How do fronts affect phytoplankton communities over oceanic regions? Contrasting responses across trophic regimes, seasons and front diagnostics

Abstract. Oceanic fronts are key features structuring marine ecosystems, yet their influence on phytoplankton communities remains difficult to quantify at regional scale. In particular, how phytoplankton responses to fronts vary across environmental conditions is still poorly understood, and may depend on the way fronts are detected. Here, we investigate how fronts affect phytoplankton biomass and community composition across the Mediterranean Sea, a region characterized by strong seasonal variability and contrasted trophic regimes. Using 23 years (1998–2021) of satellite-derived chlorophyll-a and pigment-derived phytoplankton groups data, we quantify phytoplankton responses to fronts across seasons and biogeochemical provinces, from productive northwestern regions to ultra-oligotrophic eastern basins. Fronts are identified using either the measure of an heterogeneity index (HI) or of Finite-Size Lyapunov Exponents (FSLE), applied, in both cases, to different data sources (satellite data or high-resolution fields reconstructed from machine-learning). This approach allows us to assess the sensibility of observed ecological responses to the choice of front detection method. We show that fronts are consistently associated with enhanced phytoplankton biomass by 4 % to 35 % and reshaped community composition across the Mediterranean Sea, with the strongest effects observed in oligotrophic regimes and during stratified periods. In particular, fronts are associated with an enrichment in diatoms by up to 20 % and a relative decrease in prokaryotes, suggesting enhanced nutrient supply. However, the magnitude and consistency of these responses vary depending on the diagnostic used, with weaker to no signals detected using FSLEs. These results demonstrate that phytoplankton responses to fronts are both environmentally controlled and method-dependent. They highlight the key role of fronts in structuring ecosystems in nutrient-limited regions, and support the idea that changes in oceanic fronts should be considered when investigating ecological reorganization under future climate change scenarios, particularly in climate change hotspots such as the Mediterranean Sea.