The Canadian Arctic Shelf Exchange Study (CASES) is an international effort under Canadian leadership to understand the biogeochemical and ecological consequences of sea ice variability and change on the Mackenzie Shelf (www.giroq.ulaval.ca/cases). The extent and thickness of Arctic sea ice vary considerably from year to year and over decadal time scales. Assessing the effects of present variability in sea ice cover on Arctic marine ecosystems and regional climate requires a substantial improvement in our understanding of the links between freshwater and sea ice, sea ice and atmosphere, and sea ice and biogeochemical fluxes. The need for data is particularly strong for the shallow coastal shelf regions (30% of the Arctic basin) where variability in the extent, thickness and duration of sea ice is most pronounced and where Arctic marine food webs are most vulnerable to change.

In this paper we present the scientific objectives and overview of results from the CASES field experiment. The new Canadian Research Icebreaker (CCGS Amundsen) entered the field area in September 2003 and completed a one-year over wintering experiment in August, 2004. Experiments were coordinated around 9 subgroups with over 200 scientists participating from 9 countries. In CASES we examined physical aspects of the ocean-sea ice-atmosphere system from the bottom of the ocean to the top of the atmosphere; and from a biological perspective for everything from viruses to whales. We will focus our presentation on an overview of the experimental design, examples from field sampling, and a discussion of early results. We will place a particular emphasis on the role sea ice plays in moderating aspects of the marine ecosystem in the Southern Beaufort Sea both within the Cape Bathurst polynya complex and over the continental shelf associated with the Mackenzie River inflow. We conclude the paper with a presentation of the evolution of CASES into ArcticNet – a new Network of Centres of Excellence recently funded by the Government of Canada.