Climate change has, since the industrial revolution, begun to dramatically change various ecosystems on earth. Unprecedented droughts and heatwaves cause forest fires to decimate woodlands and the flora and fauna that reside there, while sea ice disappears and the wildlife dependent on the ice for hunting or habitats along with it. And, among it all, the largest ecosystem on this planet—the world’s oceans—is undergoing rapid and radical changes. With oceans covering about 70% of the earth’s surface, they are responsible for absorbing at least 30% of carbon dioxide (CO2) released into the atmosphere by human activity (Watson, Shuster, et al., 2020, p. 1). This pollutant, CO2, is responsible for a variety of environmental catastrophes including ozone depletion, and general warming of the planet, and, as atmospheric carbon dioxide levels continue to rise, it results in higher levels of acidity in the ocean.
Causes of Ocean Acidification
In order to understand how ocean acidification occurs, it is first essential to understand the pH scale. pH is how scientists measure how basic or acidic a substance is on a scale of 0-14, with zero being the most acidic, seven being neutral, and 14 being the most basic. A substance becomes more acidic with an increase of hydrogen ions (H+) and becomes more basic with the decreased concentration of H+. The pH scale is also logarithmic, meaning that something with a pH of 3 is 10-times more acidic than something with a pH of 4.
Figure 1: pH Scale and Climate Change Indicators: Ocean Acidity. The United States Environmental Protection Agency. 2016.
As more CO2 enters the atmosphere, “more CO2 is being driven into the surface seawater, increasing the concentration of aqueous CO2,” (Hurd, Beardall, et al., 2020, p. 2). When CO2 reacts with water (H2O), the compounds break down into bicarbonate (HCO3-) and H+. This free hydrogen ion that results from the reaction of CO2 and water is what lowers the pH of the ocean water; it can also react with carbonate (CO3-) and form additional HCO3- molecules. Thus, when CO2 levels increase in seawater, HCO3- and H+ increase along with them, while the concentration of carbonate (CO3-) decreases (Hurd, Beardall, et al., 2020, p. 2).
Since the late 18th century, the mean surface ocean pH has dropped 0.1 units from 8.2 to 8.1… This change is roughly equivalent to a 30% increase in the concentration of hydrogen ions (Logan, 2010, p. 1).
While this may seem like a small change, a 30% increase in H+ over the last 200 years has made the oceans more acidic than any other time in the past 300 million years; it is an unprecedented and dramatic change in a relatively short period of time that is expected to rise by 0.5% per year (Guinotte, Fabry, 2008, p. 320). Additionally, “recent evidence suggests ocean acidification was the primary driver of past mass extinctions and reef gaps, which are time periods on the order of millions of years that reefs have taken to recover from mass extinctions,” (Guinotte, Fabry, 2008 p. 321-326). The effects of ocean acidification will be far-reaching and potentially catastrophic due to the effects on the oceanic food chain.
Figure 2: Ocean Acidification. ©Encyclopædia Britannica, Inc. 2020
Effects of Ocean Acidification
Ocean acidification has had a profound effect on marine calcifiers: oceanic organisms that use calcium and carbonate to build a shell or a skeleton such as coral reefs, oysters, and other bivalves. As previously stated, the concentration of carbonate that is available in the oceans is decreasing as the pH of seawater increases, and marine calcifiers that rely on this carbonate suffer as a result. The dissolution of coral reefs is a prime example of this as corals are calcifying organisms that, with the help of other calcifying algae, produce CaCO3 to form the reef structure (Andersson, Gledhill, 2013, p. 324). When the water is undersaturated with calcium carbonate, the minerals that are stored in the reefs will begin to dissolve back into the environment through a process known as bioerosion, leaving the reefs with lower structural integrity (Doney, Busch, et al., 2020, p. 93).
Figure 3: Pterapod Shell Dissolved in Seawater. Wikimedia Foundation, Inc. 2011.
