When scientists and policy-makers gathered in Poland this month for the United Nations Climate Change Conference, significant attention was paid to the effects of increased greenhouse gases on the oceans. The threats of sea level rise and warming sea temperatures, including the latter’s impact on coral reefs by causing bleaching, received a major focus.

A lesser-known impact of the rise in carbon dioxide levels will be “ocean acidification”, a term coined just five years ago. Early evidence suggests acidification could have as great an impact on ocean ecosystems – and, by extension, MPAs – as the other threats. Disconcertingly, a recent study on the northwest coast of the U.S. showed ocean acidification may be occurring much faster than expected. This month, MPA News examines ocean acidification and its potential impacts, as well as how MPA practitioners can plan ahead.

What is ocean acidification?

Earth’s oceans absorb carbon dioxide, or CO2 – the most common greenhouse gas, created by the burning of fossil fuels. Since the Industrial Revolution, which marked the start of the current rise in atmospheric greenhouse gas levels, the oceans have absorbed approximately half of the CO2 emitted by human activities. Without this long-term storage, the greenhouse gas concentration in the atmosphere would be much higher, and the planet much warmer. However, absorbing the CO2 causes changes in ocean chemistry, namely lowering the pH of seawater and decreasing the concentration of carbonate ions.

It works like this: When the CO2 is absorbed, it reacts with the water to form carbonic acid, which then releases hydrogen ions. These freed hydrogen ions reduce the water’s pH – in other words, the water becomes more acidic. (For an explanation of pH, see the box “What is pH?” at the end of this article.) Normally seawater is slightly alkaline, with a pH of 8.06. As seawater moves toward the acid end of the pH scale, its pH measurement will decline. Under scenarios from the Intergovernmental Panel on Climate Change, ocean pH by the year 2100 could drop as low as 7.76. That would represent a 30% increase in acidity, as the pH scale is logarithmic.

Some of the hydrogen ions released by carbonic acid also bind to carbonate ions in the water, making them unavailable for use by species that need them. Less carbonate makes it more difficult for many marine organisms – including corals, calcareous phytoplankton, mussels, snails, and sea urchins – to form calcium carbonate, their major mineral building block. To make matters worse, when carbonate concentrations fall too low, calcium carbonate that has already formed starts to dissolve. As a result, marine organisms have a harder time either creating new, or maintaining old, skeletal material. (Some people have compared ocean acidification to osteoporosis, the disease of reduced bone density in humans.)

The effects of decreased calcification rates have been studied most with coral reefs. Coral skeletons are made of calcium carbonate, and ocean acidification poses a direct threat to the foundation of reef ecosystems. A new report published by the Global Coral Reef Monitoring Network, Status of Coral Reefs of the World: 2008, states most of Earth’s coral reefs could disappear within 40 years due to acidification and other factors. (The report is at http://iucn.org/index.cfm?uNewsID=2408.) In August 2008, reef scientists gathered in Hawai’i to craft strategies to address the threat of acidification. The resulting Honolulu Declaration focused not only on the need to reduce CO2 emissions but on how to manage reef ecosystems in ways to aid their survival (see the article “The Honolulu Declaration…” in this issue).

As for impacts on non-reef organisms, many economically important species will be affected by acidification. Mollusks, including clams, mussels, and other shellfish, will have difficulty building their shells. Tim Wootton, a biologist at the University of Chicago, found in his long-term study of a mollusk community in the northwest U.S. that his water samples over eight years had acidified at a rate 20 times faster than what he had expected. According to his computer models, increased pH at his study site will likely lead to substantial declines in the number of mussels and large barnacle species, and increases in the populations of what those species eat, like algae and smaller barnacles. (Wootton’s paper, “Dynamic patterns and ecological impacts of declining ocean pH in a high-resolution multi-year dataset”, appeared in the 24 November 2008 edition of the journal Proceedings of the National Academy of Sciences.)

Wootton says other studies on the U.S. west coast offer similar acidification results. “There is reason to suspect that the strong decline in pH is not limited to our research site,” he says. “More generally, it seems likely that the trends we are seeing are at least a feature of the larger northeastern Pacific.”

Flexible MPA boundaries may be needed

Judy Kildow, a social scientist and policy analyst at the Monterey Bay Aquarium Research Institute in the U.S., says the compromising of shelled creatures as projected by Wootton could affect a wide array of marine life. “Marine mammals that feed on these shelled creatures will have to compete with other predators for other remaining food, and may not fare well,” she says. “As some creatures disappear, others will fill the niches. What is clear is that the ecosystem will look different in 20-50 years, with many familiar commercial species gone and the likelihood that far less desirable creatures will replace them. So it is not just coral reefs and mollusks that will be affected but a much larger range of marine creatures, from phytoplankton to marine mammals. Everything in the oceans will be affected in some way.”

