For years, researchers have warned that the increasing acidity of the oceans is likely to create a whole host of problems for the marine environment. Most of these warnings were predictions for future decades as well as theories about possible impacts based on experiments under artificial conditions. Now, scientists have discovered proof that ocean acidification is having significant impacts on an Antarctic marine snail, Limacina helicina antarctica, in its natural habitat. This tiny snail hails from a suborder called the “sea butterflies” because they flap their feet like wings as they swim. They’re also known as pteropods. Whatever you want to call them, they are an important component of marine food webs, and provide food for a variety of species.
Sea butterflies are vulnerable to ocean acidification because they form their transparent shells using aragonite, a form of calcium carbonate that is present in seawater. When CO2 concentrations increase in the atmosphere, more CO2 dissolves into seawater. Increasing CO2 decreases the pH of seawater, making it more acidic and thus promoting the dissolution of calcium carbonate (learn more about this process here). With less aragonite available, sea butterflies, corals, crustaceans, mollusks, and other organisms that need calcium carbonate to form their shells are expected to experience significant problems. The ocean has already become about 30% more acidic (a drop of 0.1 pH units) since pre-industrial times. Scientists had previously predicted that the Southern Ocean would be among the first to show the negative impacts of acidification because it has a relatively low saturation or level of aragonite compared to other parts of the ocean. They estimated that undersaturation of the entire Southern Ocean would occur on a seasonal basis between 2030 and 2038 if CO2 emissions continue increasing at their current rate.
However, the researchers studying the sea butterflies found that the water in some of their samples was already undersaturated with respect to aragonite due to a combination of ocean acidification and the upwelling of deep ocean water with high concentrations of CO2 to upper layers. Examining sea butterflies from these areas under a high-powered microscope, they found that they showed signs of serious shell dissolution. Samples from areas with abundant aragonite were not found to be experiencing these problems. It’s unknown what weaker shells mean in terms of survival for these creatures, but it could make them unable to defend themselves against predators. This in turn could lead to depletion of their populations, removing an important food source for many other species. If the predictions of undersaturation on a broad scale are correct, we don’t have much time before we will see these effects throughout the Southern Ocean. The researchers also note that warming temperatures drive more upwelling of deep water, which will further increase acidity and decrease saturation in the areas of the ocean where many aragonite-using species live.
It’s clear that acidification is no longer a theoretical problem. Although the fate of Antarctic snails might seem like a minor concern compared to rising sea levels, the authors of the study explain that pteropods are an important part of the carbon cycle, and changes in their population size might affect those processes. We might be able to build barriers to prevent major cities from flooding, but we will have much more trouble changing the chemistry of the entire ocean. The future health of the oceans depends on all of us paying attention to the plight of the humble sea butterfly.
(For more information on what one U.S. state is doing to address ocean acidification, click here.)