Volcanic Seeps Reveal a Slow-Motion Crisis for Coral Reefs

Imagine being able to view the future of our oceans without waiting a century. Scientists have achieved this by analysing tropical coral reefs near underwater volcanic CO₂ seeps, which create natural gradients of acidity. These unique environments serve as a real-world laboratory, allowing researchers to observe how marine life copes with the changing seawater chemistry expected in the coming decades.

The study specifically measured the aragonite saturation state (Ω_Ar)—a key chemical metric indicating how easily marine organisms can access the minerals needed to build their skeletons. The findings offer a sobering correction to previous theories: rather than hitting a specific 'tipping point' or sudden cliff-edge, reef communities degrade in a linear, progressive fashion as acidity rises.

As the water became more acidic, the complex, habitat-forming corals and crustose coralline algae (CCA)—the hard, pink algae that acts as the reef's cement—retreated significantly. In fact, the cover of these vital builders dropped by over 50 per cent as conditions worsened. In their place, the landscape was colonised by non-calcareous brown and red algae, shifting the ecosystem from a vibrant city of stone to a field of weeds.

Crucially, the team observed that significant damage occurs even at slight drops in saturation from present-day levels. Their projections for the year 2100 show that the health of these ecosystems remains strictly tied to our CO₂ emissions, with higher output causing progressively severe deviations from the healthy reefs we recognise today.