Nutrients in Europe's transitional, coastal and marine waters

Eutrophication in marine, coastal and estuarine ecosystems, caused by human activities that introduce excess nutrients into water, lead to harmful effects. To address this, Europe adopted an integrated strategy to reduce inputs. Nutrient levels have significantly declined between 1980 and 2023, yet eutrophication remains a significant problem in the Baltic, Black and Greater North Seas and some coastal areas of the Mediterranean Sea. Over 91-94% of the assessed time-series show no significant change. Progress has been made to reduce nutrient inputs, specifically nitrogen, yet more effort is necessary for phosphorus.

Figure 1. Trends in dissolved inorganic nitrogen and orthophosphate concentrations in transitional, coastal and marine waters in Europe, for two periods; Pre 2000 (1980-1999) and Post 2000 (2000-2023)

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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. The excessive availability of nitrogen and phosphorus accelerates 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, decreasing oxygen.

Emissions of nutrients on land originate from urban areas, dispersed settlements, industrial areas, 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. Eutrophication is still a significant problem in the Baltic, Black and Greater North Seas and in some parts of the Mediterranean Sea (WISE Marine).

The distribution of nutrient trends in European waters (Figure 1) displays data from 1980 to 2023, divided into two periods: pre and post year 2000. The main difference between the two periods is the increased available datasets post 2000. 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.

For orthophosphate (DIP), the number of assessment areas recording significant increases is greater than the recorded decreasing trends in the Baltic and Black Seas. The Greater North Sea has a significant decreasing trend. Due to limited time series data, many areas cannot be assessed, or trends for both DIN and DIP cannot be identified.

These results show 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 effort is required to lower these emissions to waters.

See more information about relevant legislation in the Supporting information.

Figure 2. Number of time series available showing increasing, decreasing or no trends in dissolved inorganic nitrogen and orthophosphate concentrations for each regional sea for two time periods; Pre 2000 (1980-1999) and Post 2000 (2000-2023)

The availability of time series data for the assessment of nutrient levels varies across regional seas. The number of time series present for the Baltic and Greater North Seas are larger than the Mediterranean and Black Seas. Figure 2 illustrates the distribution of trends based on available time-series data across regional seas and all of Europe’s seas.

Since 2000, DIN concentrations have been decreasing in the Greater North and Baltic Seas. However, before 2000, declines were observed in the Black Sea. Fewer increasing trends in DIN, post 2000, suggest reductions in nutrient inputs as a result of EU policy implementation. Decreasing trends have become less common for DIP after 2000, with increasing trends observed in both the Baltic and Black Seas (11% each). This rise in DIP concentrations is likely due to phosphorus release from sediment under anoxic conditions.

DIP levels have continued to decline in the Greater North Sea. However, the number of assessed trends is much less than pre 2000 for the Black Sea. The availability of time series data is crucial for assessing the state of European seas and evaluating the effectiveness of measures taken to achieve good status.

Supporting information

DefinitionMethodology

The primary data source for this indicator is the OceanCSI_1980 - 2023 dataset (ICES 2023), which compiles information from ICES, EMODnet, and the WISE SoE – Water Quality (WISE-6). The data handling processes—extraction, selection, aggregation, trend analysis and plotting—are performed using the R programming language.

Data flagged as bad quality (bad value, probably bad value), uncertain quality (value in excess, uncertain value, missing value), or duplicates are excluded, and the 99.9% percentile of the highest values are sorted out. The analysis considers only data from depths of less than 10 meters and during the winter months, which include January to March for stations in the Baltic Sea east of 15E and January to February for all other stations.

Average values are calculated using a combination of estimated marginal means (EMM) and arithmetic means, where the EMM were used for time series containing data from both different years and months and the arithmetic means were used for time-series with yearly observations. Initial calculations of average annual concentrations are done by location and by month-year to standardise observations across different depths and time (days, hours or even minutes in some cases).

Geographical classification: sea region, coastal or offshore and station

Stations are geographically defined by their longitude and latitude in decimal degrees. All geographical positions in the data are mapped to a marine (sub) region based on coordinates. The data often lack reliable and consistent station identifiers, which can lead to fragmented time series. To improve time series aggregation, data are grouped into 1.375km squares for coastal stations (within 20km of the coastline) and 5.5km squares for open water stations. While this procedure does not eliminate the risk of incorrect data aggregation or the breaking of time series due to minor positional shifts, it significantly reduces these issues. These aggregated clusters are designated as ‘locations’.

EEA grid used https://www.eea.europa.eu/en/datahub/datahubitem-view/a35e113a-fe10-4e20-9c49-7dc5b47b16b5?activeAccordion=1083504.

Trend analysis

Temporal trends are analysed based on the yearly averages for each time-series within a grid cell. A linear regression model (lm) is then applied to identify trends over time, with the slope indicating whether the trend is positive or negative. To assess statistical significance (p.value < 0.05), a robustness test is conducted. Additionally, the Mann-Kendall test is used to validate the trends. Based on the Mann-Kendall results, trends are classified as "increasing," "decreasing," or "no trend." If the initial test deems a model "not robust," the trend is automatically set to "no trend." Only stations with at least five years of data are included in the analysis.

