Progressive changes in coral reef communities with increasing ocean acidification.

Area of Science:

• Marine Biology
• Oceanography
• Climate Science

Background:

• Increasing atmospheric CO2 causes ocean acidification, altering seawater chemistry.
• Predicting marine ecosystem impacts requires empirical field data.

Purpose of the Study:

• Quantify changes in tropical coral reef benthic communities along a natural CO2 gradient.
• Determine the aragonite saturation state (ΩAr) thresholds for significant community shifts.
• Predict future reef states under various emissions scenarios.

Main Methods:

• Field surveys at 37 stations with varying CO2 exposure from submarine volcanic seeps.
• Measurement of aragonite saturation state (ΩAr).
• Quantification of benthic community composition (coral, algae cover, diversity).
• Development of response curves to predict future reef states.

Main Results:

• Declining ΩAr led to reduced cover of reef-building corals and crustose coralline algae (CCA).
• Non-calcareous brown and red algae proliferated as ΩAr decreased.
• Significant declines in coral and CCA cover occurred at ΩAr levels of 3.2 and below.
• Coral diversity (adult and juvenile) rapidly decreased with declining ΩAr.
• Future reef states are predicted to deviate significantly from present-day reefs, especially under higher emissions scenarios (SSP3-7.0).

Conclusions:

• Ocean acidification directly impacts coral reef community structure and function.
• Thresholds for significant community shifts exist, with some impacts observed even at current ΩAr levels.
• Future CO2 emissions will determine the trajectory of coral reef ecosystems, with higher emissions leading to greater degradation.