We already know that pH varies naturally across the oceans of the world. In some sites, it varies more in a single day than global oceans are likely to face in a century.
But cold water corals live in deep water, are slow growing, and hard to study.
Six years ago, experts in cold water corals were telling us how they would be likely to fall victim to ocean acidification first, and that they believed this for good reasons but with little experimental data. But about a year ago data came out (by one of those same experts) showing that rather than being the badly affected, cold water corals adapted to effectively very high levels of CO2 and possibly even increased their calcification rates. Eight days after the pH was changed suddenly, the corals did worse. But when the experiment was continued for six months, the results turned right around. The researchers pointed out how useful longer studies are: “This is the first evidence of successful acclimation in a coral species to ocean acidification, emphasizing the general need for long-term incubations”. The paper is called “Acclimation to ocean acidification during long-term CO2 exposure in the cold-water coral Lophelia pertusa.” The pH fell as low as 7.75 in the long term study (from the normal pH of about 8.1).
It’s highly unlikely the atmospheric levels of CO2 will reach 1,000 ppm in the next couple of centuries, but if they did, it appears that at least one major and widespread species of cold water coral can adapt within six months. Co2 feeds plant life above the water, and atmospheric levels were much higher during the time that corals evolved. That doesn’t guarantee anything, but suggests scientists could have been more cautious in predicting a disaster when they didn’t have the data.
In 2006 experts thought Cold Water Corals would be some of the first to suffer
There were reasons to fear for the corals of the deep cold depths. It was thought they were particularly sensitive to acidification, so much so, one researchers suggested they could disintegrate like “chalk in acid”… De Speigel 2006.
“Cold-water corals could be the first organisms to fall victim to the acidification of the oceans,” says [Ulf] Riebesell, whose dire prediction is supported by a study that appeared earlier this week in the professional journal Frontiers in Ecology and the Environment – a publication of the Ecological Society of America. In the study (link is in PDF format), experts working with John Guinotte at the Marine Conservation Biology Institute in Bellevue, Washington report that 70 percent of the current habitat of coral branches may no longer be suitable for these organisms by as early as the end of this century.
“Imagine dripping hydrochloric acid onto chalk,” says André Freiwald of the University of Erlangen-Nuremberg, one of the co-authors of the study. “The chalk would disintegrate immediately; the corals could face a similar fate.”
Lophelia and its relatives form their skeletons from aragonite, a mineral form of calcium carbonate. But aragonite is highly soluble and very sensitive to changes in pH levels. The problem for cold-water corals is that it is precisely in their habitat — deep, cold seawater — where high pressure and low temperatures already make the water more acidic than it is closer to the surface.
Note that Ulf Riebesell was one of the researchers interviewed in that de Speigel article in 2006 to raise awareness of potential problems with cold water corals.
Caveats: It’s just one coral type (though a widespread and “at risk” one). Other corals may react differently. Warmer water (if that happens) may also have an effect — increasing the need for food (as the corals grow faster). But presumably increasing coral growth would remove extra carbonic acid and bicarbonate, acting as yet another negative feedback.
It’s noteworthy that normally the Great Barrier Reef varies by 8 or 9 degrees along it’s full length from summer to winter.
The real threat to reefs is more likely to be trawling or overfishing.
Ocean acidity has increased by 30% since preindustrial times due to the uptake of anthropogenic CO2 and is projected to rise by another 120% before 2100 if CO2 emissions continue at current rates. Ocean acidification is expected to have wide-ranging impacts on marine life, including reduced growth and net erosion of coral reefs. Our present understanding of the impacts of ocean acidification on marine life, however, relies heavily on results from short-term CO2 perturbation studies.
Here we present results from the first long-term CO2 perturbation study on the dominant reef-building cold-water coral Lophelia pertusa and relate them to results from a short-term study to compare the effect of exposure time on the coral’s responses. Short-term (one week) high CO2 exposure resulted in a decline of calcification by 26-29% for a pH decrease of 0.1 units and net dissolution of calcium carbonate.
In contrast, L. pertusa was capable to acclimate to acidified conditions in long-term (six months) incubations, leading to even slightly enhanced rates of calcification. Net growth is sustained even in waters sub-saturated with respect to aragonite. Acclimation to seawater acidification did not cause a measurable increase in metabolic rates. This is the first evidence of successful acclimation in a coral species to ocean acidification, emphasizing the general need for long-term incubations in ocean acidification research. To conclude on the sensitivity of cold-water coral reefs to future ocean acidification further ecophysiological studies are necessary which should also encompass the role of food availability and rising temperatures.
If this study tells us anything for sure, it’s that we don’t know a lot about marine life. The corals considered to be the “most vulnerable” turned out to not decalcify at all under more acidic conditions.
Other posts on Ocean Acidification
- The chemistry of ocean pH and “acidification”
- Scripps blockbuster: Ocean acidification happens all the time — naturally
- Ocean Acidification — a little bit less alkalinity could be a good thing
Form, Armin U. and Ulf Riebesell, “Acclimation to ocean acidification during long-term CO2 exposure in the cold-water coral Lophela pertusa,” Global Change Biology, 18, 843-853, 2012.[ abstract]
UPDATE: Many commenters rightly point out that “Ocean Acidification” is a hyperbolic term, not in keeping with accurate scientific descriptions. They are correct, but search engines look for keywords, and no one is searching for “reduced alkalinity”. Hence I chose to use the common term so this post will turn up in common searches. Obviously the oceans are not acidic, not likely to become acidic, and the term is misleading.