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Eutrophication in marine, coastal and estuarine ecosystems is caused by human activities that introduce excess nutrients into water, leading to harmful effects. To address this, Europe adopted an integrated strategy to reducing nutrient inputs. Nutrient levels have significantly declined between 1980 and 2021, yet eutrophication remains a significant large-scale problem in the Baltic, Black and Greater North Seas and some coastal areas of the Mediterranean Sea. Although progress has been made to reduce nutrient inputs, specifically nitrogen, more effort is necessary, particularly for phosphorus.
Figure 1. Trends in dissolved inorganic nitrogen and orthophosphate concentrations in Europe’s transitional, coastal and marine waters, 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 and marine activities . Emissions of nutrients on land originate from urban areas, dispersed settlements and industrial areas, as well as farms and agricultural practices. Sea-based sources include aquaculture, dumping of dredged material and discharge from ships. All emissions of nutrients to air contribute to atmospheric deposition and transboundary pollution.
The 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. Eutrophication impacts marine organisms by reducing light availability and water quality, thus oxygen decreases.
EU legislations aimed at taking an integrated approach in reducing the input of nutrients to aquatic systems to reduce the risk of eutrophication and achieve good status for marine waters are the Water Framework Directive and the Marine Strategy Framework Directive. Both set targets in relation to nutrient levels.
The also relevant Nitrates Directive requires EU Member States to monitor the quality of waters, sets criteria for definition of Nitrate Vulnerable Zones and promotes better management of agricultural activities to limit emissions of nutrients. Further, the Urban Waste Water Treatment Directive requires EU Member States to collect and treat wastewater in urban settlements. The latest requirement from 2024 includes systematic monitoring of microplastics in emissions from urban wastewater treatment plants and in the sludge.
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: Oslo-Paris Convention (OSPAR), Helsinki Convention (Helcom), Barcelona Convention (UNEP-MAP) and Black Sea Convention.
The number of assessment areas recording significant decreasing trends in dissolved inorganic nitrogen levels (DIN) of the Baltic and Greater North Seas is greater than the number recording increasing trends (Figure 1). 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. Eutrophication is still a large-scale problem in the Baltic, Black and Greater North Seas and in some parts of the Mediterranean Sea (WISE Marine). Due to limited time series data, many areas cannot be assessed or trends for both DIN and P cannot be identified.
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 still increase in some regions (e.g. Baltic Sea). Main sources of phosphorus are agricultural fertilisers, manure, and organic wastes in sewage and industrial discharges; more is required to lower these emissions to waters.
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 2017-2021
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 larger than the Mediterranean and Black Seas.
Figure 2 shows an overall decrease in 14% of the DIN time series, an increase in 2%, with 85% of time series showing no significant trend. Eleven percent of the P time series shows a decrease, with 7% having an increase and 82% showing no significant changes.
DIN concentrations in the Greater North and Baltic Sea seas have been decreasing over 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 in P are observed in the Greater North Sea.
The availability of time series data is critical for both assessing the state of European seas and assessing the effectiveness of measures taken to achieve good status. In this context, efforts should be made specifically to increase the spatial coverage of monitoring and fill in the data and knowledge gaps in all regional seas.