Less Ice, More Food

Phytoplankton blooms gain nutrients from glacial melting.

A large tabular iceberg in the Amundsen Sea, Antarctica

Chris Payne (University of British Columbia)

Within the icy waters surrounding Antarctica are hot spots of life where the ocean remains atypically open much of the year. Known as polynyas, these mostly ice-free coastal waters host massive phytoplankton blooms, attracting numerous marine mammals and birds and consuming huge amounts of climate-changing carbon dioxide. What drives the growth of these valuable phytoplankton blooms at the base of the marine food chain?

To answer this question, instead of enduring Antarctica’s harsh and isolated conditions, Kevin R. Arrigo and two colleagues at Stanford University have relied on the increasing amount of satellite data on the region. They examined how six environmental factors—sea surface temperature, polynya size, length of time polynya waters remained mostly ice-free, width of continental shelf beneath polynyas, light availability, and the melting rate of neighboring glacial ice shelves—influenced the abundance and productivity of phytoplankton in forty-six Antarctic polynyas from 1998-2014.

Surprisingly, neither light availability nor the number of ice-free days contributed much to the magnitude of blooms. Instead, the dominant factor spurring phytoplankton productivity in polynyas was the melting rate of ice sheets. Arrigo’s team knew that dissolved iron was an important nutrient limiting phytoplankton growth, but did not realize that melting glaciers could remove this limitation by releasing iron. Wider continental shelves, where iron in seabed sediments can become resuspended and available to phytoplankton, also contributed to phytoplankton abundance.

The six analyzed factors explained approximately 80 percent of the difference in phytoplankton abundance and productivity among polynyas. Yet as a warming climate speeds Antarctic glacial melting, Arrigo predicts that “the waters around Antarctica, home to so many large populations of birds, seals, and whales, are likely to become even more productive.” How that change ultimately impacts the marine food web in these highly variable hot spots remains to be seen. (Journal of Geophysical Research: Oceans)

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