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Chlorophyll-a, a key indicator of marine primary production and water quality, has shown mixed trends since 1980. While declines in areas, noticeably in the Kattegat Strait and Northwest of Ireland, indicate some improvement, most locations (94.5%) show no significant change. Where trends are significant, improved conditions outnumber declining ones, particularly in the Greater North Sea (10.7% improving vs. 6.8% declining) compared to the Baltic Sea (2.6% improving vs 2.8% declining). Despite some progress, ongoing efforts and better monitoring are essential for more comprehensive assessments.
Chlorophyll-a (chl-a) is an indicator of phytoplankton biomass and reflects the level of primary production in marine waters in response to nutrient and light availability. The excessive enrichment of waters by nutrients (either from anthropogenic or natural sources) leads to eutrophication, which can cause ecosystem degradation, harmful algae blooms, oxygen deficiency in bottom waters and loss of biodiversity. The analysis of chl-a values and their change over time is key to assessing progress towards better marine and coastal water quality, in line with the EU's integrated approach to reducing the input of nutrients to aquatic systems to reduce the risk of eutrophication and achieve good status for marine waters. The Marine Strategy Framework Directive (MSFD) and the Water Framework Directive (WFD) set targets in relation to nutrient levels and under the European Green Deal, the EU biodiversity strategy 2030 , Zero Pollution Action Plan and Farm to Fork Strategy further set ambitious targets for reducing nutrient loses from agriculture. At the international level, the Regional Sea Conventions, i.e., the Oslo-Paris Convention (OSPAR), the Helsinki Convention (HELCOM), the Barcelona Convention (UNEP-MAP) and the Black Sea Convention (Bucharest Convention), each outline measures aimed at reducing nutrient loads and impacts.
Average values of chl-a naturally vary substantially across transitional, coastal and offshore water bodies, as well as regional seas and seasons. This makes it difficult to set absolute values for boundary conditions in water quality that are applicable at European scale. Nonetheless, changes in concentrations over time in each geographical position (‘trends-at-locations’), is a relative measure that allows assessing the evolution of chl-a across European waters, independent of their natural chl-a values.
A trend analysis of chl-a concentrations by location in European waters between 1980 and 2021, identified significant trends at 264 locations. Of these, 150 locations, most noticeably in the Kattegat Strait and around North-Western Ireland, revealed decreasing trends (i.e., improving conditions) while the remaining 114 locations showed
increasing trends. All other 4,529 locations (94.5% of cases), showed no significant trend.
These results are coherent with those found for nutrients in European waters suggesting that measures introduced to reduce nutrient inputs to transitional and coastal ecosystems have been effective to a certain extent, but more action is needed.
|Total number of time series
|Greater North Sea
|IBI* and Celtic Seas
Trends in chl-a vary across locations in Europe’s regional seas. Most locations show no significant trends, ranging between 82.5- 97.1% by marine region. Increasing trends are still observed in several locations in all regions except the Black Sea, where a limited number of observations were available. Most of the significant trends are found in the Baltic, Greater North and Celtic Seas regions.
The share of increasing and decreasing trends differs between regions, with almost equal amounts (2.6% vs 2.8%) found in the Baltic Sea and a higher proportion of decreasing trends in the Greater North Sea (10.7% vs. 6.8%), suggesting that the level of eutrophication is dependent on local conditions. Nonetheless, the number of locations with decreasing trends exceeded the number of locations with increasing trends in all sea regions.
While the spatial coverage of the analysis has improved, including several new coastal segments mainly in the North-East Atlantic and Macaronesia, the coverage in some regions, such as parts of the Mediterranean and Black Seas, remains limited. These gaps continue to impede widespread assessments and more effort is needed to maintain and promote monitoring programmes with regular time series.