By Brent Fewell
As first reported by Amena Saiyid over at BNA, EPA announced this week that it plans to establish a technical working group to begin reviewing the causes of ocean acidification, as announced in a letter from EPA’s Nancy Stoner, Acting Assistant Administrator for the Office of Water, to the Center for Biological Diversity in response to a request for EPA to establish new water quality criteria. Now, some may take a dim view of this development, given the political rancor over climate change, but I think the decision is timely and well founded.
Setting aside for a moment the heated debate over hotter temperatures and sea level rise from human activities, the increasing concern over ocean acidification from atmospheric carbon emissions is something that is very real and for which a much stronger scientific case can be made. While the longterm risks and implications are still plagued with scientific uncertainty, the science behind ocean acidification is fairly straight forward and well established. So, how will increased carbon emissions affect the ocean? And why should we be concerned? Glad you asked.
As carbon emissions continue to rise – an indisputable trend – the oceans will absorb more CO2, as part of a natural cycle. Carbon in the ocean comes from numerous sources, including atmospheric uptake, as depicted in the NOAA diagram below. When CO2 dissolves, it reacts with water to form a balance of ionic and non-ionic chemical species: dissolved free carbon dioxide (CO2), carbonic acid (H2CO3), bicarbonate (HCO−3) and carbonate (CO2−3). The more CO2 we emit, the more CO2 the oceans will absorb. Some of these extra carbonic acid molecules react with a water molecule to produce bicarbonate and hydrogen ions, thus increasing the ocean’s acidity (H+ ion concentration). Over the last century, it is estimated that ocean pH has dropped by 0.1 units on the logarithmic scale, which represents a 30 percent increase in H+ ions. If carbon emissions continue to rise as projected, ocean pH is estimated to drop by another 0.3 to 0.5 by 2100. This doesn’t sound like a big change in ocean chemistry, but the biological effects could be quite noticeable.
Most of the lower trophic marine organisms (corals, calcareous phytoplankton, mussels, snails, sea urchins, crabs, etc.) depend upon calcium carbonate to build their shells and exoskeleton. As carbonic acid increases so does the amount of corrosive hydrogen ions, impeding shell growth and, in some cases, causing the shell to simply dissolve away. The negative effects of natural acidification from volcanic activity have been well documented in marine systems. These effects are also being better understood in controlled environments. John Reis, a scientist at Woods Hole Oceanographic Institution, has conducted some very interesting research on the impact of increased CO2 on aquatic organisms, and found mixed results, boasting both winners and losers. “The wide range of responses among organisms to higher CO2—from extremely positive to extremely negative—is the truly striking thing here,” Ries noted. Some species, such as cold water lobster were not only not negatively impacted, but actually exhibited increased growth during Ries’s experiments (as seen in this picture).
In waters containing more CO2, organisms have more raw material (carbon) to use for shells. But they can only benefit from the high CO2 if they can convert the carbon to a form they can use to build their shells and can also protect their shells from dissolving in the more acidic seawater.
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As expected, in the highest CO2 used, the shells of some species, such as conchs—large, sturdy Caribbean snails—noticeably deteriorated. The spines of tropical pencil urchins dissolved away to nubs. And clams, oysters, and scallops built less and less shell as CO2 levels increased.
However, two species of calcifying algae actually did better at 600 ppm (predicted for the year 2100) than at present-day CO2 levels, but then they fared worse again at even higher CO2 levels. Temperate (cool-water) sea urchins, unlike their tropical relatives, grew best at 900 ppm, as did a temperate limpet. Crustaceans provided the biggest surprise. All three species tested—the blue crab, American lobster, and a large prawn—defied expectations and grew heavier shells as CO2 swelled to higher levels.
“We were surprised that some organisms didn’t behave in the way we expected under elevated CO2,” said Anne Cohen, second author on the Geology paper. “Some organisms were very sensitive [to CO2 levels], but there were a couple [of species] that didn’t respond ’til it was sky-high—about 2,800 parts per million. We’re not expecting to see that [CO2 level] any time soon.”
Ries and colleagues found that species with more protective coverings on their shells and skeletons—crustaceans, the temperate urchins, mussels, and coralline red algae—are less vulnerable to the acidified seawater than those with less protective shells, such as conchs, hard clams, and tropical urchins.
All of the test organisms continued to create new shell throughout the experiment, Ries said, but some suffered a net loss of shell because older, more massive portions of their shells dissolved under the highest CO2 conditions.
Although our understanding of the effects of more carbon on the oceans is still evolving, the bottom line is that more is not better. The loss of corals and other ocean critters less tolerant to additional carbon will invariably destabilize oceanic ecosystems, and adversely affect even those more tolerant species, as Ries notes:
[T]he predicted rise in CO2 over the coming centuries could cause changes in marine ecosystems—particularly those composed largely of shell-builders, such as tropical coral reefs. Moreover, even organisms that appear to benefit from the elevated CO2 may suffer from the decline of less tolerant species upon which they depend for food or habitat.
This destabilization would invariably affect tourism and commercial fisheries around the globe dependent upon the oceans for food, jobs and recreation. And although the science behind ocean acidification is simple and straight forward, the solution is less so. While some will continue to advocate for more aggressive policy solutions, ranging from a carbon-free economy to a progressive carbon tax, others continue to tout other solutions such as ocean fertilization or CCS which will help mitigate and slow down the worst of the impacts being predicted.
As I’ve said before, this stuff isn’t simple and there are no easy solutions. Notwithstanding, I’m hopeful that EPA’s workgroup will take into consideration the best available science and do what is necessary to persuade the public that whatever decision flows from the Agency’s review, will be balanced and based on sound science.