Abstract This study investigates two decades of surface biogeochemical change in the Mediterranean Sea (2001–2020) using the high-resolution Mediterranean biogeochemical reanalysis. This study focuses on monthly to decadal variations in the carbonate system, nutrients, and phytoplankton indicators, and includes a detailed assessment of coastal processes near the Po River mouth as one region with the highest variability in biogeochemical parameters. Basin-wide, surface dissolved inorganic carbon (DIC) and total alkalinity (TAlk) increased between 2001 and 2010 and 2011–2020 (DIC: 2277.78 to 2292.32 µmol/kg; TAlk: 2610.52 to 2623.47 µmol/kg), while surface partial pressure of CO₂ (spCO₂) rose from 41.23 to 42.72 Pa (+ 3.6%). These changes reflect enhanced CO₂ uptake and ongoing shifts in water-mass properties under a warming and stratifying climate. From 2011 to 2020 compared to 2001–2010, nutrient concentrations showed moderate declines (NO₃ −0.72%, NH₄ −1.16%, PO₄ −4.7%), accompanied by small decreases in chlorophyll-a (− 2.7%) and phytoplankton carbon (− 4.5%), consistent with basin-scale oligotrophication. The Po mouth analysis highlights the river’s regulatory role: discharge correlates positively with chlorophyll, NO₃, NH₄, PO₄, and phytoplankton carbon, but negatively with spCO₂, revealing nutrient-driven productivity pulses that locally enhance CO₂ drawdown. Overall, the Mediterranean surface layer suggests a possible dual behavior of carbon enrichment and nutrient depletion, reflecting a shift toward warmer, more stratified, and increasingly oligotrophic conditions. Graphical Abstract
Javad Babagolimatikolaei (Fri,) studied this question.