Space-time variability of phytoplankton biomass, diversity and production over the last 27 years in the Mediterranean Sea

Phytoplankton dynamics in the Mediterranean Sea were investigated over the period 1998–2025 using a suite of Copernicus Marine Environment Monitoring Service satellite products, jointly analysing surface phytoplankton biomass (CHL), phytoplankton size classes (PSC), and integrated primary production (PP) alongside key physical drivers such as sea surface temperature (SST), photosynthetically available radiation (PAR), and mixed layer depth (MLD). Empirical Orthogonal Function (EOF) analysis was applied independently to each variable to characterize variable-specific modes of variability, and in a multivariate configuration (MEOF) to identify coherent modes of shared physical–biological variability. The dominant univariate EOF patterns capture the basin-scale annual cycle and reveal a light–temperature-driven control: PP peaks in summer in phase with PAR and SST, while CHL and MLD peak in winter, reflecting the nutrient–light trade-off characteristic of oligotrophic systems. Secondary univariate modes highlight the sub-basin spring bloom dynamics of the northwestern Mediterranean, associated with episodic deep convection in the Gulf of Lion and the ensuing nutrient supply, with biomass accumulation as the downstream biological response. The MEOF analysis identifies two physically coherent modes of coupled variability: a dominant annual light–temperature mode and a secondary spring-bloom mode linking winter mixing, spring restratification, and enhanced biological production.