Shannon Bohman
Sea ice has become an important index of Arctic health in the midst of a warming regional climate. Its prominence is due in part to its visibility: there are few pieces of evidence as straightforward and convincing to the general public as satellite images displaying dwindling ice from year to year. There is also sound scientific support for the ice’s importance: sea ice and climate exist in a careful balance, each one impacting—while simultaneously being impacted by—the other. This relationship is especially evident in the seasonal ice zones (SIZ) of the Greenland Sea, which grow during the Arctic winter and wane during summer. Ice has an exceptionally high albedo, meaning it is an effective reflector of solar radiation. As such, the ice mediates Arctic warming at a rate dependent on its thickness.
Below the surface
There is a third element to consider in the climate-ice equation: the organisms that exist beside and below the ice. The Greenland Sea SIZ is host to annual blooms of phytoplankton, microscopic marine algae that are the foundation of the oceanic food web and carbon cycle. These phytoplankton use atmospheric carbon dioxide and sunlight to produce carbohydrates and oxygen, both of which are critical ingredients for life processes. A recent paper published by the American Geophysical Union reported that the location, timing, and extent of production of these photosynthetic powerhouses may far exceed previous expectations.
Limitations of traditional observation methods
Over the last decade, scientists have had to expand their perception of where algae can exist in seasonal ice zones. It was originally believed that blooms could only occur at the edges of ice floes, seemingly the only place with enough sunlight to support growth. This notion was contradicted by Arrigo et al. in a 2011 paper that documented a massive algae bloom underneath ice in the Chukchi Sea. This study reported peak phytoplankton biomass at the ice/seawater interface under some of the thickest ice while collecting data over a four-day period in early July. In contrast, traditional observational methods that rely on ships and satellites are temporally and spatially limiting. In order for a ship to access a SIZ, researchers must wait until the ice breaks up in late spring or summer. Heavy ice cover in conjunction with rapid light attenuation in Arctic waters prevents satellites from observing even a few meters below the surface. Without data from deep water, sub-ice water, or winter water, researchers could not piece together the whole story of annual Arctic phytoplankton blooms.
New technology, new insight
The story grew more complete in 2018 when Mayot et al. reported using Biogeochemical-Argo floats to overcome the technical challenges posed by researching the icy waters of the Greenland Sea year-round. Their floats sampled vertically underneath sea ice during two annual cycles, collecting data on the water’s temperature, salinity, nutrient composition, and biomass concentration. The team identified three peak phytoplankton activity phases during each annual SIZ cycle: The first phase, occurring during late winter and early spring, takes place directly underneath the ice; the second takes place at the edge of the ice during spring; and the third phase comprises an open-water subsurface algae bloom during summer.
When in bloom
These activity phases are determined by the abundance of sunlight and nitrates. As surface waters warm in the spring, they become less dense and less prone to mixing with the colder water below. Relatively heavy pieces of organic matter begin to sink, creating a nutrient maximum a couple of meters below the surface. Springtime sunlight and sub-surface nutrients drove the phytoplankton out from underneath the ice into the open water during the warmer months, behavior the scientists expected to see. What makes these results surprising is the relative rate of production of the activity phases: summer was not found to be the productive season for phytoplankton in the Greenland Sea. Most of the annual production occurred in equal parts under the ice and at the ice’s edge, with a much smaller contribution coming from summertime open-water blooms. Late winter and spring are therefore the most fruitful seasons for phytoplankton, a departure from the traditional school of thought on Arctic algae blooms. Without the Biogeochemical-Argo floats’ year-round data collection, this fact would have gone unrealized.
Climate change
It is especially important to have correct measurements of phytoplankton biomass and production in the face of climate change, since any estimation of future algal activity demands an accurate starting point. The Greenland Sea is already responding to new climatic pressures: warmer temperatures lead to thinner ice and melt pools, which are dark pools of water on top of the ice. Sea ice that is thin and wet has a lower albedo than normal sea ice, so the Greenland Sea is absorbing more solar radiation and heating at an accelerating rate. Increased solar radiation also makes the Greenland SIZ more hospitable for sub-ice algae blooms, which serve to darken the ice and perpetuate warming even further. The positive feedback loop becomes even more dire in light of Mayot et al.’s findings. Phytoplankton underneath and at the edge of the sea ice—now known to be the Greenland Sea’s most lucrative producers—are most susceptible to the effects of decreasing ice thickness. Higher initial counts of those populations suggest an even more drastic amplification of algae blooms and their associated effects.
Disruption of ecosystems
While an increase in the ocean’s most important primary producer might at first seem beneficial to the oceanic food web, the true ecological effect of these intensifying blooms is not straightforward. Consumers who take advantage of the new sub-ice food may experience a population boom of their own, posing other ecological threats as unsustainable ripples are sent throughout the food web. With such large and complex biological system, any rapid disturbance in primary production has implications muddied by deterministic chaos. Even a small change in the first layer of the web may have drastic, unforeseen consequences down the line. Understanding when, where, and to what extent algae blooms occur in the region is an important first step in predicting and managing the effects of climate change on the Arctic.
SOURCES
https://eos.org/project-updates/bringing-biogeochemistry-into-the-argo-age
https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2018JC014271
https://www.sciencedaily.com/releases/2019/01/190108125411.htm