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Ocean Acidification Reduces Habitat for Antarctic Organisms

Source: Negrete-García, G., N. S. Lovenduski, C. Hauri, K. M. Krumhardt. and S. K. Lauvset (2019), Sudden emergence of a shallow aragonite saturation horizon in the Southern Ocean. Nature Climate Change, doi:10.1038/s41558-019-0418-8

The ocean absorbs about one-third of the excess carbon dioxide (CO2) in the atmosphere from the burning of fossil fuels. After being taken up by the ocean, this CO2 causes chemical reactions that make seawater more corrosive – a process known as ocean acidification. This is bad news for the millions of tiny organisms in the ocean that make their shells from a chemical compound called aragonite, which is a form of calcium carbonate. As the ocean becomes more acidic, it becomes harder for these calcifying organisms to form and maintain their shells.

The aragonite saturation horizon is the depth in the ocean where calcium carbonate readily dissolves. Organisms that need calcium carbonate to construct their shells – including corals, pteropods, and foraminifera – must live above the saturation horizon. As the ocean takes up more carbon dioxide and acidifies, however, the saturation horizon gets shallower. This reduces the viable habitat for calcifying organisms like the pteropod in the image below. Pteropods, or small marine snails, are a key part of the food web and are particularly vulnerable to ocean acidification.

Unhealthy pteropod with dissolving shell ridges showing the effects of ocean acidification (NOAA Fisheries Collection via Wikimedia Commons)

A recent study led by Gabriela Negrete-García from Scripps Institution of Oceanography, uses a climate model to predict changes in the saturation horizon over the next century under different CO2 emission scenarios. The study focuses on the Southern Ocean, which surrounds Antarctica and is very vulnerable to ocean acidification because carbon dioxide dissolves more easily in cold water.

The Southern Ocean is a remote and harsh environment, so there is limited available data from the region. Therefore, studying the Southern Ocean saturation horizon is difficult using observations alone. The researchers in this study used data from the Community Earth System Model (CESM) to predict the evolution of the saturation horizon over the next century. CESM is a complex climate model that integrates physical, chemical, and biological processes. Rather than relying on a single forecast, the model was run many times with slightly different inputs. Together, these different model runs make up an ensemble. By comparing the individual model runs, or ensemble members, scientists can test how accurate the forecast is.

Humpback whales in the Southern Ocean eat krill, which in turn eat pteropods and other calcifying organisms that will be impacted by changes to the saturation horizon (Hugh Venables)

Results from all the ensemble members show that the Southern Ocean saturation horizon will become significantly shallower over the next century. If current carbon dioxide emission rates are not reduced, then the saturation horizon will decrease from 1,000 meters in the present-day ocean, to only 83 meters by 2100. The timing of this shallowing varied across the different ensemble members, which may be why it wasn’t detected in past studies that analyzed the ensemble average.

In some locations, the shallowing of the saturation horizon occurs in a single year. This rapid reduction in habitat for calcifying organisms could cause major changes to Southern Ocean food webs and ecosystems. For example, pteropods (which can only survive above the saturation horizon) are an important food source for krill, which in turn support whales, and other large marine animals.

There are several factors not accounted for in the CESM model. First, it is possible that marine organisms could adapt to changes in the environment. This will depend on how fast the changes occur. Second, ocean circulation may also change, which could impact the saturation horizon as well. Therefore, monitoring the calcium carbonate saturation horizon with observations will be crucial in allowing fisheries and conservation groups to plan for and respond to ecosystem changes.


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