Progressive changes in coral reef communities with increasing ocean acidification

Ocean acidification from increasing atmospheric CO₂ 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 CO₂ 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 CO₂ emissions, with higher emissions causing larger deviations from the reefs of today.