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.

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

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).

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

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.