europe.
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.
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, as is currently the case in OSPAR. In HELCOM, minimum concentrations are assessed. 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 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. Long stretches of coastal and marine waters are not covered by the analysis because of this lack of data. Most of the available time series concern the Greater North Sea and the Baltic Sea, in particular the Kattegat, the Dutch, German and Danish parts of the North Sea and the central and western part of the Baltic Sea. In the other seas, data are more limited, particularly areas along the north-western coast of Spain, the northern part of the Adriatic Sea, waters near the Balearic Islands and the north-western shelf area of the Black Sea.
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. Low oxygen concentrations generally occur in deeper waters, as vertical mixing will diminish the occurrence of oxygen depletion in shallow waters.
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. Therefore, it may be more appropriate to assess assessment areas instead of the 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 those which are currently available. There may, therefore, be a requirement to expand the monitoring network to all regions to ensure long-term changes are captured.
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HELCOM assesses 17 open sea area sub-basins in addition to the WFD coastal water bodies in its eutrophication assessment. The oxygen debt indicator has significantly increased between 1990 and 2012, indicating that oxygen conditions are deteriorating (see http://www.helcom.fi/baltic-sea-trends/indicators/oxygen-debt/).
For the Black Sea, CMEMS has developed three complementary indicators: oxygen penetration depth, oxygen penetration density and vertically integrated oxygen content (von Schuckmann et al. 2018). The indicators are based on work by Capet et al. 2016. https://marine.copernicus.eu/sites/default/files/wp-content/uploads/2020/02/BLKSEA_OMI_HEALTH_oxygen_odo2_trend-hq.png; https://marine.copernicus.eu/access-data/ocean-monitoring-indicators/black-sea-oxygen-trend
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