Frontal dynamics and phytoplankton decoupling in the Alborán Sea from high-resolution satellite data

The Alborán Sea, in the westernmost Mediterranean, is dominated by oceanic fronts that sustain high primary production through strong physical–biological coupling. Whether this linkage is changing under current warming trends remains poorly understood. Here, we analyse the seasonal and interannual variability of sea surface temperature (SST, 2008–2024) and chlorophyll- a (chl- a , 1998–2024) fronts using 1-km resolution satellite data. Applying the Canny Edge Detector algorithm, we derived two complementary metrics: frontal gradient (i.e., sharpness) and frontal probability (i.e., detection frequency). SST-derived frontal metrics exhibit a consistent seasonal cycle and have intensified over time, whereas chl- a concentration and frontal features show more variable seasonal patterns and long-term declines. This divergence indicates a progressive decoupling between physical frontal structures and phytoplankton response in the Alborán Sea. We suggest that increasing stratification and shifts in bloom phenology may limit the capacity of surface fronts to sustain phytoplankton growth, even where frontal activity persists. These findings provide early evidence of ecosystem vulnerability and underscore the importance of analysing physical and biological dynamics together at fine scales. • Long-term SST and chlorophyll fronts were mapped using 1-km satellite data. • SST-derived frontal gradients and probability increased in the Atlantic Jet system. • Chlorophyll-a concentration and chlorophyll-derived frontal probability declined. • Divergent SST- and chlorophyll-front trends indicate weakening coupling at the surface.