Decoupling of high-resolution surface pH and DO reveals temporal algal bloom dynamics on the East China Sea.

ABSTRACT

The stoichiometric ratio between seawater CO2 and dissolved oxygen (DO) during phytoplankton metabolism holds significant importance in evaluating ecological and biogeochemical processes. We collected high-resolution underway temperature, salinity, DO, and pH data in the East China Sea inner shelf in May 2017. Our results revealed high pH (8.36) and supersaturated DO (171 %) in the outer Changjiang Estuary, indicating the occurrence of an algal bloom event. They were significantly correlated with a regression slope of 0.0029, which roughly followed the Redfield ratio. In contrast, a much higher ratio (0.0088) manifested in a low-salinity patch on north of the Changjiang Estuary, featuring a pH of 8.40 and oxygen saturation of approximately 123 %. The substantially faster air-sea equilibrium rate of O2 than CO2 probably caused such decoupling, offering insight into the temporal evolutions of algal bloom. Theoretically, a steeper regression slope implies an earlier onset of algal bloom. An end-member mixing model was constructed to exclude the physical mixing influences on dissolved inorganic carbon (ΔDIC) and DO (ΔDO). Furthermore, we conducted simulations to explore the temporal variations of ΔDIC-ΔDO regression slope with time. Comparing slopes derived from simulation and mixing model suggested that the biological signal of the decoupled waters likely preceded our observations by 6-10 days. Satellite results captured high-Chl a waters southwest of the low-salinity patch a week before our observation, potentially transported northward by prevailing southwest wind. Given that oxygen and pH are frequently measured in aquatic environments, their combined assessment could be a valuable method for assessing temporal algal bloom dynamics.