Researchers say ocean acidification may have dramatic effects on phytoplankton.

Phytoplankton, tiny plants which live in the ocean, form the foundation of the marine food web and play an important role in the regulation of Earth’s climate. These wonderful organisms create their own food using sunlight, nutrients and carbon dioxide drawn down from the atmosphere, releasing oxygen as a beneficial waste product. This process, known as photosynthesis, helps regulate the climate by removing heat-trapping carbon dioxide gas from the atmosphere. In addition, certain species of phytoplankton (Charlson et al., 1987) contribute to the formation of clouds, an important component of the climate system which influences global temperature. As a result of human activities such as deforestation and the burning of coal and oil for electricity production, these organisms now face multiple environmental changes caused by rising atmospheric carbon dioxide levels. These include ocean acidification, rising sea-surface temperatures and reduced nutrient availability. Ocean acidification, where excess atmospheric carbon dioxide dissolves into seawater, is sometimes referred to as the “other carbon dioxide problem”, global warming being its counterpart.

A team of researchers from MIT (Massachusetts Institute of Technology), the University of Alabama at Birmingham, and elsewhere has carried out an important study, recently published in the journal Nature Climate Change (Dutkiewicz et al., 2015), which charted phytoplankton growth rates under more acidic ocean conditions. The team gathered data from previous laboratory and field experiments on ocean acidification and used them in a numerical model which simulated changes in phytoplankton over the course of the 21^st century. The model was run assuming a business as usual scenario with regards to carbon emissions (IPCC, 2013).

Phytoplankton species are not interchangeable. Each has different effects on nutrient recycling and the flow of energy through the marine food web. For their study, the team organised common phytoplankton species, including coccolithophores, diatoms and the picoplankton Synechoccocus into six functional groups and then measured growth rates under more acidic ocean conditions.

Stephanie Dutkiewicz, a principal research scientist at MIT and the paper’s lead author, says she was “actually quite shocked by the results” (Chu, 2015). The model predicts pernicious changes in phytoplankton communities through the 21^st century which may have serious knock-on effects further up the marine food chain. For instance, diatoms may form the base of a food chain that feeds a polar bear whereas picoplankton such as Synechoccocus may form the base of one that feeds a relatively small fish.

There was a wide range of responses between phytoplankton species within and between functional groups, with some “losers” decreasing in abundance or even dying out while other “winners” increased in abundance. The higher carbon dioxide scenario appears to result in increased competition between species. According to the researchers, such a significant change will dramatically affect global populations of phytoplankton. Such a change could drastically alter local fishery resources and affect people who rely on seafood for sustenance.

“Normally, over evolutionary time, things come to a stable point where multiple species can live together,” Dutkiewicz says. “But if one of them gets a boost, even though the other might get a boost, but not as big, it might get out-competed. So you might get whole species just disappearing because responses are slightly different.”

The team also found that the majority of phytoplankton species migrate polewards in the higher carbon dioxide scenario, which is expected to occur as waters become warmer. This would likely add to the implications for food chains and local fisheries. Some out-competed species, however, migrate towards the equator where they can establish their communities.

An important result predicted by the model was that global phytoplankton biomass (total amount of all phytoplankton) changed very little, although regional changes were substantial. This is good news for carbon sequestration, oxygen production and possibly, therefore, climate regulation.

The team’s results show that changing carbon dioxide levels alone can have profound impacts on phytoplankton community structure and that ocean acidification is the greatest driver of change in the 21^st century.

These results give an unsettling prediction of how the marine ecosystem may be affected by ocean acidification. However, to get a more accurate picture, Dutkiewicz says more experiments are needed to determine how important competition between species is as the ocean becomes more acidic.

References:

Charlson, R. J., Lovelock, J. E., Andreae, M. O. and Warren, S. G. (1987) ‘Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate’, Nature, vol. 326, no. 6114 [Online], Available at file:///C:/Users/Patrick_2/Desktop/Lovelock%20%20Oceanic%20phytoplankton,%20atmospheric%20sulphur,%20cloud%20albedo%20and%20climate..htm (Accessed 01 July 2015).

Chu, J. (2015) MIT News [Online], Available at http://newsoffice.mit.edu/2015/ocean-acidification-phytoplankton-0720 (Accessed 26 July 2015).

Dutkiewicz, S., Jeffrey Morris, J., Follows, M. J., Scott, J., Levitan, O., Dhyrman, S. and Berman-Frank, I. (2015) ‘Impact of ocean acidification on the structure of future phytoplankton communities’, Nature Climate Change [Online], Available at http://www.nature.com/nclimate/journal/vaop/ncurrent/full/nclimate2722.html#access (Accessed 25 July 2015).

IPCC (2013) Summary for Policymakers. In: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.