1. INSU-CNRS, UMR 8187, LOG, Laboratoire d'Océanologie et des Géosciences; Université du Littoral Cote d'Opale, ULCO; Wimereux France
2. Department of Ecology and Genetics/Limnology; Uppsala University; Uppsala Sweden
3. Estonian Marine Institute; University of Tartu; Tallinn Estonia
4. Department of Environmental Sciences; University of Virginia; Charlottesville Virginia USA
5. Now at Department of Ecology and Environmental Sciences; Umeå University; Umeå Sweden