1. Center for Limnology, University of Wisconsin, Madison, WI 53706, USA.
2. Departments of Civil and Environmental Engineering, and Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
3. Department of Zoology, University of British Columbia, Vancouver, BC V6T 1W5, Canada
4. Department of Zoology, University of Alberta, Edmonton, AB T6G 2E9, Canada
5. Norwegian Institute for Water Research, Box 173 Kjelsas, 0411 Oslo, Norway