1. 2Institute for Water and Wetland Research (IWWR), Department of Animal Ecology and Ecophysiology, Radboud University Nijmegen, P. O. Box 9010, NL-6500 GL Nijmegen, The Netherlands, Netherlands Centre for Biodiversity Naturalis, P.O. Box 9517, NL-2300 RA Leiden, The Netherlands
2. 1Monitor Taskforce, Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology (NIOO-CEME), P. O. Box 140, NL-4400 AC Yerseke, The Netherlands;, Email: s.wijnhoven@nioo.knaw.nl
3. 3Monitor Taskforce, Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology (NIOO-CEME), P. O. Box 140, NL-4400 AC Yerseke, The Netherlands