Ocean acidity is altering coral reefs and helping algae take over

Coral reefs may look tranquil at first glance, but clear water often hides the slow rise in ocean acidity that reshapes entire ecosystems.

To understand these changes, scientists are turning to settings that reflect real-world conditions, not just tanks or models.

Volcanic seeps in Papua New Guinea now offer that chance. Here, CO2 bubbles rise through living reefs and alter the water in predictable gradients. That unusual setting reveals how future oceans may affect corals.

Corals under rising acidity

Research teams report steady declines in coral strength as more CO2 enters seawater. Rising acidity reduces aragonite saturation, which matters because corals use aragonite to build their skeletons. Even small drops make construction harder.

Field data from 37 stations show early declines across many coral groups once saturation begins to fall. Massive Porites manage to hold their ground, but most branching and plate-forming species lose cover with each step along the CO2 gradient.

Dr. Katharina Fabricius, a coral researcher at Australian Institute of Marine Science (AIMS) in Townsville and senior author on the paper described the value of the seeps.

“These unique natural laboratories are like a time machine,” said Dr. Fabricius. Those gradients reveal limits that standard experiments never show.

Early damage to corals

Young corals show even sharper responses. Juvenile density drops quickly with minor chemical shifts. That matters because juveniles keep reefs growing after storms or bleaching events.

Once juveniles thin out, recovery slows. The number of coral genera also decreases along the gradient, and the loss appears early rather than late. These changes happen before any “collapse point,” which surprised many researchers who expected something abrupt.

Calcareous algae also fall fast. Crustose coralline algae, which help coral larvae attach, decline by more than half at moderate saturation loss. Some groups vanish completely once saturation falls toward levels expected later this century.

Their disappearance removes important settlement surfaces, which further slows coral renewal.

Algae grow as corals fall

Soft algae respond very differently. Brown and red non-calcareous algae spread across exposed areas, especially in places where corals once dominated.

Higher CO2 boosts their photosynthesis and removes competition from weakened calcifying species.

Over time, these algae cover more rock, trap sediments and block space for coral settlement. This shift occurs smoothly along the gradient, not in sudden jumps. The pattern echoes results reported across other volcanic seeps around the world.

Macroalgal biomass changes in a more complex way. Calcareous algae shrink as saturation falls, but non-calcareous algae remain stable or even increase. In some places, one group declines while another doubles.

The balance between these types decides how much structure a reef keeps. Once soft algae dominate, structure starts to flatten.

Simpler reefs with less complexity

Structural complexity offers hiding places for fish and invertebrates. Branching species provide that structure, yet those groups show the strongest declines.

Even a small drop in aragonite saturation reduces the cover of complex corals. Habitat complexity scores fall sharply once saturation passes key points identified in the paper.

This loss pushes many reef animals into open water, where survival becomes harder.

Massive Porites continue to survive, but their rounded forms cannot replace the intricate shapes of Acropora or Pocillopora. The reef becomes simpler and less protective.

Communities become dominated by a smaller set of tolerant organisms. That shift reduces the range of species that reefs once supported.

A massive global problem

The study compares several climate futures. Low emissions keep many present-day features intact. Moderate scenarios still show major changes in sensitive coral groups by 2100.

High emissions cause steep declines in structural corals, crustose coralline algae and many other calcifying species. Non-calcareous algae expand strongly in those futures.

The scientists behind the study emphasize that these projections include only acidification, not warming or bleaching. Those pressures act together, which means stronger real-world impacts.

Dr. Fabricius warned about the scale of the issue. “Ocean acidification is a massive global problem, which has been understudied and underreported to date.”

Ocean acidity alters corals

Some patterns described in the study already match observations on the Great Barrier Reef. Contemporary reefs show reduced crustose coralline algae and fewer coral juveniles in lower aragonite regions.

These early signs confirm that acidification is not just a future concern. Present-day chemical shifts already alter community structure.

Volcanic seep sites are imperfect models of the future since they do not include warming. Even so, they provide rare insight into how organisms react after generations under high CO2.

They reveal steady changes across small differences in water chemistry, showing that even moderate emissions will reshape many reefs.

The fate of coral reefs

Rising CO2 nudges coral reefs toward simpler, algae-dominated states. The corals that build the reef weaken first, and few juveniles manage to settle and grow.

Calcareous algae fade while soft algae flourish, and the reef’s structural complexity steadily declines. These shifts unfold slowly but with striking consistency.

Volcanic seeps offer a glimpse of what lies ahead if emissions don’t decline. They reveal communities that adjust to new chemistry yet shed the features that once supported abundant marine life.

Ultimately, the fate of reefs will be determined by choices made far above the waterline.

The study is published in the journal Communications Biology.

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