Policy/environmental relevance

The risk of eutrophication and the goal of achieving good status for coastal and marine waters are addressed by the Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD).

The Comission Decision (EU) 2017/848, developed under the MSFD Common Implementation Strategy (CIS), outlines key elements for assessing eutrophication under Descriptor 5. The primary criterion (D5C1) focuses on nutrient concentrations. The related objective is: Nutrient concentrations are not at levels that indicate adverse effects of eutrophication. Threshold values should be set to assure Good Environmental Status (GES) and be harmonised with the WFD.

Commission Decision (EU) 2018/229 summarises the results of the third phase of the WFD intercalibration exercise, providing different threshold values for chlorophyll-a depending on the regional sea and water typology (EU, 2018). Nevertheless, threshold values and indicators are different for different regions, depeding on water typologies. Threshold values, indicators and classification systems are currently still hard to be used in an assessment on the pan-European scale. Several EU directives such as the Nitrates Directive and the Urban Waste Water Treatment Directive are directly addressing human activities and are aiming to reduce the loads of nutrients emitted in aquatic systems.

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.

In addition, international initiatives and policies such as the Regional Sea Conventions — the Oslo-Paris Convention (OSPAR), the Helsinki Convention (Helcom), the Barcelona Convention (UNEP-MAP) and the Black Sea Convention — also outline measures that aim to reduce the loads and impacts of nutrients and include countries beyond the EU.

This indicator assesses trends in nutrient levels in European transitional, coastal and marine waters to provide an indication of whether measures taken to reduce the risk of eutrophication are on track to be effective. It does not provide information on whether or not policy objectives have been achieved, as threshold values (boundaries between good and bad condition) were not available for each location.

Accuracy and uncertainties

No uncertainties have been quantified. The data availability is variable across the different regional seas, with higher data coverage over space and time in the North Sea and the Baltic Sea. Hence, the accuracy of the assessment is more precise for those regions. All observations have been gathered from large databases with differences between methods for sampling and analysing which cannot be identified or quantified but may influence the results in some areas. The databases are also known to include faulty data, which slips though the first data cleaning process, therefore a second cleaning was performed by removing the 99.9 % percentile of the highest values in each region. The risk of removing potential correct data by this extra cleaning was estimated to be very low and would to have negligible effect on the assessment, as the number of excluded data was very low compared to the total number of observations, and the extreme values and potential faulty observations were sorted out which could have had a potential effect on the assessment.

Geographical comparability

The natural ranges of nutrient values vary greatly depending on the geographical characteristics of monitored locations. Nutrient concentrations tend to be higher in coastal waters due to river discharges and direct nutrient inputs. Thus, geographical comparisons of results should be made only within regional seas and between similar water types (i.e. transitional, coastal, or offshore locations).

Comparability over time

The natural ranges of nutrient values also vary greatly depending on the season. However, as assessments have been made on a yearly basis and only for winter months (January to March for Baltic Sea stations east of 15E and January to February for all other stations), temporal comparability is possible. It is also important to note that the temporal range of data at different locations varies depending on the availability of data, although all locations have at least five-yearly observations.

Data sources and providers

Metadata

DPSIRTopicsTagsTemporal coverageGeographic coverage

TypologyUN SDGsUnit of measureFrequency of dissemination

References and footnotes

<div class="csl-bib-body" style="line-height: 1.35; "> <div class="csl-entry">EEA, 2019, 'Nutrient enrichment and eutrophication in Europe’s seas — European Environment Agency', ( <a key=link-1 href=https://www.eea.europa.eu/publications/nutrient-enrichment-and-eutrophication-in rel="noopener"> https://www.eea.europa.eu/publications/nutrient-enrichment-and-eutrophication-in </a>) accessed May 12, 2022.</div> </div>
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<div class="csl-bib-body" style="line-height: 1.35; "> <div class="csl-entry">Stigebrandt, A. and Andersson, A., 2020, 'The Eutrophication of the Baltic Sea has been Boosted and Perpetuated by a Major Internal Phosphorus Source', <i>Frontiers in Marine Science</i> 7 ( <a key=link-1 href=https://www.frontiersin.org/articles/10.3389/fmars.2020.572994/full rel="noopener"> https://www.frontiersin.org/articles/10.3389/fmars.2020.572994/full </a>).</div> </div>
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<div class="csl-bib-body" style="line-height: 1.35; "> <div class="csl-entry">Commission Decision (EU) 2017/848 of 17 May 2017 laying down criteria and methodological standards on good environmental status of marine waters and specifications and standardised methods for monitoring and assessment, and repealing Decision 2010/477/EU (Text with EEA relevance. ), 2017, OJ L.</div> </div>
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<div class="csl-bib-body" style="line-height: 1.35; "> <div class="csl-entry">Commission Decision (EU) 2018/229 of 12 February 2018 establishing, pursuant to Directive 2000/60/EC of the European Parliament and of the Council, the values of the Member State monitoring system classifications as a result of the intercalibration exercise and repealing Commission Decision 2013/480/EU (notified under document C(2018) 696)Text with EEA relevance., 2018, OJ L.</div> </div>
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