Chlorophyll in Europe's transitional, coastal and marine waters

Chlorophyll-a concentrations, an indicator of phytoplankton biomass and eutrophication, show mixed trends across Europe's seas. Improvements have been observed in some areas, notably the Kattegat Strait and northwest of Ireland. Decreasing trends are more frequent in the Greater North Sea and the Black Sea, while increasing trends are more common in the Baltic Sea. Overall, these findings suggest modest improvements while continued monitoring and measures to reduce eutrophication pressures remain important, particularly in the context of climate change.

Figure 1. Average concentrations and trends of surface chlorophyll-a in Europe's transitional, coastal and marine waters

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Chlorophyll-a (chl-a) is an indicator of phytoplankton biomass and reflects primary production (PP) in marine waters, which is influenced by nutrient and light availability. Phytoplankton play a key role in the ocean carbon cycle by absorbing atmospheric carbon dioxide, thereby helping to mitigate climate change.

Chl-a concentrations naturally fluctuate across marine environments and seasons. Persistently high concentrations may indicate large phytoplankton blooms. Eutrophication, driven by excessive nutrient inputs from human activities and natural sources, can trigger harmful algal blooms and cause oxygen depletion in near-bottom waters. These conditions can degrade marine ecosystems and result in biodiversity loss.

Monitoring changes in chl-a concentrations over time helps assess progress towards improved water quality and EU policy objectives under the Water Framework Directive (WFD) and the Marine Strategy Framework Directive (MSFD). These directives aim to achieve good ecological and good environmental status in Europe’s waters, respectively. The European Green Deal supports these objectives through targets to reduce nutrient use in agriculture and nutrient losses to the environment.

Analysis of average summer chl-a concentrations across Europe's seas during 2018 and 2024 shows high concentrations in the Baltic Sea and in some coastal areas of the North Sea and Mediterranean Sea (Figure 1). The natural variability of chl-a concentrations makes it difficult to establish uniform water quality standards; however, examining ‘trends-at-locations’ provides a useful way to assess changes in chl-a levels over time, independently of their natural concentrations.

Trend analyses conducted before and after 2000, the year the WFD entered into force, show no statistically significant changes at most monitoring locations. After 2000, decreasing trends were concentrated in areas with relatively high summer chl-a concentrations, particularly around Denmark, in the Kattegat Strait and northwest of Ireland, suggesting improved conditions. Increasing trends were identified in parts of the Baltic Sea, the Bay of Biscay, the Mediterranean Sea and several other coastal areas. Although monitoring data increased after 2000, trend assessment remains constrained in many locations by insufficient data.

Figure 2. Trends in chlorophyll-a concentrations in Europe's transitional, coastal and marine waters, for two periods: Pre 2000 (1980-1999) and Post 2000 (2000-2024)

Trends in chl-a concentrations vary across Europe's seas. In the Baltic Sea, increasing trends were more common than decreasing trends, accounting for 4% and 2%, respectively. In contrast, the Greater North Sea had more decreasing than increasing trends (6% versus 1%), while in the Black Sea, 9% showed decreasing trends, with none increasing. Overall, decreasing trends were slightly more frequent than increasing trends across all assessed regions (3% versus 2%), suggesting modest improvements where significant changes were observed. Trends by country are also available.

These findings are consistent with trends reported in two other eutrophication indicators assessed over the same period: nutrient and oxygen concentrations. Together, they suggest that measures to reduce nutrient inputs may be improving conditions in some areas. However, continued efforts are needed to address persistent eutrophication pressures, particularly where conditions remain problematic.

Despite increased monitoring coverage, substantial gaps remain in several marine regions. These gaps limit large-scale assessments and highlight the need for sustained and harmonised monitoring programmes to provide long-term time series for management and policy evaluation.

Supporting information

Definition

This indicator shows the geographical distribution and trends in mean summer concentrations of chlorophyll-a (µg/l) in the upper 10 meters of the water column during the assessment period 1980-2024, in Europe's seas.

Chlorophyll-a levels serve as a biological indicator of the direct effects of nutrient enrichment and, consequently, eutrophication. The purpose of this indicator is to illustrate the effectiveness of regulatory measures, such as the Water Framework and Marine Strategy Framework directives, aimed at reducing nutrient discharges and their influence on phytoplankton levels.

