Explaining blooms of phytoplankton growing under Arctic sea ice29 March 2017
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 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 research has been published in Science Advances, and involved scientists from the University of Reading and the University of Oxford.
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. These attract fish, which attract larger predators and provides food for indigenous communities living in the Arctic.
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.
"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." - Dr David Schroeder, University of Reading
“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 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% 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.
“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 coauthored by David Schroeder, Daniela Flocco and Danny Feltham from the University of Reading, and David Rees Jones and Sarah Iams from the University of Oxford. It was supported in part by the National Science Foundation.
Dr Schroeder said: "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."
C. Horvat, D. Rees Jones, S. Iams, D. Schroeder, D. Flocco, D. Feltham (2017). 'The frequency and extent of sub-ice phytoplankton blooms in the Arctic Ocean'. Science Advances. doi: 10.1126/sciadv.1601191
Picture credit: Dark areas of Arctic sea ice show where conditions have become suitable for algae and phytoplankton underneath the ice. Courtesy of NASA