UC Santa Cruz

The California Current System, situated off the US West Coast, experiences natural exposure to acidified and oxygen-poor conditions due to coastal upwelling, which brings low pH, low oxygen water from depth to the nearshore environment. The addition of anthropogenic ocean acidification and hypoxia (OAH) is therefore pushing conditions below biological thresholds, resulting in a variety of harmful effects ranging from behavior impacts to shell dissolution and mortality. It is therefore important to characterize the progression of ocean acidification and hypoxia in the California Current, where exposure to corrosive and hypoxic conditions is spatially variable and episodic in nature, making it a challenge to describe these patterns and their biophysical drivers through observational data alone. Here, a high resolution (~3 km) coupled physical-biogeochemical model is used to characterize the recent and projected spatial and temporal patterns in exposure to reduced pH and oxygen conditions, along with their physical and biogeochemical drivers. Results from Chapter 1 demonstrate that historical (1988-2010) alongshore variability in pH and oxygen is driven by a complex interplay of upwelling and primary production, modulated by the alongshore and cross-shore regional circulation. Results from Chapter 2 establish that historical variability in the interannual severity of exposure to corrosive conditions is driven by combined changes in source water properties and upwelling intensity, respectively associated with decadal basin scale variability and interannual regional scale forcing. Chapters 3 and 4 utilize downscaled regional climate projections to investigate the future (2000-2100) progression of ocean acidification and hypoxia hot spots, the emergence of these features, and their implications for marine resource management. Results from Chapter 3 highlight that where and when hot spots and refugia for pH and oxygen emerge depends on the metrics used to quantify them. If one is managing for OAH and cares about where and when conditions become distinct from their historical range, the projections suggest hot spots will be located in areas of historically weaker upwelling due to their narrow range of variability. In contrast, if one is managing for OAH and cares about where and when conditions will drop below specific biological thresholds, the projections suggest hot spots will be located in areas of historically stronger upwelling due to their lower baseline pH and oxygen conditions. Chapter 4 synthesizes information from the projections and displays it in an online interactive management tool, where users can explore future OAH change based on their species or region of interest. As a whole, these four chapters provide the first comprehensive mechanistic description of the physical and biogeochemical drivers shaping historical and future alongshore and interannual OAH variability in the central California Current region, while disseminating this information to marine resource managers in an accessible format.