Surface Chlorophyll-a Trends in the Mediterranean Sea: Seasonal Bloom Dynamics and Drivers

Abstract

Chlorophyll-a (Chl-a) is the most widely used proxy for phytoplankton biomass, bloom intensity, and trophic status in marine ecosystems. We analyzed 27 years (1998–2024) of 1 km-resolution satellite-derived Chl-a data across the Mediterranean Sea and evaluated long-term trends, bloom phenology, and seasonal variability in relation to sea surface temperature, mixed-layer depth, and nutrient dynamics. A Mediterranean-specific Chl-a algorithm was also used to accurately represent both open and optically complex coastal waters. Before assessing trends, we compared three central-tendency metrics to account for the right-skewed nature of Chl-a distributions. The geometric mean best represents typical conditions by reducing the influence of extreme values and was therefore selected as it enhances the robustness of basin-scale trend detection. Trend analysis revealed a basin-wide decline in surface Chl-a over the 27-year study period, with negative trends across all seasons and localized increases in some coastal areas. The longer satellite record reconciles previously reported spatially heterogeneous trends. The clustering of Chl-a variability delineated a gradient from productive, seasonally dynamic regions to persistently oligotrophic regimes. Regionally, the spring bloom magnitude declined in the Gulf of Lion (northwestern Mediterranean) and the southern Adriatic Sea, whereas biomass in the Levantine Sea (eastern basin) remained persistently low. Overall, these patterns suggest a shift toward increased oligotrophy, as warming-induced stratification suppresses nutrient resupply. By combining long-term satellite observations, robust Chl-a statistics, and phenological analysis, this study provides an update and comprehensive assessment of phytoplankton variability and its drivers in the Mediterranean Sea.