Abstract
Ocean acidification from increasing atmospheric CO(2) is progressively affecting seawater chemistry, but predicting ongoing and near-future consequences for marine ecosystems is challenging without empirical field data. Here we quantify tropical coral reef benthic communities at 37 stations with varying exposure to submarine volcanic CO(2) seeping, and determine the aragonite saturation state (Ω(Ar)) where significant changes occur in situ. With declining Ω(Ar), reef communities displayed progressive retractions of most reef-building taxa and a proliferation in the biomass and cover of non-calcareous brown and red algae, without clear tipping points. The percent cover of all complex habitat-forming corals, crustose coralline algae (CCA) and articulate coralline Rhodophyta declined by over 50% as Ω(Ar) levels declined from present-day to 2, and importantly, the cover of some of these groups was already significantly altered at an Ω(Ar) of 3.2. The diversity of adult and juvenile coral also rapidly declined. We further quantitatively predict coral reef community metrics for the year 2100 for a range of emissions scenarios, especially shared socio-economic pathways SSP2-4.5 and SSP3-7.0. The response curves show that due to ocean acidification alone, reef states will directly depend on CO(2) emissions, with higher emissions causing larger deviations from the reefs of today.