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 post-2000. More areas are improving than declining in the Greater North Sea and the Black Sea, while the Baltic Sea displays a near balance in trends. Notably, over 95% of the assessed locations show no significant change. These findings highlight the need for ongoing efforts to improve monitoring and mitigate the risk of eutrophication amid a changing climate.

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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.

Naturally, chl-a levels fluctuate across different marine environments and seasons. However, 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. This creates 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 (WFD) and the Marine Strategy Framework (MSFD) directives. 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 2030 and the Farm-to-Fork strategies, and Zero Pollution Action Plan.

An analysis of the average summer chl-a concentrations across Europe's seas between 2018 and 2023 reveals areas with higher levels in the Baltic Sea and along coastal areas in the North Sea and Mediterranean Sea. The natural fluctuation of chl-a concentrations poses a challenge when setting uniform water quality standards across Europe's seas. Yet, examining ‘trends-at-locations’ offers a relative measure to evaluate the evolution of chl-a levels independent of their natural concentrations.

Trends in chl-a concentrations, analysed pre and post 2000 (the year of WFD implementation), show no significant changes across most locations. Pre-2000, most significant trends are seen in the Baltic Sea and the Greater North Sea. Post-2000, decreasing trends are mainly seen in areas with high average summer chl-a levels, around Denmark, the Kattegat and Northwest of Ireland, indicating improvements. Increasing trends are seen in the Baltic Sea, Bay of Biscay and several other coastal regions. Many locations lack sufficient data to determine trends.

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 (2000-2023)

Trends in chl-a concentrations vary regionally. In the Baltic Sea after 2000, the proportion of increasing versus decreasing trends is almost balanced (4% vs 5%). Yet, the Greater North Sea has more decreasing (10% vs 2%) and the Black Sea has only decreasing or no trends, which is positive. Despite variations, the number of time-series with decreasing trends surpasses those with increasing trends in all assessed regions (4% vs 3%).

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. This suggests that current measures to reduce nutrient inputs are somewhat effective but highlight the need for further action to maintain and improve water quality, particularly in problem areas.

While the spatial coverage of this analysis has expanded, particularly after 2000, many areas still lack comprehensive coverage. This limits broad assessments and highlights the need for enhanced and consistent monitoring efforts to maintain regular time series data for more effective management.