Persistent mid-water column hypoxia in a temperate fjord of the northeast Pacific Ocean

Coastal waters are increasingly susceptible to low-oxygen conditions where circulation or nutrient inputs have been altered by anthropogenic activities, leading to increased occurrence of coastal hypoxia worldwide. Low-oxygen conditions also occur naturally in coastal settings, typically in bottom water or landward of sills where circulation is weak. Here we identify and characterize a persistent, low-oxygen layer in the mid-water column of a remote fjord of the northeast Pacific Ocean that uncharacteristically lacks an entrance sill. This feature is sustained by weak reoxygenation processes constrained by fjord and watershed morphology and geography, and expands seaward into a neighboring channel during summer and autumn. Marine CO2 system conditions characteristic of adverse habitat for local calcifiers also occurred within the low-oxygen layer, demonstrating that such features are multi-stressor habitats more typical of deeper bottom waters or eutrophied settings. Seasonal movement of this layer overlapped habitat margins for some sensitive local species, indicating risk from interannual variability or response to ongoing climate change. These results provide an improved understanding of fjord habitat that can be applied to other fjords where similar characterization of morphological features and water properties can be evaluated.