Ocean acidification (OA) driven by increasing anthropogenic CO₂ is expected to impact the productivity of marine phytoplankton, yet the extent and even direction of these changes are not well constrained. Here we investigate the responses of phytoplankton community composition and productivity to OA across the (sub)tropical western North Pacific. Consistent reductions in primary production were observed under acidified conditions in the North Pacific Subtropical Gyre and the northern South China Sea, whereas no significant changes were found at the northern boundary of the subtropical gyre. Shifts in the abundance of small (< 20 µm) eukaryotic phytoplankton appeared to drive the overall community responses of primary production to OA, which was closely related to variations in nutrient supply. Extrapolating these results to global (sub)tropical oceans predicts a potential decrease of 5.1 Gt C yr^-1 in primary production in low Chl-a oligotrophic regions, which are anticipated to experience both acidification and stratification in the future.

Ocean acidification,eukaryotic phytoplankton,primary production,the (sub)tropical ocean