The growth of phytoplankton in the water column beneath Arctic sea ice was historically considered to be negligible, largely due to the light attenuation caused by the snow-covered sea ice.  Recent observations and model results are in direct contrast to these old assumptions.  In fact, the Regional Arctic System Model (RASM) estimates that as much as 63% or more of the total pelagic primary production in the Arctic Ocean and interior seas occurs beneath sea ice.  This source of carbon is completely unaccounted for by pan-Arctic satellite observations that return no data in areas covered by sea ice.  In addition, RASM has shown that the under-sea ice growth of phytoplankton is highest in June and July each year, which is typically earlier than the timing of most shipboard expeditions that have occurred in the previous decades.  Phytoplankton growth is lower in the central Arctic Ocean and higher in the Chukchi, East Siberian, and northern Barents seas, as well as in the eastern part of Fram Strait.

Annually recurrent phytoplankton blooms are present beneath sea ice and show an increasing trend of 5% per decade over the previous four decades.  These blooms are initially light-limited in the spring, but as summer approaches the available light increases due to increases in daylength, melting of snow, and melt pond presence, as well as the thinning of sea ice.  Later on in the year, nutrient limitation controls the intensity and length of the phytoplankton blooms.  RASM shows that under ice blooms in the Western Arctic develop in situ, however blooms in the Eastern Arctic tend to be more affected by advection beneath the sea ice due to persistently low light availability. 

We plan to continue our investigations into the importance of melt ponds for phytoplankton growth, by quantifying their impact on under ice light availability.  We are also leading an effort that will intercompare results from multiple regional and global climate model on the growth of phytoplankton beneath sea ice in order to further reduce uncertainty in current estimates of this source of primary production in the Arctic that is largely missing from available observational datasets.  This effort is one of the projects that is being developed as part of the Consortium for the Advancement of Marine Arctic Science (CAMAS). 

This presentation contributes to the High Latitude Application and Testing of Earth System Models (HiLAT)-RASM project by using our modeling capabilities, which are run on world-class supercomputers, to advance understanding of the Arctic ecosystem.