# Progressive changes in coral reef communities with increasing ocean acidification

## TL;DR

Ocean acidification is causing gradual changes in coral reef communities, with fewer calcifying organisms and more non-calcareous algae as CO2 levels rise.

## Contribution

The study provides empirical evidence of progressive coral reef community shifts with decreasing aragonite saturation state, without clear tipping points.

## Key findings

- Coral and calcareous algae cover declined by over 50% as aragonite saturation state dropped from current levels to 2.
- Non-calcareous algae increased in biomass and cover with decreasing aragonite saturation.
- Coral diversity, including juveniles, rapidly declined with lower aragonite saturation.

## Abstract

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 (ΩAr) where significant changes occur in situ. With declining ΩAr, 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 ΩAr levels declined from present-day to 2, and importantly, the cover of some of these groups was already significantly altered at an ΩAr 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.

In situ examinations of coral reef benthic communities along a gradient of CO2 exposure show a decline in calcareous taxa and a proliferation of the non-calcareous taxa. Community changes expected by the end of century will depend on atmospheric CO2 emissions.