1. Habitat Ecology Section, Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, B2Y 4A2, Canada, Atmospheric Environment Service, Environment Canada, 4905 Dufferin Street, Downsview, Ontario, M3H 5T4, Canada, and Fisheries and Oceans Canada, Central and Arctic Region, Freshwater Institute, 501 University Crescent, Winnipeg, Manitoba, R3T 2N6, Canada