Impacts of Typhoon on Phytoplankton and CDOM in the South China Sea Revealed by BGC-Argo Floats and Satellite Composite Analysis

The impacts of typhoons on phytoplankton have been studied for decades, but most studies have relied on satellite-derived sea-surface chlorophyll (Chl), which is now recognized as an inadequate proxy of phytoplankton biomass. Moreover, typhoon effects on colored dissolved organic matter (CDOM) remain poorly understood. Using Biogeochemical Argo (BGC-Argo) float observations from two typhoon cases and 20-year satellite measurements from 235 typhoons (2003–2022), we analyzed phytoplankton and CDOM responses to typhoons in the South China Sea. Our results demonstrated the vertical redistribution process, driven by mixing entrainment or typhoon-induced upwelling, rapidly elevated surface Chl and CDOM by transporting subsurface high-concentration waters upward. Phytoplankton growth lagged 3–5 days due to light limitation until clear skies returned, and surface biomass increased by only ∼14.5%, lower than the surface Chl increase (∼20%). The variance of sea-surface Chl was dominated by vertical redistribution and photoacclimation, largely decoupled from biomass. These results underscore that studies relying on surface Chl substantially mischaracterized the response dynamics of phytoplankton. Moreover, CDOM exhibited a secondary peak following the typhoon passage, coinciding with maximum biomass response, indicating newly produced CDOM from the phytoplankton bloom. Future work should focus on quantifying the contributions of various processes and refining response periods by incorporating additional BGC-Argo data and improving synergy between in situ observation and marine biogeochemical models.