AGU Fall Meeting: the (pH) lowdown on ocean acidification

Ocean acidification is often overlooked as a problem in favour of its more famous parent, climate change. But it’s receiving plenty of attention at the AGU Fall Meeting in San Francisco.

Whilst most information on the effects of acidification is based on modelling or lab experiments over limited time periods, Adina Paytan of the University of California Santa Cruz has been looking at whole ecosystems – the natural submarine springs, or “ojos”, that occur along Caribbean coastlines. Formed when rainwater travels through limestone caves under land and discharges into the sea via faultlines, these springs have a low pH, making them a natural laboratory for studying the effects of acidification on ocean-dwelling species over long timescales.

To her surprise, Paytan found that three coral species were able to grow under the low pH conditions near the springs, despite a scarcity of the carbonate ions that corals normally need to form their calcite shells. Near the springs the pH was around 7.6 whilst further away, where nine coral species were discovered, the pH was 8.1.

“It’s encouraging that certain corals can survive,” Paytan told reporters, “but it’s only a few species and they’re not reef-building. They tend to grow slowly in patchy colonies.”

Paytan speculates that the higher nutrient concentration of the springwater may aid growth of the coral’s symbiotic photosynthesizing algae, providing the coral with more energy and the ability to grow under less optimal conditions. She is currently investigating this theory further. CT scans of samples drilled from the corals revealed that the organisms formed less dense skeletons when the surrounding water contained fewer aragonite ions (a form of carbonate). Under these conditions, the corals also suffered more boring by clams and worms. As a result, the corals may be less robust and more susceptible to damage by hurricanes.

Acidification researcher Nina Keul of the Alfred Wegener Institute in Bremerhaven, Germany had also had a surprise – her lab tests showed that a species of foraminifera found in Northern Germany actually grew faster in more acid water. This is in contrast to previous studies, although Keul’s specimens were at an earlier stage of their development than those used in other work. The mechanism for the increase is not yet clear; Kent speculated that a lower pH may make it easier for the forams to expel hydrogen ions formed during their shell-making process.

Robert Riding of the University of Tennessee Knoxville, meanwhile, has been looking to the past. His analysis of cores drilled from a reef in Tahiti in 2005 indicates that natural acidification associated with past climate changes weakened the bacterial crust that glues reefs together. At some periods, these calcifying bacteria formed layers up to 20 cm thick and could make up as much as 80% of the reef framework. But acidification led to a lower abundance of the bacteria, making the reefs less strong. Riding explained that this creates a double whammy – natural acidification has weakened reefs and manmade acidification will now weaken them again.

 


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