Laura Nielsen for Frontier Scientists
The Arctic hosts a complex ecosystem, sensitive to the alterations in our changing world. Algae is part of that biome, growing in strands which hang down from the edge of ice floes. New conditions have caused an explosion in the growth rate of the algae Melosira arctica, which will influence Arctic life in ways we can't predict with certainty.
2012's summer season saw the lowest Arctic ice extent on satellite record. Arctic sea ice volume has also dropped. The satellites ICESat (National Aeronautics and Space Administration, 2003-2008) and CryoSat-2 (European Space Agency, 2010-current) use clever methods to measure sea ice volume. The satellites helped discover that between 2003 and 2012, Arctic sea ice volume declined by 36% in autumn and 9% in winter.
In summer 2012 scientists with the Alfred Wegener Institute for Polar and Marine Research headed to the Central Arctic to investigate what lay under the region's freezing waters aboard the research vessel Polarstern. What they found was a surprise. Algal blooms of Melosira arctica had flourished, growing under the sea ice in never-before-recorded massive blooms. Many of the strands of algae grew to a staggering 5 meters long. Melosira arctica was found to have been responsible for almost half of the area's 2012 primary production (in this case, creation of plant matter by converting solar energy, carbon dioxide, and water in a process called photosynthesis). Plants and phytoplankton, microscopic plant-like organisms like algal diatoms, are responsible for primary production in the Arctic waters near ice floes.
Once, multi-year ice dominated. At thicknesses that might reach 10-15 meters, the ice was a formidable shield to sunlight. So thick that even summer warmth could not dispel it, the multi-year ice lingered, grinding against other floes, becoming hummocked and dense. The large area of light-colored ice on the ocean increased the area's albedo, or reflective quality, sending sunlight back into space instead of absorbing it into the dark-colored open ocean. Because of warming global temperatures, less ice survives through summer. That means an Arctic ocean increasingly dominated by first-year ice, which is thinner and smoother.
The thinner ice, with wide, shallow melt-ponds atop its surface, allows more sunlight into the Epipelagic zone, the top layer of the ocean where enough light penetrates to support photosynthesis. With so much light available, the algae flourished. In a region where nutrients are fairly scarce, that means the algae claimed nutrients that might have otherwise gone to other phytoplankton organisms.
The algal chains grew hanging from the ice floes above, but then the first-year ice melted in the summer heat. With diameters up to 50 centimeters, the new, long, well-fed Melosira arctica chains were heavy enough to sink quickly through the water. They landed about 4000 meters below on the sea floor, in the dark, stealing energy away from the upper ocean layers and depositing it below. When the research team investigated the sea floor via Remotely Operated Vehicle and undersea cameras, they documented the remains: masses of algae deposits covered up to 10% of the sea floor.
Where the algae fell, populations of sea cucumbers and brittle stars exploded: they feasted on the algae feast delivered to their deep dark zone. There on the seafloor, Melosira arctica also made a fine meal for bacteria. In the process of breaking down the algae, bacteria use oxygen. This lessens the amounts of oxygen available to the sea floor community, creating anoxic (low oxygen) conditions. Few living things tolerate a fully anoxic habitat. In contrast the seabed where no algae had fallen was found to be aerated (contain oxygen) down to a depth of 80 centimeters, making it clear this was an unusual event.
If the huge quantities of algae growing in the Arctic's summer continue as a trend, it might be good news for humans and the planet. Oceanic plants take carbon dioxide from the atmosphere as they photosynthesize and grow. Then falling away to the seafloor, Melosira arctica could remove that carbon to the seabed, adding to the ocean's capacity to act as a carbon sink, a system which removes carbon dioxide from the atmosphere.
Still, nothing guarantees the summer of 2012 represents a new trend. Explosive growth requires nutrients, and there are not many to go around in Arctic waters. If it continues proliferous growth, this particular kind of algae may deplete the nutrients available to the point that it starves itself, and other organisms. That threatens the food chain based around plankton, which support fish, seals, birds, whales, and even humans. Meanwhile heightened anoxic conditions could vastly alter the seabed, threatening clams and crustaceans, meaning in turn a threat to walruses and other creatures which feed on them. While research missions like this help us learn more, the Arctic ecosystem is still one we poorly understand, and it now faces an imbalance. We have yet to see what will come of it.
Alfred Wegener Institute for Polar and Marine Research: "Study published in Science: Rapid changes in the Arctic ecosystem from surface to depth during ice minimum in the summer of 2012," February 14, 2013. http://www.awi.de/en/news/press_releases/detail/item/study_published_in_science_rapid_changes_in_the_arctic_ecosystem_from_surface_to_depth_during_the_i/?tx_list_pi1%5Bmode%5D=6&cHash=41c20b47a82125bff0b68f92efcc6557
American Geophysical Union: "CryoSat-2 mission reveals major Arctic sea-ice loss," February 13, 2013. http://www.agu.org/news/press/pr_archives/2013/2013-04.shtml