Is EBUS shelf hypoxia imported or home-grown? Observations from northern California

The California Current System (CCS) is a highly productive Eastern Boundary Upwelling System (EBUS) characterized by wind-driven coastal upwelling that delivers oxygen-poor, high-pCO2 waters to nearshore ecosystems. While regional models and observations demonstrate the progression of ocean acidification (OA) and deoxygenation, exposure to hypoxia and OA stressors over the shelf is expected to be heterogeneous. For central CCS shelf waters there are limited time-series data on subsurface dissolved oxygen (DO) and carbonate system parameters. Hypoxia is due to physical processes that supply oxygen-poor source waters and/or local biogeochemical processes that drawdown DO in situ, which work together to produce intermediate to severe hypoxia over the shelf. We addressed this question by comparing water properties across space (glider & ship survey data) and across time (moored sensors) in the central-CCS. We monitored DO concentrations from 2014 to present over the northern California shelf, a region dominated by coastal upwelling and where the influence of river plumes is seasonally absent. Temperature and salinity data were used to distinguish water types, revealing a seasonal cycle in upwelling source water. In general waters at the shelf-edge site exhibit DO concentrations with values on the threshold of mild hypoxia (≤ 2.5 ml/L). Over the shelf, recurrent intermediate hypoxic events (≤ 1.4 ml/L) were observed, with occasional observations of severe hypoxia (≤ 0.5 ml/L). Seasonal fluctuations can be explained by a combination of source water variability and seasonality in the stratification and organic loading of shelf waters. However, hypoxia is not uniform across our study region – marked events were observed at different sites at different times, indicating a more complex space-time mosaic of shelf hypoxia (and likely also OA extrema). Linking observations with numerical modeling will help to describe the mosaic of oxygen variability across upwelling shelves and elucidate how short duration hypoxic events may be exacerbated by global deoxygenation.