Clarifying Water Column Respiration and Sedimentary Oxygen Respiration Under Oxygen Depletion Off the Changjiang Estuary and Adjacent East China Sea

The Changjiang Estuary and its adjacent East China Sea are among the largest coastal hypoxic sites in the world. The oxygen depletion in the near-bottom waters (e.g., meters above the seabed) off the Changjiang Estuary is caused by water column respiration (WCR) and sedimentary oxygen respiration (SOR). It is essential to quantify the contributions of WCR and SOR to total apparent oxygen utilization (AOU) to understand the occurrence of hypoxia off the Changjiang Estuary. In this work, we analyzed the delta O-18 and O-2/Ar values of marine dissolved gas samples collected during a field investigation in July 2018. We observed that the delta O-18 values of dissolved oxygen in near-bottom waters ranged from 1.039 to 8.457 parts per thousand (vs. air), generally higher than those of surface waters (-5.366 to 2.336 parts per thousand). For all the sub-pycnocline samples, the delta O-18 values were negatively related to O-2 concentrations (r(2) = 0.97), indicating apparent fractionation of delta O-18 during oxygen depletion in the water column. Based on two independent isotope fractionation models that quantified the isotopic distillation of dissolved oxygen concentration and its delta O-18, the mean contributions of WCR and SOR to total near-bottom AOU were calculated as 53 and 47%, respectively. Beneath the pycnocline, the WCR contribution to the total AOU varied from 24 to 69%, and the SOR contribution varied from 31 to 76%. The pooled samples beneath both the pycnocline and upper mixed layer indicated that WCR contributions (%) to total AOU increased with increasing AOU (mu mol/L), whereas SOR% - AOU had the reverse trend. We propose that the WCR% and SOR% contributions to the total AOU of the sub-pycnocline waters are dynamic, not stationary, with changes in ambient environmental factors. Under hypoxic conditions, we observed that up to 70% of the total AOU was contributed by WCR, indicating that WCR is the major oxygen consumption mechanism under hypoxia; that is, WCR plays a vital role in driving the dissolved oxygen to become hypoxic off the Changjiang Estuary.

Automated summary

The study revealed that oxygen depletion in the near-bottom waters off the Changjiang Estuary is caused by water column respiration and sedimentary oxygen respiration, with contributions of 53% and 47% respectively to the total apparent oxygen utilization. Below the pycnocline, the contribution of water column respiration to the total AOU varied from 24% to 69%, while sedimentary oxygen respiration varied from 31% to 76%.