Last week, the National Academies of Science released a report on research of what has been called “the other carbon problem”—ocean acidification. Excess carbon in the atmosphere has been lowering the ocean’s pH (increasing its acidity), which has the potential to severely alter the ocean’s chemistry. The NAS report says that we’re way behind in studying this problem, which wasn’t even fully recognized until recently. Just how far behind we are is made clear by a paper that will be released this week by PNAS, which reveals that two species of commercially harvested shellfish are likely to already be suffering increased mortality due to ocean acidification.
The basics of ocean acidification are fairly simple. Roughly a third of the CO2 emitted by human activity has ended up dissolved in the oceans; some fraction of that has combined with water to form H2CO3, a weak acid. These reactions take place quickly enough that a drop in pH has been apparent in some long-term monitoring stations. Overall, current estimates are that the pH of the oceans have dropped by 0.1 units (pH is a logarithmic scale) since the beginning of industrial carbon emissions.
This may sound minor, but as the NAS notes, it is expected to cause a “suite of changes in ocean chemistry.” Chief among them is a change in the availability of carbonate ions, which corals and shellfish use to build reefs and shells, respectively. With reduced access to the raw material for their homes, it’s possible that these animals, which provide ecosystem services and food to humans around the globe, might fall under increased stress, and be more prone to population crashes.
According to the new paper, however, those concerns are already past their sell-by date: significant shellfish species are already having problems with ocean acidification.
Most of the studies of acidification’s impacts have been done in prospective studies, in which a population of animals are exposed to an environment that represents likely future atmospheric CO2 concentrations. For example, we’re a bit above 390 parts-per-million CO2, so a study might set up an environment where the levels are 750ppm, which we could hit by the end of the century. These studies have generally found that shellfish don’t do well in this environment, suffering from malformations, loss of shell material, and increased mortality.
The authors of the new paper saw these as well, since they tested two species, the quahog clam and bay scallop, in concentrations of 750 and 1500 ppm. At the 750ppm level, basic shell structures like the hinge were severely malformed, while the surface of the shell had holes that were apparent when it was examined via scanning electron microscopy. There was also a significant drop in the viability of the larvae, and those that did survive were developmentally delayed compared to those raised at today’s concentrations. Matters got worse at the higher levels.
The interesting twist in the new work is that the authors also run the experiment under preindustrial CO2 levels of about 250ppm (actual levels were closer to 280ppm). For both species of shellfish, the mortality was much lower and development proceded more quickly. For the quahog, viability doubled (from 20 percent to 40 percent), while for the bay scallop, viability went from 43 percent to 74 percent. The animals made major developmental milestones more quickly—metamorphosis at day 14 occurred in half the animals at preindustrial CO2 levels, but that dropped to less than seven percent at modern levels.
The authors helpfully point out that they’ve eliminated predation in their lab conditions. If the animals were subject to being eaten, the weaker shells that form at higher CO2 levels would almost certainly increase the mortality.
Overall, they suggest that population crashes in bivalves have been ascribed to a number of stresses, like overfishing and pollution, but it’s possible that ocean acidification has also been at work in these cases. Given that the Earth has experienced higher CO2 levels in the past, why are they being hit so hard now? According to the paper, it’s actually been over 24 million years since levels are likely to have been this high, and many shellfish have diversified more recently than that; any changes in CO2 in the intervening time have also been far more gradual than the current pace.
The fact that we seemed to have completely missed existing problems arising from ocean acidification provides some perspective to last week’s NAS report, which suggests that the federal government has “taken positive initial steps” by setting up a program to study ocean acidification, but needs to make up for decades of neglect. We’ve already got an infrastructure in place to track changes in the ocean, but it was set up before acidification was widely recognized, so the equipment wasn’t designed to monitor it.
The report suggests a number of priorities when it comes to future research and monitoring—”commercially important mollusks” like the ones in the study receive mention. And it points out that international cooperation will be needed to put an effective monitoring system in place. Finally, it emphasizes the need for a set of data storage and sharing standards to ensure that any results are accessible to the wider research community.
Even though we may have missed some of the impacts of this change in ocean chemistry, further changes are likely to occur throughout this next century, and coming to grips with them as soon as possible may enable us to adapt to or mitigate the most severe problems.
PNAS, 2010. DOI: 10.1073/pnas.0913804107.
John Timmer, ars technica, 20 September 2010. Article.