Exploring origin of oxygen-consuming organic matter in a newly developed quasi-hypoxic coastal ocean, the Bohai Sea (China): A stable carbon isotope perspective

Oxygen Deficit Area Spatial-temporal Heterogeneity in Bohai Sea: Formation and their Drivers

“…At present, the oxygen deficit degree of stations A2 and A4 is slightly lower than that of other hypoxic areas in the world, but if the current trend continues, the oxygen deficit degree of stations A2 and A4 will be further aggravated. Currently, both horizontal and vertical transport in the oxygen-depleted region of the Bohai Sea have been studied (Zhao et al, 2017;Zhang et al, 2022). This study further supplements the control mechanism of vertical transport on bottom layer DO, indicating that the intensity of stratification, the duration of stratification and the onset time of stratification enhancement jointly affect the oxygen deficit in the bottom layer.…”

“…These results indicate that the occurrence of stratification impedes the vertical transport of DO from the surface to the bottom (Zhao et al, 2017). Moreover, previous studies have shown that water column stratification often occurs before hypoxia (Zhang et al, 2022). Therefore, the occurrence of stratification is not only an important physical condition but also a prerequisite for the occurrence of oxygen deficit.…”

“…Apparent oxygen consumption (AOU) is generally considered to be related to community metabolism (Zhai et al, 2019), assuming that water starts from a fully saturated state and ignoring the effects of air-ocean exchange and water mixing. AOU > 0 indicates a dissolved oxygen deficit in the water column, and AOU < 0 indicates supersaturation of dissolved oxygen in the water column (Chen et al, 2022). AOU is calculated by Eq.…”

“…This means that when the degree of oxygen deficit is the highest, the corresponding stratification strength is not the strongest, which means that when the stratification strength reaches a certain strength, even though it continues to increase, the underlying oxygen deficit degree does not necessarily continue to decrease. In particular, Zhang et al (2022) showed that the longer the duration of the pycnocline, the lower the DO concentration in the bottom layer.…”

“…Stratification is not only necessary for the formation and maintenance of oxygen deficits in the Bohai Sea but also plays an important role in low oxygen regions such as the Changjiang Estuary, Mississippi Estuary and Chesapeake Bay (Rabouille et al, 2008;Irby et al, 2016), so a comprehensive stratification analysis is needed. By comparing the effects of stratification on YRE and QHD, it can be seen that not only the intensity and duration of stratification should be considered when analysing the effects of stratification on oxygen-depleted regions (Zhao et al, 2017;Zhang et al, 2022), but the influence of the initial time of water stratification enhancement on the degree of oxygen deficit in different regions should also be taken into account.…”