Eutrophication in marine, coastal and estuarine ecosystems is caused by human activities that enrich waters with nutrients, creating harmful effects. To address this, Europe takes an integrated approach to reducing nutrient inputs. Levels of nutrients have significantly decreased between 1980 and 2021 in the Baltic and Greater North Seas where eutrophication is still a large-scale problem. Time series data were not sufficient to identify total trends in all regional seas. Although progress has been made to reduce nutrient inputs, specifically nitrogen, more effort is necessary, particularly for phosphorus. Better monitoring and reporting are needed to produce more comprehensive assessments.

Figure 1. Trends in dissolved inorganic nitrogen and orthophosphate concentrations in transitional, coastal and marine waters in Europe, 1980-2021
Trends in dissolved inorganic nitrogen and orthophosphate concentrations in transitional, coastal and marine waters in Europe, 1980-2021

Eutrophication in marine, coastal and estuarine ecosystems is a consequence of anthropogenic nutrient over-enrichment, with nitrogen and phosphorus coming from land-based sources, marine activities and atmospheric deposition, as well as fluxes from neighbouring water bodies . The resulting excessive availability of nitrogen and phosphorus accelerates the growth of microalgae and higher forms of plant life, with indirect negative effects on aquatic organisms and water quality.

The EU takes an integrated approach to reducing the input of nutrients to aquatic systems to reduce the risk of eutrophication and achieve good status for marine waters. EU legislation aimed at addressing this issue includes the Water Framework Directive and the Marine Strategy Framework Directive, both of which set targets in relation to nutrient levels. The Nitrates Directive and the Urban Waste Water Treatment Directive are also relevant. The EU Biodiversity Strategy to 2030 , Farm to Fork and Zero Pollution Action Plan are the main policies under the EU Green Deal setting ambitious targets for the reduction of nutrients from agriculture. International initiatives and policies that outline measures aimed at reducing nutrient loads and impacts include the Regional Sea Conventions: the Oslo-Paris Convention (OSPAR), the Helsinki Convention (Helcom), the Barcelona Convention (UNEP-MAP) and the Black Sea Convention.

Eutrophication is still a large-scale problem in the Baltic, Black and Greater North Seas and in some coastal areas of the Mediterranean Sea (WISE Marine). In the Baltic and Greater North Seas the number of assessment areas recording significant decreasing trends in dissolved inorganic nitrogen levels (DIN) is greater than the number recording increasing trends. For orthophosphate (P), the number of assessment areas recording significant increases is greater than the recorded decreasing trends in the Baltic Sea, whereas the Greater North Sea has a significant decreasing trend. However, due to limited time series data many areas cannot be assessed or the trends for both DIN and P cannot be identified.

Overall, these results show some significant decreasing trends in the levels of nutrients in regional seas and areas where nutrient management strategies have been implemented. However, phosphorus concentrations are still increasing in some regional seas (e.g. the Baltic Sea) and more needs to be done to tackle this issue.

Figure 2. Number of time series available showing increasing, decreasing or no trends in dissolved inorganic nitrogen and orthophosphate concentrations for each regional sea during the period 1980-2021
Number of time series available showing increasing, decreasing or no trends in dissolved inorganic nitrogen and orthophosphate concentrations for each regional sea during the period 1980-2021

The availability of time series data for the assessment of nutrient levels varies across regional seas, with the number of time series present for the Baltic and Greater North Seas much larger than the Mediterranean and Black Seas.

The assessment shows an overall decreasing trend in 14% of the DIN time series, increasing in 2%, with 85% showing no significant trend. Eleven percent of the P time series shows a decreasing trend, with 7% increasing and 82% show no significant changes. DIN concentrations in the Greater North and Baltic Sea seas have been decreasing during the last four decades, reflecting the effects of reductions in nutrient inputs as a consequence of EU policy implementation. The increasing trends of P concentrations observed in some parts of the Baltic Sea are likely related to phosphorus release from sediment under anoxic conditions. Decreasing trends are observed in the Greater North Sea.

The availability of long time series data is clearly important for assessing the effectiveness of measures taken to achieve good status for Europe’s seas. Therefore, efforts should be made to increase the spatial coverage of the assessment and fill the gaps in some regional seas.