1. Centro de Estudos do Ambiente e do Mar and Departamento de Biologia, Universidade de Aveiro, Campus Universitario de Santiago, 3810-193 Aveiro, Portugal
2. School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK and Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99775 USA