Daily to weekly impacts of mixing and biological activity on carbonate dynamics in a large river-dominated shelf

Large eutrophic river plumes can lead to hypoxic near-bottom water during summer. However, how the carbonate system in this stratified water column varies at a daily-to-weekly scale is still unclear. At the end of the first severe El Niño Southern Oscillation event in the 21st century during 2015/2016, high temperature, high salinity water was observed in the middle of the Pearl River plume on the northern South China Sea shelf over 6 d (July 24–29, 2016). We deployed a sensor pack (conductivity, temperature, pressure, and dissolved oxygen [DO]) along the water column each hour and took discrete samples, including total alkalinity and dissolved inorganic carbon every 3 h, to calculate pH. We observe a pH reduction rate of 0.011 pH unit·d−1 and an oxygen consumption rate of 4.4 μmol kg−1·d−1 in the near-bottom water. The temporal variations in calculated net community production rate and excess DO (measured DO - saturated DO) implies the switch in the dominance of net respiration to net photosynthesis in the near-surface water during this mixing event. We suggest that both net photosynthesis and net respiration were in the water with oversaturated DO on a short-term scale. The pH reduction and oxygen consumption rates in this study could help to estimate the level of coastal acidification and hypoxia better. •We surveyed at a fixed station for 6 d at the end of ENSO 2015/2016 in summer.•High temperature and salinity water intruded the river plume on day 3.•Net respiration changed to net photosynthesis in the near-surface water on day 3.•The southwesterly monsoon was disturbed, and coastal upwelling was relaxed.•Bottom water continuously reflected a pH reduction and oxygen consumption.