Legislatively, the Water Framework Directive (WFD), defines target chlorophyll concentrations and ranges for various water types and categories, including coastal and transitional water bodies, as outlined in Commission Decision (2018/229).

Similarly, the Marine Strategy Framework Directive (MSFD), sets threshold values (TVs) for coastal waters aligned with those of the WFD, extending these beyond coastal waters to ensure consistency. These TVs are established by Member States through (sub)regional cooperation.

Methodology

The primary data source for this indicator is the OceanCSI_198020231 ICES dataset, which compiles information from ICES, EMODnet, and the WISE SoE – Water Quality (WISE-6).

The analysis considers only data from depths of less than 10 meters and during the growing season months, which spans June to September for stations in the Baltic Sea north of 59N and May to September 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).

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 (limits; Chl-a 69.3mg/l).

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’.

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 5 years of data are included in the analysis.

Methodology for gap filling

Not applicable.

Policy/environmental relevance

The analysis of chlorophyll-a values and their change over time is key to assessing progress towards improved marine and coastal water quality in line with EU policy objectives, such as those under the Marine Strategy Framework Directive (MSFD) and the Water Framework Directive (WFD). The WFD requires the achievement of good ecological status or the good ecological potential of transitional and coastal waters across the EU. The MSFD requires the achievement or maintenance of good environmental status in European sea basins and establishes, under descriptor D5 on eutrophication, a primary criterion (D5C2) to ensure that ‘chlorophyll concentrations are not at levels that indicate adverse effects of nutrient enrichment’. 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.

Other EU directives are also related to the control of eutrophication by aiming to reduce the loads and impacts of the nutrients, and include: the Urban Waste Water Treatment Directive aimed at reducing pollution from sewage treatment works and certain industries; the Nitrates Directive aimed at reducing nitrate pollution from agricultural sources and the Integrated Pollution Prevention and Control Directive aimed at controlling and preventing the pollution of water from industry. In addition, the EU Biodiversity Strategy 2030, Farm to Fork Strategy and Zero Pollution Action Plan are main policies under the EU Green Deal setting ambitious targets for reducing nutrients from agriculture.

EU policies and legislation also support the implementation of the Regional Seas Conventions and Action Plans (RSCAPs) — the Oslo Paris Convention (OSPAR), the Helsinki Convention (HELCOM), the Barcelona Convention (UNEP-MAP) and the Bucharest Convention, which also outline measures that aim to reduce the loads and impacts of nutrients.

Measurements of chlorophyll-a are used as a biological indicator of the direct effects of eutrophication. It is part of a set of indicators that focus on the state of and pressures acting on Europe's seas. In terms of impacts and pressures, this indicator is closely linked to the marine indicators 'Nutrients in transitional, coastal and marine waters’ and ‘Oxygen concentrations in European coastal and marine waters’. Eutrophication may also interact with other anthropogenic stressors such as overfishing or the introduction of invasive alien species, further impacting marine ecosystems and fisheries, in particular in a changing climate, and is thus also related to the indicators Ocean acidification, 'Status of marine fish and shellfish stocks in European seas', 'Marine non-indigenous species in Europe's seas', and 'Changes in fish distribution in European seas'.

This indicator assesses trends in Chlorophyll-a levels in Europe's 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 assessed for each location.

Accuracy and uncertainties

Methodology uncertainty

In the current application, two growing seasons are distinguished, one for the northern part of the Baltic Sea (June-September) and one for the southern part of the Baltic Sea, the North-East Atlantic, the Mediterranean Sea and the Black Sea (May-September). It is questionable whether using one growing season for all waters that range geographically from the Mediterranean and the Black Sea to the North Sea and the Baltic Sea is appropriate.

Geographical comparability

The natural ranges of chlorophyll-a values vary greatly depending on the geographical characteristics of monitored locations. Chlorophyll-a concentrations tend to be higher in coastal waters where nutrient concentrations are higher 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 chlorophyll-a values also vary greatly depending on the season. However, as assessments have been made on a yearly basis and only for summer months, 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

EU, 2008, Directive 2008/56/EC of the European Parliament and of the Council of 17 June 2008 establishing a framework for community action in the field of marine environmental policy (Marine Strategy Framework Directive), OJ L 164, 25.6.2008, p. 19-40.

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EU, 2017, 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, OJ L 125, 18.5.2017, p. 43-74.

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