Seasonal Variability of Phytoplankton Following Extreme Aerosol Events in the Western Subarctic Pacific

Phytoplankton growth in the western subarctic Pacific is strongly influenced by the supply of bioavailable iron (BFe), which is primarily regulated by atmospheric deposition that was most prominent in spring, weak in summer, and negligible in other seasons. The seasonal phytoplankton responses on these events were investigated using a coupled physical-biogeochemical model. In fall and winter, light limitation and BFe dilution caused by deep mixing suppressed phytoplankton growth following such events. However, as the mixed layer shallowed in spring, BFe concentrations in the upper ocean increased sharply, triggering a rapid increase in diatom biomass. In contrast, summer exhibited no significant enhancement of diatom biomass due to mesozooplankton grazing pressure, while picoplankton instead dominated the phytoplankton increase. These findings underscore the importance of accounting for local physical and biological factors when projecting the impacts of atmospheric deposition on marine biogeochemical cycles under global climate change.