Overview: Ocean Acidification in Alaska
Ocean acidification is the term used to describe the long-term lowering of average global ocean pH. As concentrations of atmospheric carbon dioxide (CO2) increase, some of this CO2 is absorbed by the ocean. This triggers a series of chemical reactions that ultimately lead to lower pH and decreased concentration of carbonate ions in the oceans.
Since the Industrial Revolution, the average pH of surface ocean waters has decreased by 0.11 units, from 8.21 to 8.10. This may not seem significant, but the pH scale is logarithmic, meaning a decrease of a single pH unit represents a tenfold increase in acidity. The 0.1 drop in pH that we have seen corresponds to a 30% increase in ocean acidity.
The chemistry of ocean acidification is accepted and well understood by the scientific community. However, the biological impacts of ocean acidification on marine organisms and ecosystems are much more uncertain. A great deal of research is now being directed at understanding ocean acidification and its current and potential effects all over the world, including Alaska.
How Will Alaska Be Affected by Ocean Acidification?
Alaska’s coastal waters are especially susceptible to ocean acidification compared to lower latitude oceans. First, cold water can absorb more CO2 than warm water, so our cold northern waters tend to be naturally high in CO2. This means that Alaska and other high latitude seas will likely see the effects of ocean acidification sooner than areas farther south. Additionally, natural factors such as high productivity, glacial melt, and upwelling increase the potential for regions around Alaska to be vulnerable to ocean acidification.
Alaska experiences large surface blooms of phytoplankton each year. Phytoplankton consumes CO2 through photosynthesis. When phytoplankton individuals die, they sink to the ocean floor and are decomposed by bacteria, which releases CO2 and lowers pH in the deep ocean.
In some parts of Alaska, deep CO2-rich water is churned up onto coastlines by natural, wind-driven upwelling. This upwelling mixes with the surface waters and lowers pH at the surface.
Glacier melt produces very dilute freshwater, and has low concentrations of the carbonate that helps the ocean buffer against lower pH. In the summer months when glacier melt is high, the surface waters of the ocean have a weaker buffer to changes in pH. This makes it easier for CO2 to decrease pH levels in the surface of the ocean.
A number of natural factors can influence sensitivity to ocean acidification in Alaska’s seas at different depths and at different times of the year. Ocean acidification is not uniform, and the range and degree of its impact can vary.
Biological Impacts of Ocean Acidification
A consequence of increasing CO2 in the ocean is that it becomes harder for carbonate to mineralize. This is important, as many organisms, including crabs and mollusks, use carbonate to build their shells. Because Alaskans rely on the marine ecosystem, primarily because of the strong commercial and subsistence fishing sectors, ocean acidification poses a high risk for Alaska.
Calcifying plankton, such as pteropods, foraminiferans, coccolithophores, echinoderm larvae, and bivalve larvae, are at the base of the food web. If they were unable to form shells as a result of ocean acidification, this could have serious effects on species further up the food chain, including commercially valuable fish such as salmon, Alaska pollock, and halibut.
In Alaska, pteropods are of particular concern because they are an important food source for commercially valuable pink salmon, herring, and Alaska pollock. Pteropods can compose up to 50% of the juvenile pink salmon diet. Laboratory studies conducted on pteropods analyzed how shells would respond to projected pH levels for the year 2100. Results showed that when pteropods were placed in seawater with projected lower pH levels, their shells slowly dissolved.
Scientists have also documented pteropod shells dissolving in the wild from corrosive, low-pH waters along the US West Coast. A large decline in pteropods would mean salmon and other species would have to compete more strongly for limited food supplies. This could result in a decline in fish populations, or individual fish could be smaller.
In Alaska, deep-sea cold-water corals are widespread and diverse, and they support many commercial crab and fish species. The species composition of coral reefs is likely to shift to those that can tolerate lower acidity. These changes could impact the many organisms that rely on corals for shelter and food, and could result in detrimental effects on commercial and subsistence fish and shellfish species in Alaska.
Even a small reduction in pH makes it just a little bit harder for calcifying organisms to perform normal body functions. Research on red king and Tanner crabs has shown their growth and survival rates go down in response to a significant drop in pH. Ocean acidification will impact species differently, and some species may even benefit from the change. In general, organisms that use calcium carbonate to form shells are more likely to be negatively impacted by ocean acidification.
Ocean acidification has the potential to change species composition and abundance, which can significantly alter the marine food web in Alaska. The extent to which ocean acidification is currently affecting Alaska fisheries, and how it will affect fisheries in the future, are both important issues that we know relatively little about. But scientists and agencies are gaining a greater understanding of how ocean acidification will affect Alaska as well as the rest of the world. Learn more about current research conducted in Alaska.