LAUSR

Naturally acidified environments, such as those caused by volcanic CO 2 venting, reveal how complex coral reef ecosystems may respond to future ocean acidification conditions. Few of these sites have been described worldwide, and only a single such site is known from the Caribbean. Herein, we have characterized an area of volcanic acidification at Mayreau Island, St. Vincent and the Grenadines. Despite localized CO 2 enrichment and gas venting, the surrounding area has high hard and soft coral cover, as well as extensive carbonate frameworks. Twice daily extremes in acidification, in some cases leading to undersaturation of aragonite, are correlated with tidal fluctuations and are likely related to water flow. Corals persisting despite this periodic acidification can provide insights into mechanisms of resilience and the importance of natural pH variability on coral reefs.