1. Dept of Aquatic Ecology; Netherlands Inst. of Ecology (NIOO-KNAW), Droevendaalsesteeg 10; NL-6708 PB Wageningen the Netherlands
2. Laboratoire Interdisciplinaire des Environnements Continentaux (LIEC), Univ. de Lorraine; Metz France
3. Dept of Ecology and Biodiversity; Utrecht Univ.; Utrecht the Netherlands