1. Ecology Laboratory, Plant Biology and Ecology Department, Science and Technology Faculty, University of the Basque Country, P.O. Box 644, 48080 Bilbao, Spain
2. Norwegian Institute for Water Research, Gaustadalléen 21, NO-0349 Oslo, Norway
3. University of Oslo, Department of Biology, Marine Biology, P.O. Box 1066, Blindern, NO-0316 Oslo, Norway
4. School of Animal, Plant & Environmental Sciences, University of the Witwatersrand, Private Bag 3, WITS 2050, South Africa