Additionally, bivalve and shellfish species are at risk due to ocean acidification which has a cascading effect on human populations that rely on bivalve fisheries for both employment and food: bivalves such as mollusks make up 9% of the total fishery production. “Ocean acidification causes decreases in bivalve reproduction, survival of juvenile bivalves, or delayed maturation of adults and can alter recruitment, harvestable biomass, maximum sustainable yield, and economic value of shellfish fisheries,” (Doney, Busch, et al., 2020, p. 96). Without the calcium carbonate needed to successfully build their shells and combined with the bioerosion that occurs if they are actually able to make their shells in water that is undersaturated with calcium carbonate, the shellfish biomass is expected to decrease by 25% over the next 50 years according to the National Oceanic and Atmospheric Administration. This will eventually have an effect on the population of fish that rely on bivalves for food, further damaging the food web.
Coral reefs and shellfish populations are only two examples of how ocean acidification will impact marine ecosystems, and, how those impacts will trickle up to affect human populations. With fragile food chains that begin in the ocean, it is imperative that CO2 emissions start to decrease in the coming years to try and curb the harsh effects they have on the planet. The effects of ocean acidification are just beginning to manifest, and as the pH of the oceans is predicted to continue to fall in the coming decade, drastic measures must be taken to protect the planet and its oceans.
Andersson, A. J., & Gledhill, D. (2013). Ocean Acidification and Coral Reefs: Effects on Breakdown, Dissolution, and Net Ecosystem Calcification. Annual Review of Marine Science, 5(1), 321–348. https://doi.org/10.1146/annurev-marine-121211-172241 Doney, S. C., Busch, D. S., Cooley, S. R., & Kroeker, K. J. (2020). The Impacts of Ocean Acidification on Marine Ecosystems and Reliant Human Communities. Annual Review of Environment and Resources, 45(1), 83–112. https://doi.org/10.1146/annurev-environ-012320-083019 Guinotte, J. M., & Fabry, V. J. (2008). Ocean Acidification and Its Potential Effects on Marine Ecosystems. Annals of the New York Academy of Sciences, 1134(1), 320–342. https://doi.org/10.1196/annals.1439.013 Hurd, C. L., Beardall, J., Comeau, S., Cornwall, C. E., Havenhand, J. N., Munday, P. L., Parker, L. M., Raven, J. A., & McGraw, C. M. (2020). Ocean acidification as a multiple driver: how interactions between changing seawater carbonate parameters affect marine life. Marine and Freshwater Research, 71(3), 263. https://doi.org/10.1071/mf19267 Logan, C. A. (2010). A Review of Ocean Acidification and America’s Response. BioScience, 60(10), 819–828. https://doi.org/10.1525/bio.2010.60.10.8 Watson, A. J., Schuster, U., Shutler, J. D., Holding, T., Ashton, I. G. C., Landschützer, P., Woolf, D. K., & Goddijn-Murphy, L. (2020). Revised estimates of ocean-atmosphere CO2 flux are consistent with ocean carbon inventory. Nature Communications, 11(1). https://doi.org/10.1038/s41467-020-18203-3
Cover Image: Watson, J. (2017). Ocean Acidification. [Illustration]. Image retrieved from https://jacklynwatson.com/sketch/ocean-acidification-sketch/ Figure 1: Climate Change Indicators: Ocean Acidity. The United States Environmental Protection Agency. (2016). [Illustration]. Image retrieved from https://www.epa.gov/climate-indicators/climate-change-indicators-ocean-acidity#%20 Figure 2: Ocean Acidification. ©Encyclopædia Britannica, Inc. (2020). [Illustration]. Image retrieved from https://www.britannica.com/science/ocean-acidification. Figure 3: Pterapod Shell Dissolved in Seawater. Wikimedia Foundation, Inc., Wikipedia®. (2011). [Photograph]. Image retrieved from https://commons.wikimedia.org/wiki/File:Pterapod_shell_dissolved_in_seawater_adjusted_to_an_ocean_chemistry_projected_for_the_year_2100.jpg