Sedimentary and water column contributions to oxygen depletion considering mixing in a dynamic coast

Abstract

Estuarine and coastal hypoxia is a worldwide problem and its occurrence and development mechanism attract great public and scientific attention. The δ¹⁸O approach is a useful option in revealing the mechanism, but the mixing interference on distinguishing water column respiration (WCR) and sedimentary oxygen respiration (SOR) remains a challenge. We conducted a survey south off the Changjiang Estuary, using dissolved oxygen concentration ([O₂]), dissolved oxygen δ¹⁸O, and shipboard respiration incubations to address this question. We observed changes in [O₂] and δ¹⁸O over two days (30m: +14.6 μmol L⁻¹ day⁻¹ and -0.57 ‰ day⁻¹, 40–50m: −24.8 μmol L⁻¹ day⁻¹ and +1.73 ‰ day⁻¹). These changes were decomposed into five budget items based on the respective concentration-(¹⁶O¹⁶O) and isotope-(¹⁸O¹⁶O) box models: WCR, SOR, shallow mixing, deep mixing, and lateral advection. On day 2, the SOR∗% assessed by multiple approaches was 61 % for 50m, comparable to the solo δ¹⁸O-approach assessed SOR% (64 %), but at 30m the SOR∗% (37 %) was significantly lower than SOR% (54 %), corresponding to a stronger mixing ratio at 30m (1.5) relative to 50m (0.6). Our results showed that WCR was the main cause of oxygen depletion at middle layer contributing 63 % of AOU, whereas for near-bottom waters SOR's role instead was more significant (61 %). This research contributes to a more detailed understanding of coastal oxygen budget and depletion, and provides scientific base for coastal hypoxia management and environmental policy making.