Ocean Acidification – The full story:

Ocean acidification has been termed by some as “the other CO2 problem”.

It is a direct result of excess carbon dioxide emitted into the atmosphere (from the burning of fossil fuels) being taken up by the world’s oceans. In fact, our oceans have absorbed more than one-third of the CO2 we have emitted. Without the oceans acting as a carbon sink our planet would be warming at an even faster rate than it is now.(1) However, the oceans pay a price for the increased CO2 that they store. It changes the fundamental chemistry of the oceans resulting in a decline in ocean pH (a measure of how acidic or basic a solution is) and a decrease in the concentration of carbonate ion.

What does a more acidic ocean mean for wildlife?

 The effects of ocean acidification on marine organisms and ecosystems has been an exploding area of research for the past decade, and we are still in the relative infancy of understanding all the mechanisms by which ocean acidification changes the marine environment and the long-term effects it will cause.

One of the biggest effects of ocean acidification is on the process of calcification.

Organisms like corals and shellfish rely on high concentrations of carbonate ions in seawater to form their calcium carbonate skeletons and shells. As the carbonate ion concentration declines as part of ocean acidification, the ability of these organisms to make their calcified structures is affected and they may experience a decrease in growth, survival, and reproduction.

Even animals that don’t use calcification are affected by ocean acidification. Marine organisms need to maintain their internal body within an optimal pH range. As the water around them becomes more acidic they must expend more energy to maintain their internal environment within an acceptable range, which can also lead to less growth and reproduction.(2,3) Ocean acidification can even interfere with the olfactory cues reef fish use to find suitable habitat or avoid predators.(4,5)

Where are we feeling the impacts?

Coral reefs:

Coral reefs provide important habitat to thousands of species, supply food for coastal communities, attract economically important tourism, and protect coasts from waves and storms. Reefs are built from the calcium carbonate skeletons of corals and are highly susceptible to ocean acidification. Low pH limits the growth rate of corals, can increase mortality rates, and decrease survival of larval or juvenile corals. The effects of ocean acidification on coral reefs are compounded by other stressors including increasing water temperatures and rising sea levels.

Shellfish Hatcheries:

Ocean acidification has already had a significant economic impact on fisheries, in particular shellfish farmers. The larval stages of shellfish species are particularly sensitive to low pH waters which can disrupt proper development of their shells, resulting in their inability to feed and resulting in high mortality rates.(8)

In places like the Pacific Northwest or Southern Massachusetts a large percentage of the local economy depends on shellfish harvest and aquaculture, making these communities particularly vulnerable to ocean acidification. It is estimated that ocean acidification has cost the oyster industry in Washington $110 million and jeopardized 3200 jobs.(9)

Arctic/Antarctic:

The Arctic and Southern Oceans are extremely vulnerable to ocean acidification and climate change. In these oceans the increase in CO2 from human sources is compounded by the cold waters (cold water holds more CO2 than warm water), CO2 rich waters upwelling from deep, and influx of freshwater from melting ice caps. Pteropods (a small marine snail) are unable to form their calcium carbonate shell in the acidified waters.(6) Similarly, Antarctic krill (a small crustacean) experience reproductive failure at low pH levels.(7) These animals are important prey species for Polar food webs, and the indirect effects of ocean acidification could be significant.

What are we doing about it?

The long-term solution is to reduce CO2 emissions through strong government legislation and reducing our personal carbon footprint by using alternative energy sources. Additionally, limiting other forms of pollution into the oceans will increase the ocean’s resiliency to acidification.

On a small scale, there are several immediate strategies that managers and industries are implementing. Seagrasses, such as eelgrass, act as carbon sinks because they capture CO2 from seawater and use it in photosynthesis. These grasses are now being planted (or existing beds are being actively protected) as a way to mitigate the local effects of ocean acidification.

The aquaculture industry is investing in the monitoring of water chemistry so they can time spawning events to align with the most favorable conditions. Some hatcheries have installed bicarbonate treatment plants to increase the pH of the seawater before it is brought into the hatchery. Others are choosing to move their operations to Hawaii, where it is less prone to strong acidification events.

References:

1. Doney SC, Fabry VJ, Feely RA, Kleypas JA (2009) Ocean acidification: the other CO2 problem. Ann Rev Mar Sci 1:169-192
2. Heisler N, (1986) Acid-base regulation in animals. Elsevier Science, University of Michigan
3. Seibel BA (2003) Biological impacts of deep-sea carbon dioxide injection inferred from indices of physiological performance. J Exp Biol 206(4):641-650
4. Munday PL, Dixson DL, Donelson JM, Jones GP, Pratchett MS, Devitsina GV, Døving KB (2009) Ocean acidification impairs olfactory discrimination and homing ability of a marine fish. Proc Natl Acad Sci USA 106(6):1848-1852
5. Munday PL, Cheal AJ, Dixson DL, Rummer JL, Fabricius KE (2014) Behavioural impairment in reef fishes caused by ocean acidification at CO2 seeps. Nature Climate Change 4, 487–492
6. Comeau S, Jeffree R, Teyssié JL, Gattuso JP (2010) Response of the Arctic pteropod Limacina helicina to projected future environmental conditions. PLoS One 5(6):e11362
7. Kawaguchi S, Kurihara H, King R, Hale L, Berli T, et al. (2011) Will krill fare well under Southern Ocean acidification? Biol Lett (7):288-291.
8. Waldbusser GG, Brunner EL, Haley BA, Hales B, Langdon CJ, and Prahl FG (2013) A developmental and energetic basis linking larval oyster shell formation to acidification sensitivity, Geophys. Res. Lett., 40, 2171–2176
9. Ekstrom JA, Suatoni L, Cooley SR, Pendelton LH, Waldbusser GG, et al. (2015) Vulnerability and adaptation of US shellfisheries to ocean acidification. Nature Climate Change 5, 207–214