Abstract
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 δ18O and O2/Ar values of marine dissolved gas samples collected during a field investigation in July 2018. We observed that the δ18O values of dissolved oxygen in near-bottom waters ranged from 1.039 to 8.457‰ (vs. air), generally higher than those of surface waters (−5.366 to 2.336‰). For all the sub-pycnocline samples, the δ18O values were negatively related to O2 concentrations (r2 = 0.97), indicating apparent fractionation of δ18O 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 δ18O, 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 (μmol/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.
Highlights
Dissolved oxygen is essential and of great importance for most marine habitats
The mean water column respiration (WCR)% that contributed to the total apparent oxygen utilization (AOU) beneath the pycnocline was 53%, and the remaining 47% was contributed by sedimentary oxygen respiration (SOR)
Given the horizontal (Figure 11) and vertical patterns (Figure 12) of WCR% and SOR%, we propose dynamic, instead of fixed, oxygen consumption mechanisms of WCR and SOR in the near-bottom waters beneath the pycnocline (Figures 10A,B, 11) that cause hypoxia off the Changjiang Estuary and adjacent East China Sea (ECS) (Figure 13): In the stratified water column, the SOR contribution to the total bottom AOU is a background-like, chronic process that exists at all times
Summary
Dissolved oxygen depletion in estuarine and coastal near-bottom waters is worsening worldwide (2001), causing negative effects on the structure of marine ecosystems and their biodiversity (Nilsson and Rosenberg, 1994; Crain, 2007). The water column is usually well stratified in summer, with hightemperature and low-salinity Changjiang diluted water at the surface and high-salinity Kuroshio-originated water prevailing at the bottom (Su, 1998).
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