The shellfish that helped make Molly Malone famous, cockles and mussels, could rapidly go into short supply due to climate change. The rising carbon dioxide levels left behind by burning fossil fuels are making our oceans more acidic, posing a threat to the ocean's shell-forming creatures.
Surface ocean acidity or pH has become 0.1 unit more acid since the industrial revolution and this trend will continue as the oceans take up the ready supply of carbon dioxide in circulation overhead. Projections suggest that pH could be 0.3 or 0.4 units more acidic by 2100.
While mopping up some of the excess carbon dioxide might help to slow CO2 induced climate change, it could also spell doom for many species, according to new research published today in Nature.
The problem for shellfish and other species including corals and sea urchins relates to ocean concentrations of carbonate ions. These provide the building block substance used by many marine organisms to produce the calcium carbonate found in shells and exoskeletons.
The animals take up the carbonate in a form called aragonite, but the more acid the oceans the lower the concentrations of aragonite available in the water, and the more difficult it becomes for these creatures to form their shells.
The researchers tested this theory on a species of small swimming snail, known as a pteropod. The scientists exposed the pteropods to simulated ocean conditions as they are expected to be by 2100. The pteropod shells showed obvious signs of thinning in as little as 48 hours when exposed to these waters, the researchers from France, the US, Britain, Tasmania, Switzerland, Germany and Belgium pointed out.
Oceans in the polar regions are likely to see the effects of acidification earlier than warmer waters and these are waters where the pteropods thrive. Worryingly, the pteropods are a key food source for animals higher up the food chain.
They can be more plentiful than krill in waters such as the Ross Sea off Antarctica, with densities of hundreds to thousands of individuals per cubic metre of water, the authors state. This makes them an important food source for large baleen whales that live in polar regions.
The pteropods are also on the menu of carnivorous zooplankton that in turn feed larger creatures. The loss of pteropods or zooplankton could have a major impact on diverse species, including north Pacific salmon, mackerel, herring, cod and many species of baleen whales, the authors say.
Other organisms that rely on the availability of aragonite could also be at risk, "including cold-water corals which provide essential fish habitat", the authors say. There are extensive growths of cold-water corals off Ireland's western seaboard.
Sea urchins would be in similar difficulties. In waters exposed to higher carbon dioxide levels, "juvenile [sea urchins] stopped growing and produced more brittle and fragile exoskeletons in a subtropical six-month manipulative experiment," the authors report.
There is substantial experimental evidence to show that increasingly acidic seawater will greatly slow calcification rates in low latitude corals which form reefs out of aragonite, they say. Many phytoplankton, the bottom rung of the marine food web, will also have trouble forming their "tests" or shells.
Unfortunately we won't have to wait too long for these more acidic conditions to emerge, the authors add. "In our projections southern ocean surface waters will begin to become undersaturated with respect to aragonite . . . by the year 2050. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously."
This "could well alter the structure and biodiversity of polar ecosystems", they warn.
