Quantifying sedimentary oxygen consumption contribution to total oxygen depletion in shallow waters via the oxygen isotope approach

To understand dissolved oxygen depletion in estuaries and coasts, it is essential to distinguish the contributions of diffusion-mediated sedimentary oxygen respiration (SOR) and water column respiration (WCR). For the present study, near-bottom waters were sampled in May, July, and October 2019 in Dinghai Bay, China, to reveal the oxygen depletion mechanism in shallow nearshore waters (3-20 m). Field observations of oxygen isotopes and endmember incubations were conducted. Dissolved oxygen concentration ([O 2 ]) and δ 18 O ranged from 157 to 241 μM and -1.34 to 3.41‰, respectively. Except for one outlier value, -1.34‰ (site TT1), there was a strong negative relationship between [O 2 ] and δ 18 O among all 3 seasons (r 2 = 0.97, p < 0.001, n = 10), suggesting the prevalence of fractionation in the process of O 2 consumption. Given the contrasting fractionation effect during SOR (fractionation factor, ε = -3.57‰) and WCR (ε = -18.58‰), the contributions of SOR and WCR to total oxygen depletion were quantified via a respiration-only model or photosynthesis-respiration model (TT1), respectively. The SOR% range was 18-96% (mean 73%), with the remaining contributed by WCR%. There was no clear relationship between SOR% and temperature or depth or [O 2 ] in the near-bottom waters, but [O 2 ] was the best indicator for SOR% when data was pooled with results from deeper waters (20-50 m) in the East China Sea. For shallow waters <20 m in Dinghai Bay, [O 2 ] was an essential control of SOR% and hence should be considered in the water column oxygen budget in coastal ecosystem modeling and management.