Ocean Acidification: More than just a sour taste (pt.2)

Acidification of our oceans is having, and will have, many impacts on marine ecosystems and processes. As mentioned at the end of part 1, excess atmospheric carbon dioxide moves into the ocean to restore the equilibrium, or balance, between the two. This occurs through a reaction that forms carbonic acid, as seen below, the Hydrogen ions (H+) from which lowers the pH.

oa

http://www.oceanacidification.org.uk/Oarp/media/images/oa_800.jpg

There is also an equilibrium between Carbonic acid and its two ions- Bicarbonate (HCO3-) and Hydrogen. As such, some of the carbonic acid breaks down into these products to restore the balance. Yet another equilibrium exists between Bicarbonate ions and its products of dissolution, Hydrogen ions and Carbonate ions (CO32-). This whole relationship can be described below:

equib1
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This increase in Hydrogen ion concentration, as already mentioned, can have detrimental effects to species using carbonates to build their skeletons, such as corals or pteropods. It makes it harder for them to ‘pull’ the carbonates out of solution and so slows growth, or if pH drops enough, as happened in the case of some pteropods, can cause the structures to dissolve.

Most people have heard of the great barrier reef- indeed, it is a tropical reef systems most people think of when talking about corals. Tropical reefs provide a habitat for many species of fish, crustaceans, arthropods and anemones, among other kinds of creatures. This means they play host to an entire ecosystem that the corals themselves are a part of- The death of the reefs will result in a possible loss of habitat for many marine organisms.

reefpicFigure 3: The kind of environment most of us think about when talking about reefs. Oceana

But as well as these tropical reefs, there are coldwater reefs at higher latitudes that are at even more immediate risk– either way, if carbon output continues at the current rate we risk losing a large part of the global marine ecosystem by 2100.

There are a few ways we can tell this will happen. Perhaps the most reliable one is a method called ‘Time-For-Space proxies’. This is where a natural source of CO2 in the water, such as a volcanic vent, lowers the pH of the surrounding area over a gradient- pH is lowest closer to the vent- and so the effects on local habitats and ecosystems of a certain pH level can be recorded:

Nature.com, J.M Hall-Spencer et al

These sort of studies  show that a drop in pH correlates with a drop in biodiversity and a temporary drop in calcifying algae.

coralline

Figure 5: A form of calcifying, or coralline algae. (Courtesy of Seafriends)

  As well as Time-for-Space proxies, there are laboratory experiments we can perform called laboratory mesocosms- essentially a series of water tanks, of which we can control factors such as pH and salinity. Studies using these environments showed the production by phytoplankton of a chemical called Dimethyl Sulphide was reduced at lower pH values. Dimethyl Sulphide is an important cloud seeder, meaning that it provides a nucleus for clouds to condense around. Without it we may have less clouds resulting in less sunlight being reflected into space and so an increased warming effect on the Earth’s atmosphere and thus oceans.

In the final part, we will go through possible solutions to this problem, and the consequences of each.

Sources:

Calcifying algae (no date) Available at: http://www.seafriends.org.nz/enviro/habitat/rsgreen.htm.

CO2 dissolution equation (no date) Available at: https://realocean.files.wordpress.com/2017/02/ff407-co2rxn.jpg.

Hall-Spencer, J.M., Rodolfo-Metalpa, R., Martin, S., Ransome, E., Fine, M., Turner, S.M., Rowley, S.J., Tedesco, D. and Buia, M.-C. (2008) ‘Volcanic carbon dioxide vents show ecosystem effects of ocean acidification: Abstract: Nature’, Nature, 454(7200), pp. 96–99. doi: 10.1038/nature07051.

Henderson, C. (2006) Ocean acidification: The <I>other</I> CO<SUB>2</SUB> problem. Available at: https://www.newscientist.com/article/mg19125631-200-ocean-acidification-the-other-co2-problem/.

Ocean acidification (no date) Available at: http://www.oceanacidification.org.uk/Oarp/media/images/oa_800.jpg.

Roberts, J.M. (no date) Researchgate. Available at: https://www.researchgate.net/publication/307910172_Cold-Water_Corals_in_an_Era_of_Rapid_Global_Change_Are_These_the_Deep_Ocean’s_Most_Vulnerable_Ecosystems.

Tropical ocean reef (no date) Available at: http://oceana.org/sites/default/files/styles/lightbox_full/public/_21.jpg?itok=mcvGetD4.

Webb, A.L., Malin, G., Hopkins, F.E., Ho, K.L., Riebesell, U., Schulz, K.G., Larsen, A. and Liss, P.S. (no date) ‘Ocean acidification has different effects on the production of dimethylsulfide and dimethylsulfoniopropionate measured in cultures of Emiliania huxleyi and a mesocosm study: A comparison of laboratory monocultures and community interactions’, Environmental Chemistry, 13(2), p. 314. doi: 10.1071/en14268.

Calcifying algae (no date) Available at: http://www.seafriends.org.nz/enviro/habitat/rsgreen.htm.

CO2 dissolution equation (no date) Available at: https://realocean.files.wordpress.com/2017/02/ff407-co2rxn.jpg.

Hall-Spencer, J.M., Rodolfo-Metalpa, R., Martin, S., Ransome, E., Fine, M., Turner, S.M., Rowley, S.J., Tedesco, D. and Buia, M.-C. (2008) ‘Volcanic carbon dioxide vents show ecosystem effects of ocean acidification: Abstract: Nature’, Nature, 454(7200), pp. 96–99. doi: 10.1038/nature07051.

Henderson, C. (2006) Ocean acidification: The <I>other</I> CO<SUB>2</SUB> problem. Available at: https://www.newscientist.com/article/mg19125631-200-ocean-acidification-the-other-co2-problem/.

Ocean acidification (no date) Available at: http://www.oceanacidification.org.uk/Oarp/media/images/oa_800.jpg.

Roberts, J.M. (no date) Researchgate. Available at: https://www.researchgate.net/publication/307910172_Cold-Water_Corals_in_an_Era_of_Rapid_Global_Change_Are_These_the_Deep_Ocean’s_Most_Vulnerable_Ecosystems.

Tropical ocean reef (no date) Available at: http://oceana.org/sites/default/files/styles/lightbox_full/public/_21.jpg?itok=mcvGetD4.

Webb, A.L., Malin, G., Hopkins, F.E., Ho, K.L., Riebesell, U., Schulz, K.G., Larsen, A. and Liss, P.S. (no date) ‘Ocean acidification has different effects on the production of dimethylsulfide and dimethylsulfoniopropionate measured in cultures of Emiliania huxleyi and a mesocosm study: A comparison of laboratory monocultures and community interactions’, Environmental Chemistry, 13(2), p. 314. doi: 10.1071/en14268.

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