Decadal shifts in hypoxia and acidification reveal changing anthropogenic pressures on bottom waters of a coastal shelf

Abstract Coastal systems provide habitat that sustains valuable shellfisheries but are subject to dissolved oxygen (DO) and/or carbonate chemistry impairment from anthropogenic pressures such as eutrophication and increasingly, climate change. Although extreme events can have disproportional negative ecological impacts, their ephemeral nature and a lack of baseline monitoring data make them challenging to characterize. Through assessments of historical records and a series of modern‐day cruises, this study documented the magnitude and extent of summer hypoxia and acidification in the coastal shelf bottom waters of an urban shelf ecosystem, the New York Bight, before and during a devastating hypoxic event in 1976 and at present. In 1974, the most severe DO (2.39 mg L −1 ) and carbonate chemistry [pH N : 7.47; aragonite saturation state (Ω Ar ): 0.45] conditions occurred as a halo around a now derelict sewage disposal site, while averaging 4.43 mg L −1 (DO), 7.84 (pH), and 1.25 (Ω Ar ) across the region that August. During the mass mortality event of 1976, extremely low DO (< 1 mg L −1 ), pH N (< 7.5), and Ω Ar (< 0.5) levels were observed across bottom waters during summer. Comparisons of modern subsurface chemistry to that of 1974—a year with ocean dumping but no mass mortality—indicated increases in bottom water DO, with evidence to suggest that ocean acidification has dampened the concomitant increases in Ω Ar over the intervening half‐century. This study highlights the impacts of ocean dumping and the threat of ocean acidification to systems that are experiencing or recovering from coastal hypoxia.