BY FiONa mclaughliN, eddY CarmaCk, aNdreY prOShuTiNSkY, riChard a. kriShField, ChriSTOpher guaY, miChiYO YamamOTO-kawai, JeNNiFer m. JaCkSON, aNd Bill williamS aBSTraCT Sea ice extent in the Arctic Ocean diminished significantly during the first decade of the 2000s, most particularly in the Canada Basin where the loss of both multiyear and first-year ice was greater than in the other three subbasins. Using data collected during basin-wide surveys conducted from 2003–2010 together with data collected during the 1990s and 2000s at one station in the southern Canada Basin, we investigate the response of the Canada Basin water column to this significant decrease in ice cover. Changes were evident from the surface down to the Atlantic layer: some changes were the result of Beaufort Gyre forcing on regional processes, others were the result of Arctic Ocean atmospheric forcing on a hemispheric scale and large-scale advection. These changes have troubling consequences for the ecosystem. iNTrOduCTiON Situated north of North America, the Canada Basin is the largest of the four subbasins of the Arctic Ocean. It is separated from the Makarov Basin by the Alpha Cordillera, and from the Amundsen and Nansen Basins by the Lomonosov Ridge (Figure 1). The Canada Basin is ringed by continental shelves on three sides: they are shallow and broad to the west and include the East Siberian and Chukchi Sea shelves, narrow along the Beaufort Shelf to the south, and slightly broader to the southeast and northeast along the edge of the Canadian Arctic Archipelago. Several canyons (Herald, Barrow, and Mackenzie) score these shelves and provide two-way communication with the basin via shelf outflow and shelfbreak upwelling (Shimada et al., 2005; Weingartner et al., 2005; Williams et al., 2006). The complex topography between the Chukchi Plateau and the Northwind Ridge adds additional constraints to basin circulation. The basin’s deepest part, the Canada Abyssal Plain where depths reach over 3,800 m, is situated to the east of the Northwind Ridge. The general circulation and composition of Canada Basin waters are unique within the Arctic Ocean in several respects. First, the atmospheric Beaufort High forces circulation, making it the only Arctic subbasin where the motion of sea ice and upper ocean is clockwise and convergent. In the past, this motion meant that sea ice remained longer within the Beaufort Gyre, where it became thicker and more ridged than sea ice elsewhere that was transported more rapidly by the Transpolar Drift toward Fram Strait (Rigor and Wallace, 2004). Second, the halocline, defined as a gradient in salinity where fresher water overlies more saline water, is more strongly stratified and has a higher nutrient content in the Canada Basin than the three other subbasins. This is because it is the only subbasin to receive