Mediterranean Phytoplankton Diversity Shifts Observed Over 25 Years

Phytoplankton Functional Types (PFTs) play distinct roles in marine biogeochemical cycles, and shifts in their relative and absolute abundances can have implications for carbon export, the silica cycle, sulfur-based climate feedbacks, and nitrogen fixation. The Mediterranean Sea, an oligotrophic basin undergoing rapid environmental change, provides a valuable setting to examine long-term trends in phytoplankton community structure. This study analyzes 25-year trends in the absolute and relative chlorophyll-based abundances of PFTs of interest to the scientific community across the open ocean surface waters of the Mediterranean Sea.The updated Copernicus Marine Service PFT retrieval algorithm was applied to the gap-free Level-4 chlorophyll concentration product (2000–2025, 1 km resolution). Trends were assessed across nine trophic sub-regions of the basin, considering both bulk chlorophyll concentration and the absolute and relative abundances of individual PFTs, as well as a chlorophyll-derived Shannon diversity index. Bulk chlorophyll concentration declined across the entire basin, accompanied by a general shift from larger to smaller organism size classes. No significant trend was detected in the Northern Adriatic Sea, likely due to spatial averaging over a highly heterogeneous region containing areas with differing, potentially opposing tendencies. In absolute terms, all PFTs showed negative trends in chlorophyll concentration, ranging from approximately 0 to −0.087µg · m−3 · year−1, with specific rates strongly dependent on local trophic conditions. In relative terms, Diatoms and Green Algae showed the most negative trends, while Prokaryotes showed the highest positive rates of increase. The chlorophyll-derived Shannon index of PFT relative abundances decreased moderately but consistently across the basin (between -0.07 and -0.138 ×10−3 year−1). Still recognizing the limitations of optical remote sensing, both in terms of its limited taxonomic identification and reduced water depth sensitivity, the observed changes in surface assemblage structure may be consistent with a weakening of the biological carbon pump strength and of the silica cycle, driven by declining Diatoms, though direct confirmation would require independent flux and export measurements. Regionally divergent trends in Haptophytes and Dinophytes may be consistent with variable DMS emission potential, with possible implications for aerosol formation and climate feedbacks. Finally, the increasing abundance of pico-cyanobacteria within the Prokaryote group could modestly favor nitrogen-fixation potential, an effect likely constrained by the phosphorus-limited nature of the Mediterranean Sea.