Protecting Ocean Ecosystems

The Catch of Climate Change: What is Ocean Acidification?

Nancy Shrodes is a volunteer for the Ocean Conservation Program at the Conservation Law Foundation. She recently graduated from Tufts University (class of 2011), majoring in Environmental Science with a focus in marine biology.

The Catch of Climate Change is an exclusive Talking Fish series that will look at the potential impacts of ocean acidification from climate change on New England’s oceans and fisheries. This post, the first in the series, will provide an introduction to the concept of ocean acidification and its potential ramifications.

Although many people are aware of climate change, the specifics concerning how it affects our oceans are complicated and often poorly understood. We have all heard about sea level rise from receding glaciers, and the threat they present to low-lying coastal communities and beautiful species like polar bears. In addition, the influx of melted glacial freshwater into our oceans could disrupt crucial ocean circulation that provides nutrients to organisms and regulates world climate, a process primarily driven by ocean temperatures and salinity. Another consequence that could have drastic effects on marine ecosystems and the economy is ocean acidification (OA).

What is ocean acidification, and how does it happen? Oceans act as a sink for carbon dioxide, absorbing 30% of the CO2 released into the atmosphere from burning fossil fuels.[1] Through a series of chemical reactions, an overabundance of CO2 can result in a lower pH, creating a more acidic oceanic environment (the lower the pH of a substance, the more acidic it is). In the last century alone, pH has already decreased by 0.1 units, a notable change in acidity, accompanied by an approximate 1°C increase in ocean temperature.[2] It is no coincidence that the onset of such rapid changes became visible after the Industrial Revolution, considering it initiated the monumental increase in greenhouse gas emissions that continues today.

While changes in temperature and pH occur simultaneously as the result of the increased release of fossil fuels, it is important to keep in mind that they are separate processes. The Intergovernmental Panel on Climate Change (IPCC) predicts a 0.3-0.5 unit drop in pH and an up to 4°C increase in ocean temperatures by 2100 if current emission rates continue.[3] These are huge changes for sensitive marine communities, occurring at faster rates than have ever occurred in the last 55 million years (which was the last time oceanic pH came close to the levels predicted for 2100).[4] Now that you are aware of some raw facts, you might be wondering:  what are the effects of such changes and why do we care?

OA causes shell degradation and inhibits many species' ability to calcify (Photo credit: NOAA)

OA is a relatively new area of study, and the research that has been done thus far has identified varied species-specific responses to increasing acidification that influence reproduction, growth, and behavior. The process of OA reduces the amount of calcium carbonate available for calcifying organisms like plankton, shellfish, and corals, which use calcium carbonate to build their hard parts. Hence, OA directly inhibits their ability to create and maintain protective shells and skeletons. In studies that looked at the development of marine organisms in conditions of warmer and more acidic waters like those predicted for the year 2100, many showed thinner, weaker, and deformed shells in organisms such as mussels, clams, oysters, and scallops. Many of these organisms are commercially valuable and at the bottom of the food web, supplying food to larger species. If OA undermines populations at the bottom of the food chain, it could result in a trophic cascade: limiting the available prey of larger marine species would potentially reduce predator population sizes, which would then have a corresponding impact on the species that rely on them for food, creating a domino effect all the way up the food chain to the sea’s top predator, humans.

There are many other potential complications for fish that could occur as a result of OA: many marine communities are threatened by coral bleaching, which degrades the coral’s skeletal structure and expels the nutrient-rich algae fish rely on; reproductive success may be challenged with changing ocean conditions; and the ability of fish to detect predators may be impeded. However, not all organisms are negatively affected by OA. In fact, some species appear to be unaffected or even increase growth when subjected to future conditions. With such a wide range of responses, relationships between species within ecosystems are bound to change. New species may dominate an ecosystem while many may face the threat of extinction from increased pressures.

In my next post, I will expand on these biological effects of OA, and the ways in which they may affect global and regional fish and shellfish populations.

[1] Feely, R.A., C.L. Sabine, K. Lee, W. Berelson, J. Kleypas, V.J. Fabry, F.J. Millero. 2004. Impact of anthropogenic CO2 on the CaCO3 system in the oceans. Science 305(5682): 362-366.

[2] Gooding, R.A., Harley, C.D.G., and Tang, E. 2009. Elevated water temperature and carbon dioxide concentration increase the growth of a keystone echinoderm. Proc. Natl. Acad. Sci. 106(23): 9316-9321.

[3] IPPC (Intergovernmental Panel on Climate Change) (2007) The fourth assessment report of the IPCC. Cambridge: Cambridge University press. (predicted future levels).

[4] Schiermeier, Quirin. 2011. Earth’s Acid Test. Nature 471: 154-156.


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