1. Department of Thematic Studies - Water and Environmental Studies, Linköping University, 58183 Linköping, Sweden, Department of Ecology, Federal University of Rio de Janeiro, 68020 Rio de Janeiro, Brazil, Center for Agricultural Research in the Pantanal - Embrapa Pantanal, Caixa Postal 109. Corumbá - MS, Brazil 79.320-900, and Department of Geology and Geochemistry, Stockholm University, 10691 Stockholm, Sweden