On the importance of Arctic pelagic phytoplankton blooms beneath a thinning sea ice regime

Monday, December 9, 2019 - 11:20
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In high-latitude environments such as the Arctic Ocean, phytoplankton growth is strongly constrained by light availability. Because light penetration into the upper ocean is attenuated by snow and ice cover, it was generally believed, until recently, that phytoplankton growth was limited to areas of open water, with negligible growth under sea ice. However, under-ice phytoplankton blooms have been reported multiple times over the past several decades. In July 2011 ICESCAPE (Impacts of Climate on EcoSystems and Chemistry of the Arctic Pacific Environment) survey observed a massive phytoplankton bloom beneath the ice in the Chukchi Sea (Arrigo et al., 2012). Observational evidence suggests that this bloom was not an isolated case, and that under-ice blooms maybe widespread on the Arctic continental shelves (Arrigo et al., 2014).

RASM is a high-resolution, fully-coupled, regional model with a domain encompassing the entire marine cryosphere of the Northern Hemisphere, including the major inflow and outflow pathways, with extensions into North Pacific and Atlantic oceans. The components of RASM include: atmosphere, sea ice, ocean, biogeochemical (BGC), and land hydrology. The ocean BGC component in RASM is a medium-complexity Nutrients-Phytoplankton-Zoo-plankton-Detritus (NPZD) model. The model has three phytoplankton categories: diatoms, small phytoplankton and diazotrophs, with explicit carbon, iron (Fe) and chlorophyll-a (chl-a) pools for each category, as well as an explicit Si pool for diatoms and an implicit CaCO3 pool for small phytoplankton.

RASM results show that under-ice pelagic chl-and primary production values can at times be very high beneath sea ice, particularly during the spring bloom. Our numerical model results are similar to observations by Arrigo et al. (2012). With this tool we are also able to see an increase in primary production over the last several decades. This increase is attributed to reduced sea ice cover, which increases light availability to the upper ocean. We conclude that under-sea-ice pelagic primary production makes up a large fraction of the total production and cannot be considered negligible. Future observational programs are needed to provide more comprehensive datasets throughout the growing season beneath the sea ice in a pan-Arctic sense.

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