Interannual variability and drivers of summertime bottom water dissolved oxygen concentrations on the coastal Mid-Atlantic Bight (USA)

Dissolved Oxygen (DO) concentrations in the New Jersey coastal shelf regularly experiences low-level depletion during late August and the early September. Strong seasonal stratification across the continental shelf restricts ventilation between the well-oxygenated surface layer and colder, more saline bottom waters. Harmful low DO events with concentrations considered hypoxic (>=3mg L-1) are rare occurrences and when present shows significant spatial and temporal variability. Discerning the timings and spatial variability of the drivers controlling coastal deoxygenation has been historically impaired due to a lack of available high-resolution data. Utilizing the last 10 years of the New Jersey Department of Environmental Protection (NJDEP) New Jersey Quality Assurance Project Plan (QAPP) autonomous glider database, comprising 89+ glider deployments representing ~371,156 vertical profiles, we have been able to discern that the persistence of stratification is the dominant factor controlling recurrent summertime hypoxia in the NJ coastal shelf region. Using a few years as model cases we studied the drivers of the shelf low DO. Prominent Summer hypoxia was present in 2015 as well as 2023, in stark contrast to years with almost consistently replete bottom water oxygen, such as 2018 and 2021. A degree of deoxygenation (<5mg L-1), nonetheless, is present every year for ~6 days between the end of August and the beginning of September. Analyses using ANOVA and GAM analyses reveal a strong difference in the strength of the driving influence of regional features, including Hudson River transport and upwelling, between northern and southern recurrent hypoxic centers.