Coupled CO₂ and O₂-driven compromises to marine life in summer along the Chilean sector of the Humboldt Current System

Abstract. Carbon dioxide and coupled CO₂ and O₂-driven compromises to marine life were examined along the Chilean sector of the Humboldt Current System, a particularly vulnerable hypoxic and upwelling area, applying the Respiration index (RI = log₁₀ pO₂pCO₂) and the pH-dependent aragonite saturation (Ω) to delineate the water masses where aerobic and calcifying organisms are stressed. As expected, there was a strong negative relationship between oxygen concentration and pH or pCO₂ in the studied area, with the subsurface hypoxic Equatorial Subsurface Waters extending from 100 m to about 300 m depth and supporting elevated pCO₂ values. The lowest RI values, associated to aerobic stress, were found at about 200 m depth and decreased towards the Equator. Increased pCO₂ in the hypoxic water layer reduced the RI values by as much as 0.59 RI units, with the thickness of the upper water layer that presents conditions suitable for aerobic life (RI>0.7) declining by half between 42° S and 28° S. The intermediate waters hardly reached those stations closer to the equator so that the increased pCO₂ lowered pH and the saturation of aragonite. A significant fraction of the water column along the Chilean sector of the Humboldt Current System suffers from CO₂–driven compromises to biota, including waters corrosive to calcifying organisms, stress to aerobic organisms or both. The habitat free of CO₂-driven stresses was restricted to the upper mixed layer and to small water parcels at about 1000 m depth. Overall pCO₂ acts as a hinge connecting respiratory and calcification challenges expected to increase in the future, resulting in a spread of the challenges to aerobic organisms.