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In 2011, researchers observed a massive bloom of phytoplankton growing under Arctic sea ice – conditions that should have been far too dark for anything requiring photosynthesis to survive.

Using mathematical modeling, researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) found that thinning Arctic sea ice may be responsible for these blooms and that the conditions that cause phytoplankton blooms have become more common. This has the potential to cause significant disruption in the Arctic food chain.

The paper published 29 March 2017 in the journal Science Advances challenges everything scientists thought they knew about phytoplankton blooms in the Arctic Ocean.

Phytoplankton shouldn’t be able to grow under the ice because ice reflects most sunlight light back into space, blocking it from reaching the water below. But over the past decades, Arctic ice has gotten darker and thinner due to warming temperatures, allowing more and more sunlight to penetrate to the water beneath. Large, dark pools of water on the surface of the ice, known as melt ponds, have increased, lowering the reflectivity of the ice. The ice that remains frozen is thin and getting thinner.

F2.largeSpatial map of the average number of days of sufficient light for sub-ice phytoplankton blooms over time. (A to C) Shading indicates the number of days in May, from 1986 to 1995 (A), 1996 to 2005 (B), and 2006 to 2015 (C), where a sub-ice bloom is permitted. (D to F) Same as (A) to (C) but for June. (G to I) Same as (A) to (C) but for July. Red boxes in (D) to (F) indicate the region of the 2011 cruise. Baffin Bay and regions with an ice concentration less than 80% at every point during each time period are colored blue. Continents are colored gray. Map courtesy of authors.

The big question was how much sunlight gets transmitted through the sea ice, both as a function of thickness, which has been decreasing, and the melt pond percentage, which has been increasing.

Chris Horvat, first author of the paper and graduate student in applied mathematics at SEAS explained: “What we found was that we went from a state where there wasn’t any potential for plankton blooms to massive regions of the Arctic being susceptible to these types of growth.”

The team’s mathematical modeling found that while the melt ponds contribute to conditions friendly to blooms, the biggest culprit is ice thickness. Twenty years ago, only about 3 to 4% of Arctic sea ice was thin enough to allow large colonies of plankton to bloom underneath. Today, the researchers found that nearly 30% of the ice-covered Arctic Ocean permits sub-ice blooms in summer months.

Horvat added, “All of a sudden, our entire idea about how this ecosystem works is different. The foundation of the Arctic food web is now growing at a different time and in places that are less accessible to animals that need oxygen.” Dr Schroeder summarized, “This study demonstrates that improving the sea ice model leads to a step forward in our understanding of how the Arctic is responding to climate change.”

The researchers hope their model will be helpful for planning future expeditions to observe these blooms and measuring the impact this shift will have on ecosystems. Read the full paper: Horvat et al. (2017) The frequency and extent of sub-ice phytoplankton blooms in the Arctic Ocean, Science Advances, 3, 3, 2375-2548. Further information can be found on the Harvard and Reading University websites.

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