The biggest threat to reefs in our lifetime

Without coralline algae - which is extremely susceptible to ocean acidification - tropical reefs as we know them would not exist. Photo: Flickr/Derek Keats
Without coralline algae – which is extremely susceptible to ocean acidification – tropical reefs as we know them would not exist. Photo: Flickr/Derek Keats

Ocean acidification is the biggest problem for coralline algae when assessed against all others. Now is the time for COP26 leaders to be courageous on the issue, argues Dr Christopher Cornwall.

Comment: With dire predictions of the impacts of climate change on humankind and Earth’s atmosphere, it’s easy to overlook what it means for our oceans.

It’s not just the atmosphere that is becoming hotter. Oceans are experiencing warmer temperatures and becoming increasingly acidic, due to the absorption by the world’s surface seawater of anthropogenically generated carbon dioxide.

This in turn has a major impact on marine species which depend on our temperate and tropical reefs to survive.

Recently-published research carried out with colleagues around the world shows that ocean acidification is the most important environmental stressor and biggest problem for coralline algae when assessed against all others.

Only by reducing our emissions to keep atmospheric CO2 concentrations below 450 parts per million will these iconic coralline algal systems retain their diversity and function.

These coralline algae provide the backbone to coastal ecosystems from the tropics to the poles. They both cement together and form their own reefs in many tropical locations.

However, coralline algae are extremely susceptible to ocean acidification. Increasing carbon dioxide concentrations cause a complicated series of chemical consequences in seawater. Overall, ocean acidification makes it harder for many marine species to grow and build their skeletons of calcium carbonate.

Early studies found that ocean acidification would have dire consequences for coralline algae. However, the field of ocean-acidification science was then rocked with discussions that effects on other taxa (i.e. fish) were less than originally proposed, and that those early studies represented more dramatic effects than would actually occur in the future.

But in our latest study in Global Change Biology, we properly quantify the effects of ocean acidification across all previous studies using a meta-analysis approach for the first time, standardising the responses of all previous experiments on the topic to assess overall effects.

We found that ocean acidification has large impacts on coralline algal calcification, abundance and recruitment in the field, partially corroborating the early finds that ocean acidification would badly impact this taxa.

However, we also discovered some inconsistencies – namely that ocean acidification does not cause bleaching in coralline algae (in the way thermal stress does for corals, for example), nor consistent changes in their ability to photosynthesise.

We found that ocean acidification will have large ramifications for these important species that span all examined ocean basins, climates and across most orders and families of coralline algae.

More intense ocean acidification, predicted under higher emissions scenarios, will result in much greater chances that coralline algae will cease to provide their important ecosystem services of reef accretion/building and settlement substrate provision.

When assessed against all other environmental drivers (such as ocean warming and light), ocean acidification had larger effects more often than the other environmental variables.

Our only chance to keep these crucial species and the reefs they support is to now limit our carbon dioxide emissions globally.

World leaders have been converging on COP26 in Glasgow to discuss implementing reduced carbon dioxide emissions. Now is the time for these leaders to be courageous and not bow to the peer pressure of those seeking short-term economic gains in utilising fossil fuels.

IdeasRoom, 28 October 2021. Full article.


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