Seawater CO2-Chemistry Variability in the Near-Shore Environment of the Southern California Bight

Seawater CO₂-chemistry in the Southern California Bight has been reasonably well characterized at seasonal resolution over the past several decades by multiple research expeditions. However, the near-shore environment (0-1 km) has largely been absent from these surveys and the drivers of seawater CO₂-chemistry variability in this region remains to be fully characterized. Here, we present near-shore seawater CO₂-chemistry data based on monthly transects conducted by small-boat between March 2017 and September 2018. Seawater samples and CTD profiles were collected at four stations equally spaced along a transect extending from Scripps Pier to the 40 m depth contour. Seawater samples were collected at 10 m depth intervals and analyzed for dissolved inorganic carbon (DIC), total alkalinity (TA), and pH, from which aragonite saturation state (Ω_arag) was calculated. The monthly surveys were paired with measurements of seawater temperature, salinity, pH, dissolved oxygen, and currents by autonomous sensors deployed at 18 m depth along the transect. Preliminary results show that fall and winter pH and Ω_arag values ranged from 7.9 to 8.1 and 1.8 to 2.9, respectively, with the lowest values measured at the bottom at the station farthest offshore. During the spring and summer, intensified upwelling transported low pH and Ω_arag seawater to the near-shore region reaching values as low as 7.69 and 0.95, respectively, at depths < 10 m. The largest spatial and temporal variability in pH and Ω_arag was observed during the spring and summer upwelling months, and was also associated with large variations in dissolved oxygen. The results show that seawater chemical conditions previously only reported for deeper environments may at times extend into the near-shore environment of the Southern California Bight. Increasing atmospheric CO₂ levels are likely to exacerbate these conditions, resulting in extended periods of low pH and Ω_arag conditions in nearshore environments.