1. Division of Biosphere Science, Graduate School of Environmental Science; Hokkaido University; Sapporo 060-0810 Japan
2. Biology Department; Northern Michigan University; Marquette MI 49855 U.S.A.
3. Laboratory of Aquatic Bioresources and Ecosystems, Field Science Center for the Northern Biosphere; Hokkaido University; Sapporo 060-0809 Japan