Using Timescales of Deficit and Residence to Evaluate Near‐Bottom Dissolved Oxygen Variation in Coastal Seas. Issue 1 (2nd January 2022)

• Record Type: Journal Article
• Authors: Zhang, Wenxia; Dunne, John P.; Wu, Hui; Zhou, Feng; Huang, Daji
• Is Part Of: Journal of geophysical research. Volume 127:Issue 1(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-01-02
• Subjects: Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2021JG006408
• Languages: English
• ISSNs: 2169-8953

Abstract

Abstract: We identify the local, vertical and lateral processes that cause bottom oxygen variation, and characterize timescales for each process using oxygen budget analysis based on a coupled physical‐biogeochemical model for the coastal seas in east China. Local oxygen deficit often occurs in the summer season due to the faster local consumption than the vertical replenishment in seasonal timescale. Lateral transport of oxygen‐rich ambient water replenishes local deficit of bottom dissolved oxygen. Competition between local deficit and lateral exchange determines seasonal hypoxia formation and sustainment. Short local consumption timescale is favorable for hypoxia formation, and transient hypoxia often forms when the local deficit and lateral exchange processes act on comparable timescales, such as the East China Sea in which the bottom hypoxia usually lasts for days. Extremely long lateral exchange timescale suggests that dissolved oxygen variation is predominantly controlled by local processes, and short local consumption timescale often causes increasingly severe seasonal hypoxia until the onset of the relaxation of local deficit, such as the bottom hypoxia in the Bohai Sea. Using these timescales to evaluate local deficit relative to vertical and lateral residence time and their variability is a convenient and potentially powerful general mechanistic framework to evaluate strategies to mitigate coastal hypoxia worldwide. Plain Language Summary: Coastal hypoxia, usually defined as aquatic dissolved oxygen concentration lower than 63 mmol m −3, has been a growing problem worldwide. Hypoxia causes detrimental effects and interrupts ecosystem functions, such as degradation of biodiversity, compression of habitat, and shrink of fish stock. Dissolved oxygen variation attributes to local oxygen consumption via biogeochemical process like degradation of organic matter, vertical replenishment via physical processes, and lateral exchange via horizontal advection. This study investigates coastal hypoxia from the perspective of timescale since these processes regulate dissolved oxygen on different timescales to compare different processes, evaluate bottom hypoxia sustainment, and isolate the relative importance of local and vertical, and lateral processes in hypoxia formation. We find this framework to have important implications for the comparative effectiveness of local ecosystem protection measures. Key Points: The timescales for local, vertical and lateral processes that regulate bottom oxygen are defined and quantified Comparable deficit and lateral exchange timescales under short local consumption timescale cause transient hypoxia Relatively longer lateral exchange timescale under short local consumption timescale favors long‐lasting hypoxia