Reef Erosion With Acidity: A 'Double Whammy'

Here's something you may not have known: tropical coral reefs are constantly building up and breaking down. In healthy ecosystems, the building will outpace the breakdown, keeping reefs growing and sturdy even as their erosion feeds predators (like worms and the unusual parrotfish).

However, in recent years, researchers have begun to worry that in the wake of elevated levels of ocean acidity our reefs are eroding faster than they can grow.

Extensive records have shown that one of the most iconic coral reefs in the world, the Great Barrier Reef, is gradually diminishing. Worse still, experts have found that the continued damage that intense ocean acidification inflicts on our reefs could be costing the world trillions of dollars in declining ecosystems and coastal infrastructure.

We have also been seeing some of the worst coral bleaching events in decades, where the symbiotic partners of corals - algae that aid in the calcification process that makes reefs strong and gives corals their vibrant colors - have been growing weak and dying off.

With these little builders on the decline, acidification and elevated ocean temperature have been blamed. However, some research has found that these algae may actually benefit from small rises in acidification, as it means that more carbon is being brought to the ocean floor for use.

However, acidification may already be spiking past this beneficial point, as it is projected that the ocean will rise in acidity by a stunning 26 percent by the end of the century. (Scroll to read on...)

So how do we know for sure when reefs are in trouble? A team of researchers at the University of Hawaii - Mānoa (UHM) are arguing that we should just keep things simple, looking for physical evidence of elevated reef erosion as it happens.

Expected Outcomes

To measure bioerosion, the team left small blocks of calcium carbonate (dead coral skeleton) on an average reef for one year. Normally, experts would just weight these blocks to determine how much it eroded at the end of the study. However, the UHM experts wanted to look deeper.

According to a study recently published in the journal Marine Ecology Progress Series, the researchers used micro-computed tomography (a high-resolution CT scan) to create detailed before-and-after imagery of each block's structure. This provided them with extremely accurate measurements of accretion and erosion rates.

They had also measured varied pH levels and other factors (resource availability, temperature, etc.) for water around the reef throughout the experiment, and found that, as was expected, ocean acidity was the strongest predictor of the growth and erosion balance. (Scroll to read on...)

A Worrisome Surprise

However, they also found evidence of a phenomenon that could spell disaster for reefs.

The researchers determined that in the wake of rising ocean acidity, the rate of erosion alone (not considered alongside rate of calcification) saw a small increase. This is big news, because it has always been assumed that acidification hurts reefs by slowing their rate of growth alone (so that natural erosion outpaces it.)

If acidification has its own impact on erosion, models for projected coral damage may be underestimating.

"We saw changes in pH on the order of meters and those small pH changes drove the patterns in reef accretion-erosion," study lead author Nyssa Silbiger added in a statement.

This suggests that it doesn't have to be a massive net increase in ocean acidity to tilt the scales towards "an erosion-dominated system in a high-CO₂ world," the researchers wrote. "This shift will make reefs increasingly susceptible to storm damage and sea-level rise, threatening the maintenance of the ecosystem services that coral reefs provide."

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