Spatial map of the average number of days of sufficient light in the Arctic Ocean 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. Graphic: Horvat, et. al, 2017 / Science Advances

By Leah Burrows
29 March 2017

(Harvard Gazette) – In 2011, researchers observed something that should be impossible — a massive bloom of phytoplankton growing under Arctic sea ice in conditions that should have been far too dark for anything requiring photosynthesis to survive. So, how was this bloom possible?

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 and more blooms in the future, and could potentially cause significant disruption in the Arctic food chain.

The research is described in Science Advances and is a collaboration between researchers from SEAS, University of Oxford, and University of Reading.

Phytoplankton underpins the entire Arctic food web. Every summer, when the sea ice retreats, sunlight hitting the open water triggers a massive bloom of plankton. The plankton plumes attract fish, which attract larger predators and provide food for indigenous communities living in the Arctic.

Phytoplankton shouldn’t be able to grow under the ice because ice reflects most sunlight back into space, stopping 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 today is thin and getting thinner.

“Our 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?” said Chris Horvat, first author of the paper and a graduate student in applied mathematics at SEAS. “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 percent of Arctic sea ice was thin enough to allow large colonies of plankton to bloom underneath. Today, the researchers found that nearly 30 percent of the ice-covered Arctic Ocean permits sub-ice blooms in summer months.

“The meter decline in sea ice thickness in the Arctic in the past 30 years has dramatically changed the ecology in that area,” said Horvat. “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.”

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.

This research was co-authored by David Rees Jones, Sarah Iams, David Schroeder, Daniela Flocco, and Daniel Feltham. It was supported in part by the National Science Foundation.

Solving the mystery of the Arctic’s green ice

Evolution and variability of the pan-Arctic likelihood of sub-ice phytoplankton blooms over time. (A to C) Percentage by area of the Arctic Ocean that has greater than 80% ice concentration and permits growth for at least three consecutive days in May (A), June (B), and July (C). Red dashed lines are linear fits to the data. Graphic: Horvat, et. al, 2017 / Science Advances

ABSTRACT:In July 2011, the observation of a massive phytoplankton bloom underneath a sea ice–covered region of the Chukchi Sea shifted the scientific consensus that regions of the Arctic Ocean covered by sea ice were inhospitable to photosynthetic life. Although the impact of widespread phytoplankton blooms under sea ice on Arctic Ocean ecology and carbon fixation is potentially marked, the prevalence of these events in the modern Arctic and in the recent past is, to date, unknown. We investigate the timing, frequency, and evolution of these events over the past 30 years. Although sea ice strongly attenuates solar radiation, it has thinned significantly over the past 30 years. The thinner summertime Arctic sea ice is increasingly covered in melt ponds, which permit more light penetration than bare or snow-covered ice. Our model results indicate that the recent thinning of Arctic sea ice is the main cause of a marked increase in the prevalence of light conditions conducive to sub-ice blooms. We find that as little as 20 years ago, the conditions required for sub-ice blooms may have been uncommon, but their frequency has increased to the point that nearly 30% of the ice-covered Arctic Ocean in July permits sub-ice blooms. Recent climate change may have markedly altered the ecology of the Arctic Ocean.

The frequency and extent of sub-ice phytoplankton blooms in the Arctic Ocean



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