MEPS 728:1-14 (2024)  -  DOI: https://doi.org/10.3354/meps14502

ABSTRACT: 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 ([O₂]) and δ¹⁸O 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 [O₂] and δ¹⁸O among all 3 seasons (r² = 0.97, p < 0.001, n = 10), suggesting the prevalence of fractionation in the process of O₂ 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 [O₂] in the near-bottom waters, but [O₂] 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, [O₂] was an essential control of SOR% and hence should be considered in the water column oxygen budget in coastal ecosystem modeling and management.

KEY WORDS: Oxygen consumption · Water column respiration · Sedimentary oxygen respiration · δ¹⁸O · Fractionation