A More or Less Certain Future: What Role for Climate Variability in Defining Coastal Ocean Acidification and Hypoxia Exposure?

Coastal ecosystems of eastern boundary current upwelling systems face especially strong exposure to ocean acidification (OA) and low oxygen stress. Oxygen and carbonate chemistry in such systems are also expected to be highly dynamic in time and space. This dynamism challenges our ability to replicate realistic exposure regimes in lab studies, discern anthropogenic changes from natural variability, and narrow projections of future changes. Using data from ship- and mooring-based time-series observations of dissolved oxygen (DO) (13 years) and carbonate chemistry parameters (5 years) from a slope to coast transect in the Northern California Current (NCC), we addressed the following questions. 1) How does exposure to OA and hypoxia (OAH) stress vary across the continental shelf and what processes modulate differences in exposure? 2) To what extent does climate variability dampen or amplify OAH stress, and diminish our ability to detect anthropogenic changes? From cross-shelf observations, we find that the severity, variability, and the strength of coupling in OAH varies strongly cross-shelf in concert with changes in respiration, and biogeochemical sensitivity to tidal and wind forcings. This suggests that realistic exposure conditions for organismal studies can differ greatly (but predictably so) even within a single ecosystem. Temporally, we find strong modulation of OAH stress by climate variability. For example the presence of the “warm blob” in 2015 in the NCC, was associated with positive DO anomalies that strongly abated hypoxia risk. Negative pCO2 anomalies equivalent in scale to current anthropogenic DIC burdens were observed and highlight the importance of climate variability in modulating OA progression. Our observations nonetheless place important constraints on the current and pre-industrial ratio of dissolved oxygen and pCO2 in the system and highlight near-term transition to no-analog state conditions in coupled OAH stress in the NCC.