Chlorophyll in Europe's transitional, coastal and marine waters

Chlorophyll-a concentrations, a key indicator of ocean health, reveal mixed trends across Europe's marine regions. Assessments show improvements in some critical areas such as the Kattegat Strait and Northwest of Ireland during 1980 to 2021. More areas are improving than declining in the Greater North Sea, while the Baltic Sea displays a near balance in trends. Notably, over 95% of the locations assessed show no significant change. These findings highlight the need for ongoing efforts to mitigate the risk of eutrophication in the face of a changing climate.

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

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

While it is natural for chl-a levels to fluctuate across different marine environments and seasons, persistently high concentrations may signal large phytoplankton blooms. Eutrophication, spurred by excessive nutrient enrichment from both human and natural sources, can trigger harmful algal blooms, creating oxygen-deficient zones in near-bottom waters, leading to ecosystem degradation and biodiversity loss.

Monitoring changes in chl-a values over time is key for assessing progress towards improved water quality in line with EU policy objectives, such as the Water Framework Directive and the Marine Strategy Framework Directive. These directives aim to achieve ‘good ecological status’ and ‘good environmental status’ of Europe’s waters, respectively. The European Green Deal supports this by introducing ambitious targets for reducing nutrient use in agriculture and losses into the environment, outlined in key policies including the EU Biodiversity Strategy 2030, Farm-to-Fork Strategy and Zero Pollution Action Plan.

The natural fluctuation of chl-a concentrations poses a challenge when setting uniform water quality standards across Europe's seas. However, examining ‘trends-at-locations’ offers a relative measure to evaluate the evolution of chl-a levels independent of their natural concentrations.

An analysis from 1980 to 2021 identified significant trends at 264 locations, with 150 displaying decreases. Many of these 150 locations are in areas with high average summer chl-a concentrations such as around Denmark, the Kattegat Strait and Northwest of Ireland, indicating improvements (Figure 1). Conversely, 114 locations, many of which also reveal high average concentrations (>6ug/l), see increasing trends, notably along the Swedish and Finnish coasts in the Baltic Sea and areas in the Greater North Sea. The vast majority, 4,529 locations, ranging from 83% to 97% in different marine regions, show no significant change. The remaining locations lack sufficient time series data to determine trends (Figure 1).

Trends in chl-a concentrations vary regionally. In the Baltic Sea, the proportion of increasing versus decreasing trends is almost balanced (2.6% increasing vs 2.8% decreasing), while the Greater North Sea has more decreasing (10.7% vs. 6.8%). Despite variations, the number of locations with decreasing trends surpasses those with increasing trends in all assessed sea regions (3.1% vs 2.4%) (Figure 2).

These findings largely align with those found for two other eutrophication indicators analysed over the same period: nutrient and oxygen concentrations in Europe's seas, suggesting that current measures to reduce nutrient inputs are somewhat effective but underscore the need for further action to maintain and improve water quality, particularly in problem areas.

While the spatial coverage of this analysis has expanded, including new coastal segments primarily in the North-East Atlantic Ocean, some areas, particularly in the Mediterranean and Black seas, still lack comprehensive coverage. This limits broad assessments and underscores the need for enhanced and consistent monitoring efforts to maintain regular time series data for more effective management.

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 1980-2022, in European 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 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 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 (limits; Chl-a 69.3mg/l). 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 as the arithmetic mean of the annual means per station and cluster. Initial calculations of average annual concentrations are done by location and by day to standardise observations across different depths and time (hours or even minutes in some cases). These daily averages are subsequently aggregated by year and location.

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 using the non-parametric Mann-Kendall test. For inclusion in the analysis, stations must have data spanning at least five years post-2006 and must have recorded data in the most recent six-year period. A p-value of <0.05 is deemed to reflect a statistically significant trend.

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'.

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.

A station-based analysis may not be the most appropriate approach. An assessment area analysis approach by regional sea where assessment areas have already been identified would provide more consistent data and allow for the use of agreed threshold levels for chlorophyll-a by the regional conventions, as is currently assessed for example by HELCOM and OSPAR.

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 and have been updated within the last six-year period (2016-2021).

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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Chlorophyll in Europe's transitional, coastal and marine waters

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

Figure 2. Trends in chlorophyll-a concentrations in Europe's seas (1980-2021)

Supporting information

Metadata

References and footnotes