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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.