Kildow points out several policy challenges associated with addressing acidification. These range from major global challenges (curbing greenhouse gas emissions) to more localized ones, like reducing other environmental stressors (such as pollution, runoff, and overfishing) that further weaken marine ecosystems and worsen any impacts of acidification. At the individual MPA level, she says, practitioners can take steps to plan ahead, including by anticipating a changing environment. “Boundaries of MPAs should not be geographic,” she says. “Instead, they should be determined by ecological indicators, such as species diversity and other critical indicators that planners seek for sustaining biodiversity.”

UNESCO, through its Intergovernmental Oceanographic Commission (IOC), has taken a leadership role in assessing the impacts of ocean acidification. The organization released a fact sheet this year that echoes Kildow’s call for flexible MPA boundaries as part of better ecosystem management. “Marine reserves are being established throughout much of the coastal oceans to preserve biodiversity and boost fishing stocks,” the fact sheet states. “Policies need to allow flexibility to shift the boundaries of these reserves as ocean chemistry and ecosystems change in response to acidification.” (The three-page UNESCO fact sheet, “The Ocean in a High CO2 World”, is available in English at http://ioc3.unesco.org/oanet/OAdocs/FactSheet_en.pdf, and in French at http://ioc3.unesco.org/oanet/OAdocs/FactSheet_fr.pdf.)

Maria Hood, Project Director of the IOC-sponsored International Ocean Carbon Coordination Project (www.ioccp.org), says much more research is needed on the impacts of acidification, including in MPAs. “Establishing baseline surveys and regular monitoring programs of the ecosystem and biodiversity will be key to detecting and understanding future changes brought on by ocean acidification,” says Hood. “The coast environment and coral reef ecosystems undergo rapid and large diurnal variations in carbonate chemistry. Monitoring will require both frequent and long-term observations of the carbonate system as well as regular ecosystem surveys. Appropriately monitored, MPAs could provide a critical early-warning system for ocean acidification impacts.” IOC is working with other organizations, including the European Project on Ocean Acidification (www.epoca-project.eu) and the U.S. Ocean Carbon and Biogeochemistry Program (www.us-ocb.org) to help coordinate global research on acidification, including setting priorities, standardizing experimental methods, and sharing data.

Remaining uncertainties surrounding the potential impacts of ocean acidification make it too early to sketch out a worst-case or best-case scenario for fish and fisheries, says Jan Helge Fosså, Chief Scientist at the Norwegian Institute for Marine Research. “The ecosystem consequences of ocean acidification are too unpredictable, and we know too little about direct physiological effects on fish species and their different life stags,” says Fosså. “But there are reasons to believe that a low pH and high CO2 can affect fish directly through their physiology and indirectly through ecosystem effects, such as changes in food quality, quantity, and timing.” In certain invertebrates and some fish, CO2 accumulation and lowered pH in animals’ bodies could result in acidosis, a build-up of carbonic acid in body fluids, according to UNESCO. This would lead to lowered immune response, metabolic depression, and asphyxiation.

“The use of MPAs in fisheries management is still under debate, but I think that MPAs can play a role in addressing acidification,” says Fosså. “It is important to maintain strong and robust fish stocks in a changing environment. By robust, I mean stocks that are not overfished and have suffered a minimal loss of genetic diversity; this allows them maximum potential for adapting to the expected and possibly irreversible changes in the environment. Although the use of MPAs in fisheries management is still under debate, they can play a role in protecting stocks that are either fished down or local and vulnerable. MPAs could help, for example, to secure reproduction and recruitment by protecting spawning grounds and nursery areas. Keeping the potential for high stock recruitment is very important.”

For links to additional fact sheets, statements, Powerpoint presentations, research programs, and other general information on ocean acidification, go to the website for the Ocean Acidification Network at www.ocean-acidification.net.

For more information:

Tim Wootton, University of Chicago, Chicago, Illinois, U.S. E-mail: twootton@uchicago.edu

Judy Kildow, Monterey Bay Aquarium Research Institute, Monterey, California, U.S. E-mail: jtk@mbari.org

Maria Hood, Intergovernmental Oceanographic Organization, UNESCO, Paris, France. E-mail: m.hood@unesco.org

Jan Helge Fosså, Institute of Marine Research, Bergen, Norway. E-mail: jan.helge.fossaa@imr.no

BOX: What is pH?

The acidity or alkalinity of a liquid is measured on a scale of 0 to 14. This measurement is called pH for “power of hydrogen”, since it measures the activity of hydrogen ions in the liquid. A pH below 7.0 is acidic, while a pH above 7.0 is alkaline (also called basic). Since pH is a logarithmic scale, a difference of one pH unit is equivalent to a ten-fold difference in hydrogen ion concentration. In other words, a solution of pH 5.0 is ten times more acidic than a solution of pH 6.0.