Reduced oxygen concentrations is used as an indicator of the indirect effects of eutrophication. Its occurrence in near seafloor waters across Europe's seas has increased in spatial extent and duration in recent decades, owing mainly to a combination of natural causes and human-induced pressures, including excess nutrient inputs and climate change. For the assessed stations, conditions deteriorated in 13%, improved in 5%, while for the majority (82%) no trend could be established. The Baltic and Black seas suffer the most from oxygen depletion, caused by restricted vertical mixing of surface waters, intensified by eutrophication and ocean warming.

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

Anthropogenic nutrient enrichment and rising water temperatures can lead to reduced oxygen concentrations, particularly in deeper water layers. Oxygen depletion 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.

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 Strategy 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, Zero Pollution Action Plan 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 dissolved oxygen (DO) concentrations (i.e., <6mg/l). For the period 2011-2022, 54% of the relative spatial area assessed in the Baltic Sea and 72% in the Black Sea showed DO concentrations less than 6mg/l (Figure 1). These areas mainly occur in the deeper, more dense water layers where the inflow or downward flux of oxygen is irregular or insufficient.

In the Mediterranean Sea, observations of oxygen depletion are more localised, occurring mainly near the Balearic Islands, the Italian Ionian coastal areas and south of Cyprus. Almost 18% of the relative spatial area assessed in this region experienced reduced DO concentrations.

DO concentrations were above the threshold value of 6mg/l for most areas assessed in the North-East Atlantic, although the region experiences some localised and short periods of oxygen deficiency.

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

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 classes: (1) below 4mg/l (includes <2mg/l class); (2) between 4 and 6mg/l and (3) above 6mg/l. Site specific longer time series (>5 years) are limited and mainly available for the North-East Atlantic Ocean and Baltic Sea. Due to the limited time series for the other regions, these are not shown.

Trends for stations with severely depleted oxygen concentrations (below 4mg/l) were mainly found for the Baltic Sea and a few Danish fjords. Of these, 27% displayed deteriorating conditions (i.e., decreasing trends), 5% improved and the remaining 68% showed no significant trend. Most stations with concentrations between 4-6mg/l were also found in these regions, with 7% deteriorating and the remaining 93% showing no trend. For Baltic Sea stations with concentrations above 6mg/l, 13% displayed worsening conditions and 5% showed an improvement, while in the North-East Atlantic, 9% deteriorated and 8% improved. For North-East Atlantic stations with low oxygen concentrations (<6mg/l), 17% displayed a deterioration while 13% improved (Figure 2).

Further actions targeting eutrophication are needed to help alleviate oxygen deficiency, and more robust datasets and monitoring networks are required to better understand the impacts of mitigation measures and effects of climate change.