Abstract

In the occurrence of hypoxia in the estuary offshore, the change of the contribution rate of sediment oxygen consumption to hypoxia is the scientific basis for the study of hypoxia mechanism, reasonable prediction and control of hypoxia. The observation of oxygen isotopes in the estuary area can reveal the regulation mechanism of the oxygen consumption process of respiration. As a unique respiration tracer, 18O can well and effectively reflect the relative change relationship between the water column and sediment respiration in the estuary area. In this study, the Aojiang Estuary and adjacent tidal flat waters in Lianjiang County, Fujian were investigated in May, July and October 2019, and the DO concentration and DO-δ18O value in the near bottom water were collected and analyzed. The results show that [O2] was the highest in spring, with δ18O values ranging from 0.58‰ to 0.72‰, δ18O values increased significantly in summer from 2.09‰ to 3.41‰, and in autumn, δ18O values ranged from 0.81‰ to 1.23‰. There was a good negative correlation between [O2] and the corresponding δ18O of all samples in the three seasons of Aojiang Estuary (r=0.34, p<0.01, n=18), suggest the fractionation during respiratory oxygen consumption among all seasons during our investigations. The apparent fractionation factors (εapp) in the three seasons of the Aojiang Estuary tidal flat ranged from -15.85‰ to -0.51‰, which were in the range of water column oxygen consumption fractionation endmember (-18.58‰) and the sediment oxygen consumption fractionation endmember (-3.57‰). Based on the pure respiration model, we quantified the contribution of sediment oxygen consumption (SOR) to the near-bottom water oxygen consumption process relative to water column respiration, which ranged from 12.3% to 81.4%. Our findings are in good agreement with the findings in the hypoxic region of the Yangtze Estuary of previous work. In addition to the [O2] of the near bottom waters, the water depth (m) in the shallow water system of the tidal flat is another main factor affecting the SOR contribution in the near bottom waters.