Seasonal and Daily‐Scale Photoacclimation Modulating the Phytoplankton Chlorophyll‐Carbon Coupling Relationship in the Mid‐Latitude Northwest Pacific

Photoacclimation is a photo‐physiological mechanism of phytoplankton associated with the modulation of the ratio of phytoplankton chlorophyll (Chl) and carbon (C), yet little is known on how it modulates Chl dynamics and the Chl‐C coupling relationship in the mid‐latitude oceans. In this study, Chl and C estimated from optical sensors on three BGC‐Argo floats, provided a record of the biomass and physiological variability at high temporal and vertical resolutions in the mid‐latitude northwest Pacific. It is found that, despite the Chl seasonal cycle being mainly dominated by variability in biomass, Chl and C became decoupled and inversely correlated in winter, due to the dominant effect of photoacclimation. Although Chl and C co‐varied in spring and autumn, photoacclimation modulated the Chl‐C coupling relationship as well, leading to a faster increase in C and Chl, respectively, in spring and late autumn. On daily and synoptic scales, Chl dynamics were also driven by photoacclimation to a large extent. Particularly, during a typhoon event, both cloud cover and mixed‐layer dynamics introduced fast changes in mixed‐layer light level, resulting in quick photo‐physiological responses of phytoplankton in modulating the variances of Chl. Similarly, during wintertime mixing‐restratification events associated with abrupt mixed‐layer changes, photoacclimation muted the Chl response, leading to a faster increase in C than Chl.