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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.
Figure 1. Occurrence of reduced oxygen concentrations in Europe's coastal and marine waters (average for the years 2011-2022)
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.
Figure 2. 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.