Daily-to-weekly Impacts of Mixing and Relaxed Upwelling on the Carbonate Dynamics at a Large-River Dominated Shelf, Summer 2016

A eutrophic large river plume can usually lead to hypoxic near-bottom water during summer. But how the carbonate system in this stratified water column is affected by an intrusion of offshore surface water and a relaxed upwelling event is still unclear. By the end of the first strong ENSO event in the 21 century during 2015/2016, an abnormal high-temperature near-surface water was observed to intrude into the Pearl River plume on the northern South China Sea shelf in our six-day fixed station (July 24-29, 2016). We deployed a sensor pack (conductivity, temperature, pressure, and dissolved oxygen [DO]) along the water column every hour and took discrete samples, including total alkalinity, dissolved inorganic carbon, and pH, every three hours over the study period. We observed a pH reduction rate and an oxygen consumption rate in the near-bottom water during the relaxation of upwelling. The daily change of calculated net community productivity rate and excess DO (measured [DO]-saturated [DO]) both imply the switch of the dominance of respiration to photosynthesis in the near-surface water during our study period. The carbonate buffer capacity decreased in the near-surface water during this offshore water intrusion. We suggest that this offshore water intrusion can reduce the resistance to atmospheric CO₂ invasion in the near-surface water. The result of this daily-to-weekly study can help to better estimate the short-term temporal variations in coastal acidification and hypoxia.