Temporal patterns and causes of deoxygenation in the Northwest Atlantic Shelf—an observation approach

Abstract

Oxygen concentrations are of fundamental importance for organisms and geochemical cycling in the oceans. Since the middle of the 20th century, oxygen concentrations have been declining in the open ocean and the coastal ocean, with implications for productive ecosystems. The northwest Atlantic (NWA) shelf from the coast of Labrador to Cape Hatteras is located near the intersection of subtropical and subpolar circulation in the Atlantic basin and is sensitive to climate variability. Here, we used in situ oxygen observations to determine the multidecadal trend of subsurface oxygen concentrations below the mixed layer on the NWA shelf from 1988 to 2019. Our results identified an overall decline in oxygen concentrations that varied across the region from 0.0 to −1.8 µmol kg−1 yr−1. These rates surpass the established Atlantic basin-wide trend. The greatest subregional oxygen loss occurred on the Eastern Scotian Shelf (−1.8 ± 0.2 µmol kg−1 yr−1). By separating and comparing the contribution of oxygen solubility and apparent oxygen utilization (AOU) to the trends, the oxygen trends on the NWA shelf were determined to be driven by changes in the AOU. In the subregions where AOU dominates, the AOU change was driven by the decreased presence of Labrador Current water and the increased presence of Gulf Stream water in the region, while the rest was unexplained by our potential predictors, implicating local biological changes and changes in ice-cover.