Increases in Arctic sea ice algal habitat, 1985-2018

In the Arctic Ocean, sea ice algae are responsible for a small but seasonally important pulse of primary production. Their persistence is threatened by the rapid loss of sea ice from the Arctic Ocean due to climate change, but this threat will be at least partially offset by the replacement of multiyear ice (MYI) with first-year ice (FYI). FYI is thinner and usually features a thinner snow cover than MYI, thus transmitting more light to support ice algal growth. We combined remote sensing, reanalysis data, and modeling products with a radiative transfer model to assess how the changing physical conditions in the Arctic altered the extent and duration of the bottom ice algal habitat over a 34-year period. Habitat was defined as areas where enough light penetrates to the bottom ice to support net positive photosynthesis. The Arctic shifted from 37% FYI in 1985 to 63% in 2018, as the 2.0 x 10(6) km(2) increase in FYI extent outpaced the 0.6 x 10(6) km(2) decrease in overall sea ice extent above the Arctic Circle. The proliferation of younger ice corresponded with a 0.08 m decade(-1) decrease in average sea ice thickness and a 0.003 m decade(-1) decrease in average snow depth. The end of the ice algal season, marked by the onset of warm summer air temperatures, moved slightly earlier, by 1.4 days decade(-1). Our analysis indicates that ice algal habitat extent increased by 0.4 x 10(6) km(2) decade(-1), or from 48% to 66% of total sea ice extent. The average ice algal growing season also lengthened by 2.4 days and shifted earlier in the year. Together, these trends suggest that net primary production in Arctic sea ice increased during 1985-2018. The most dramatic changes were localized in the Central Basin and the Chukchi Sea and were driven primarily by the declining snow cover and the shift from MYI to FYI. Although the Arctic recently became more favorable to ice algae, we expect that this trend will not continue indefinitely, as a limited amount of MYI remains.