Ocean Acidification

Over the last century, human activity has dramatically altered the natural ecosystem. Over-fishing, coastal run-0ff, and pollution are only a few ways that we are disturbing the ocean. For the most part, with adequate management and cooperation from the public, these anthropogenic activities can be controlled. However, possibly our most deleterious impact on the ocean, and all ecosystems for that matter, is a resultant of releasing carbon dioxide (CO2) into the atmosphere.

Figure 1: Data from the Mauna Loa Observatory on the island of Hawai`i. This is actual atmospheric CO2 from 1960 to 2010. For more information see http://www.esrl.noaa.gov/gmd/ccgg/trends/.

The most commonly discussed consequence of rising atmospheric CO2 is global warming. CO2 is a greenhouse gas ; when heat emitted from the sun enters our atmosphere it gets trapped and warms the Earth. Much research has shown that a warmer climate creates adverse conditions for many endangered amphibians, especially in places like Costa Rica 1 , steppe grass in the Great Plains 2, coral reefs 3, and many more organisms and ecosystems.

In the ocean, a corollary of rising sea surface temperature is sea-level rise. This is caused by two processes. Glacial melt in the polar regions is accelerating and physically adding more water to the ocean, causing the sea-level to rise. Secondly, warmer temperatures cause thermal expansion in water and consequently increases the volume of the ocean.

But what does rising CO2 have to do with ocean acidification?

Ocean acidification, “the other CO2 problem”4, is a decrease in the ocean pH. The ocean is the largest sink for CO2, so when CO2 is emitted into the atmosphere the majority is absorbed into the ocean, altering its chemistry.

More CO2 leads to more hydrogen ions (which is an acid) and less carbonate which is important for many calcifying organisms such as corals.

How does ocean acidification affect coral reefs?

Ocean acidification slows coral growth substantially and reduces the density of the coral skeleton.  The mechanisms in which ocean acidification inhibits coral growth is currently under debate  (see Jokiel 2011 5), but in general, lower pH waters are dissolving coral reefs.

Figure 2: Relationship between atmospheric CO2 and ocean acidification by Hoegh-Gulberg et al. 20073. As CO2 increases the carbonate ion (CO3=) decreases, which is necessary for coral calcification. However, see Jokiel (2011)5 for the proton flux hypothesis.

There are many studies that currently focus on the ramifications of ocean acidification on coral, fish, and invertebrate physiology.  However, there is less research available on community-level response to ocean acidification. This is my interest. Will ocean acidification affect species interaction? How will these interactions modify their ecosystem? I plan to investigate how ocean acidification influences bioerosion communities.

How might ocean acidification affect bioerosion?

Bioeroder Community

The bioeroder community is composed of all different genres of animals, plants, and fungi that have a slew of different physiologies (see my bioerosion page for more info). This diverse community of organisms will probably all respond differently to ocean acidification. Within the macroboring community in particular there are some animals that secrete calcium carbonate skeletons (such as bivalves and sponges-some sponges have calcium carbonate spicules while others have silicate, or glass), while others do not use calcium carbonate at all (such as worms). Because we know that ocean acidification has negative impacts on organisms that need calcium carbonate to survive, it is possible that the bioeroders with calcium carbonate will be negatively affected by ocean acidification while the others will not.  These different groups of organisms all erode the coral skeleton at different rates; therefore, if ocean acidification impacts the bioeroder community composition it will change the net bioerosion rate.

Bioerosion Rates

As mentioned above, ocean acidification could change the bioeroder community and alter net bioerosion rates. There are other ways that ocean acidification might increase or decrease bioerosion rates. Many boring organisms excrete acidic compounds, raising the possibility that a reduction in ocean pH could make it less metabolically costly to lower pH at the site of erosion. This would make it easier for boring organisms to erode coral skeleton and probably promote bioerosion rates. Additionally, research shows that less dense coral is more eroded than denser coral6,7, (there is one paper that has found the contrary8) and reduced coral skeletal density is expected under ocean acidification. Finally, increased CO2 can enhance the rates of photosynthesizing bioeroders, such as microborers9.

Can you help reduce the impacts of ocean acidification?

YES! You don’t have to be a marine biologist to help reduce the negative impacts of ocean acidification. First and foremost, do everything that you can to reduce your carbon footprint (check out this website for 101 ways to reduce you carbon footprint). In order to induce any major changes, we all need to do what we can to stop putting unnecessary amounts of CO2 into the atmosphere. Also, the negative effects of over-fishing, pollution, coastal run-off, and climate change all act in concert in contributing to the demise of coral reefs. If we reduce those anthropogenic activities that can be more easily managed, such as over-fishing and point-source pollution, then maybe coral reefs will have a better shot at acclimating to the new ocean conditions that we have uninvitedly created for them.

References:

1. Pounds, J., M. Bustamante, L. Coloma, J. Consuegra, M. Fogden, P. Foster, E. La Marca, K. Masters, A. Merino-Viteri, R. Puschendorf, S. Ron, G. Sanchez-Azofeifa, C. Still, and B. Young. 2006. Widespread amphibian extinctions from epidemic disease driven by global warming. Nature 439:161-167.

2. Alward, R., J. Detling, and D. Milchunas. 1999. Grassland vegetation changes and nocturnal global warming. Science 283:229-231.

3. Hoegh-Guldberg, O., P. J. Mumby, A. J. Hooten, R. S. Steneck, P. Greenfield, E. Gomez, C. D. Harvell, P. F. Sale, A. J. Edwards, K. Caldeira, N. Knowlton, C. M. Eakin, R. Iglesias-Prieto, N. Muthiga, R. H. Bradbury, A. Dubi, and M. E. Hatziolos. 2007. Coral reefs under rapid climate change and ocean acidification. Science 318:1737-1742.

4. Doney, S. C., V. J. Fabry, R. A. Feely, and J. A. Kleypas. 2009. Ocean Acidification: The Other CO2 Problem. Annual Review of Marine Science 1:169-192.

5. Jokiel, P. L. 2011. Ocean acidification and control of reef coral calcification by boundary layer limitation of proton flux. Bulletin of Marine Science 87:639–657.

6. White, J. 1980. Distribution, recruitment and development of the borer community in dead coral on shallow Hawaiian reefs. University of Hawaii at Manoa, Honolulu.

7. Glynn, P. W. 1997. Bioerosion and coral-reef growth: a dynamic balance. Pages 68-95 Life and Death of Coral Reefs. Chapman and Hall.

8. Highsmith, R. C. 1981. Coral bioerosion – damage relative to skeletal density. American Naturalist 117:193-198.

9. Tribollet, A., C. Godinot, M. Atkinson, and C. Langdon. 2009. Effects of elevated pCO(2) on dissolution of coral carbonates by microbial euendoliths. Global Biogeochemical Cycles 23:GB3008.

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