FY 2025

Seasonal and interannual variability of the aragonite saturation horizon in the California Current system of Baja California

Oliva-Méndez, N.L., J.M. Hernández-Ayón, J.A. Valencia-Gasti, R. Durazo, E. Santamaría del Ángel, S.R. Alin, and R.A. Feely

J. Geophys. Res., 130(6), e2024JC021653, doi: 10.1029/2024JC021653, View article at AGU/Wiley (external link) (2025)

Hydrographic data from cruises of the Investigaciones Mexicanas de la Corriente de California (IMECOCAL) program since 1998 were used to assess the chemical conditions associated with carbon variables in the water column in the transect “Line 100.” Seasonal climatologies along the IMECOCAL line highlight the upwelling season, during which water with different chemical characteristics is transported to the surface. Additionally, interannual events influenced the amplitude and timing of wind-driven coastal upwelling and the region's relative volumes of dominant water mass. Seasonal climatologies of pH, calcium carbonate saturation states, and dissolved inorganic carbon (DIC) concentration were estimated from hydrographic proxy variables. The strength of seasonal upwelling was reflected in the depth of the aragonite saturation horizon (ASH), which was variable nearshore: 90 m (±29 m) in spring and 133 m (±32 m) in winter. Offshore (>50 km), the effect of upwelling diminished, and the ASH was deeper and less variable (spring: 152 m ± 25 m; winter: 151 m ± 28 m). However, aragonite saturation values <1 were found at depths >250 m and were associated with Equatorial Subsurface Water (ESsW) dominance. At seasonal timescales, Subarctic Water (SAW) was found to modulate ASH depth. At interannual scales, ASH was found to be deeper (180 m) during periods of El Niño and shallower (120 m) during La Niña conditions. However, the impacts of El Niño and La Niña events give notable differences in the ASH depth.

Plain Language Summary. In the California Current System, where nutrient and carbon rich subsurface waters are brought to the surface and support a productive ecosystem, higher seawater carbon dioxide levels result in a reduction of the carbonate ion concentrations, which serve as building blocks for marine species that create calcium carbonate shells or skeletons. Therefore, it is more difficult for these organisms to form carbonate structures when the saturation states of aragonite or calcite, two forms of calcium carbonate created by marine species, are low. Here, we investigated drivers of seasonal and interannual variability of the aragonite saturation horizon (ASH; below this depth, conditions are corrosive to aragonite) since 1998 along a cross-shelf transect in the Southern California Current System to ∼220 km offshore off Ensenada using IMECOCAL surveys. We found that variability in coastal upwelling intensity and the relative dominance of regional water masses were important factors influencing the depth of the ASH. Seasonal and interannual patterns of variability of seawater carbonate chemistry have implications for how southern California Current ecosystems will be affected by human activities.