Galápagos coral reef persistence after ENSO warming across an acidification gradient

Anthropogenic CO₂ is causing warming and ocean acidification. Coral reefs are being severely impacted, yet confusion lingers regarding how reefs will respond to these stressors over this century. Since the 1982–1983 El Niño–Southern Oscillation warming event, the persistence of reefs around the Galápagos Islands has differed across an acidification gradient. Reefs disappeared where pH < 8.0 and aragonite saturation state (Ω_arag) ≤ 3 and have not recovered, whereas one reef has persisted where pH > 8.0 and Ω_arag > 3. Where upwelling is greatest, calcification by massive Porites is higher than predicted by a published relationship with temperature despite high CO₂, possibly due to elevated nutrients. However, skeletal P/Ca, a proxy for phosphate exposure, negatively correlates with density (R = -
0.822, p < 0.0001). We propose that elevated nutrients have the potential to exacerbate acidification by depressing coral skeletal densities and further increasing bioerosion already accelerated by low pH.