Aaron Takeo Ninokawa
Coral reefs are not only beautiful and cool – in addition to tourist dollars and unique diving experiences, they also provide real benefit to human society. They are home to about a quarter of the world’s fish, on which many people depend as a food source. They can form a barrier against rising sea levels and they can protect coastlines from erosion.
But thanks to all the carbon we’ve pumped into the air, coral reefs are disappearing. Fast. Part of that is heat stress, but CO2 may also affect coral’s ability to form reefs in the first place. A new experiment gives us a first look at how much this affects an entire reef ecosystem.
When oceans absorb atmospheric carbon dioxide, they acidify. This in turn lowers the concentration of carbonate ions in the water. When there is a shortage of carbonate ions in seawater, coral reefs, made of carbonates, dissolve to restore the balance. So it is obvious that an increase in carbon dioxide in the water would cause problems for the corals.
But tracking the actual effects is remarkably challenging. Experiments have been conducted in controlled laboratory settings on isolated species, but not in the open ocean where the effects of CO2 interacting with variations in light, temperature, water flow rate and nutrient availability. Only in the open ocean can all these factors be assessed together on the entire coral community. Our ability to address future damage to coral reefs depends on this knowledge.
Acid washed
A group of marine biologists took advantage of the structure of a reef on One Tree Island in the southern Great Barrier Reef to set up an experiment in an otherwise natural environment.
At low tide, the water flows over this reef from a higher lagoon to a lower lagoon. The researchers bubbled CO2 by a tank they placed in the upper lagoon to increase the concentration of carbon dioxide in the seawater flowing over the reef, bringing it to levels expected to occur later this century. They then measured the rate of coral reef calcification — i.e., the formation of more reef — at different locations after the treated water overflowed. This was compared to the velocities measured before the experiment.
CO2 enrichment reduced the calcification of the net coral reef community by 34 percent compared to background. Previous lab experiments had estimated the calcification sensitivity of corals at 15 to 28 percent. The researchers who conducted this study suggest that their new results may have a greater impact due to the presence of crustose calcified algae in the ecosystem, which can alter the balance of carbonate ions. Alternatively, the calcification rate may increase in sensitivity as the concentration of carbonate ions in the water decreases, and this experiment has revealed a snowball effect that we might eventually see in the wild.
Our challenge, they conclude, is to first understand how climate change will affect complex ecosystems such as coral reefs; then how changes in natural resources will affect human lives and societies; and finally how we can mitigate and cope with these effects. “Ultimately,” the scientists note, “just the reduction of atmospheric CO2 levels will address the challenges of ocean acidification.”
Since that won’t happen anytime soon, experiments like this can help us figure out exactly what we stand for.
Nature2018. DOI: 10.1038/nature25968 (About DOIs).
List image by Aaron Takeo Ninokawa