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Chlorophyll-a is an indicator of primary production in response to nutrient and light availability in marine waters and is key to assessing water quality. Mean concentrations of chlorophyll-a reflect the natural differences between low/mid/high biological productivity in marine regions. Trends at locations show the development of chlorophyll-a independent of the magnitude. Between 1980 and 2019, concentrations declined overall, noticeably in the Greater North and Celtic Sea regions. Declining trends show the efficiency of policy measures to reduce nutrients in these areas, however, data gaps continue to hamper widespread assessments. Significant trends were found at only 9.4% of the locations (85 decreasing and 32 increasing).
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 loss of biodiversity, ecosystem degradation, harmful algae blooms and oxygen deficiency in bottom waters. The analysis of chlorophyll-a values and their change over time is key to assessing progress towards better marine and coastal water quality, in line with policy objectives. For example, the Marine Water Framework Directive (MSFD) aims to achieve ‘good environmental status of the EU’s marine waters’ and the Water Framework Directive aims to achieve ‘good ecological status’. The EU biodiversity strategy 2030 and Farm to Fork are the main policies under the EU Green Deal that further set ambitious targets for reducing nutrients from agriculture.
Average values of chlorophyll-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 chlorophyll-a across European waters, independent of their natural chlorophyll-a values.
Trends in chlorophyll-a concentrations by location in European waters between 1980 and 2019 revealed no statistically significant trends at 1,125 out of the 1,242 locations selected. However, at 32 locations, mostly in coastal and transitional waters in the Baltic Sea, North Sea and Celtic Seas, significant increasing trends were identified, while at 85 locations, mostly in the Kattegat and the south-western North Sea and in the Celtic Sea and western Mediterranean Sea, significant decreasing trends were found.
These results indicate that trends in chlorophyll-a concentrations are similar to trends reported for nutrients in European waters. This suggests that measures introduced to reduce nutrient inputs to transitional and coastal ecosystems have been effective to a certain extent, resulting in decreases in nutrient concentrations in some areas and changes in the dynamics of chlorophyll-a concentrations.
Trends in chlorophyll-a vary across locations in Europe’s regional seas. Increasing trends are still observed in several locations in the Baltic Sea and in a few locations in the Greater North and Celtic seas. Meanwhile, other locations in these regional seas show decreasing trends. For example, the number of locations with decreasing trends in the Kattegat and the southern coasts of the North Sea exceeds the number with increasing trends. The same is true for the Celtic Seas. Likewise, one location in the Alboran Sea, one in the western Mediterranean Sea and another in the eastern Mediterranean Sea showed decreasing trends, however, it was not possible to assess trends for many locations in the Mediterranean Sea (mostly in the central and eastern subregions) due to insufficient data. Although, none of the 14 locations assessed in the Black Sea showed any significant trend, the region is highly eutrophic and it is very likely that chlorophyll concentrations remain high and are not showing an improving trend. Trends were also not available for the Outer Atlantic and Macaronesia regions. More effort is needed to maintain and promote monitoring programmes with stations where regular time series data are collected.