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.

夏季，富营养的大河羽会导致缺氧的近底水。但是，目前尚不清楚该分层水柱中的碳酸盐体系每天如何变化。在2015/2016年21世纪第一次严重的厄尔尼诺南方涛动事件结束时，在6 d内（7月24日）在南海北部陆架的珠江羽流中部观测到高温高盐度水。 – 2016年29日）。我们每小时在水柱上部署一个传感器包（电导率，温度，压力和溶解氧[DO]），并每3 h采集离散样品，包括总碱度和溶解的无机碳，以计算pH值。我们观察到pH降低率为0.011 pH单位·d ^-1和耗氧率为4.4μmolkg ^-1·d ^-1在接近底部的水中。计算的净群落生产率和过量溶解氧（测得的溶解氧-饱和溶解氧）的时间变化表明，在这种混合过程中，近地表水的净呼吸作用向净光合作用的转换。我们建议短期内净光合作用和净呼吸作用都在溶解度过高的水中。在这项研究中，pH降低和耗氧率可以帮助更好地估计沿海酸化和缺氧的水平。