Multi-level effects of sulphur–iron interactions in freshwater wetlands in The Netherlands Marlies E.W. van der Welle ⁎ , Jan G.M. Roelofs, Leon P.M. Lamers Institute for Water and Wetland Research, Radboud University, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands ARTICLE INFO ABSTRACT Article history: Received 30 May 2008 Accepted 30 May 2008 Available online 26 July 2008 Although sulphur deposition rates in Europe have considerably decreased over the last decades, sulphate concentrations in freshwater wetlands are still high, as a result of drainage, nitrate pollution, and increased sulphur loads in rivers. High sulphur fluxes may cause sulphide toxicity and eutrophication, and strongly interfere with the biogeochemical cycling of iron and phosphorus. In the present study the ecotoxicological interactions between sulphur, phosphate, iron, and trace metals in freshwater wetlands are reviewed. © 2008 Elsevier B.V. All rights reserved. Keywords: Ecotoxicology Iron Sulphur pollution Toxicity Wetlands 1. Introduction Within the stimulation program system-oriented ecotoxico- logical research the present study focused, amongst others, on the ecotoxicological interactions between metals, in particular iron, and sulphide. These interactions can be rather complex, since many factors may influence the bioavailability, and therefore the toxicity, of natural toxicants such as iron and sulphide. The main factors involved and their interactions, which were assessed in the present study, are presented in Fig. 1 and will be discussed in the following sections. 2. Sulphur pollution in freshwater wetlands Pollution of wetlands poses a serious threat to wetland ecosystem structure and functioning (Mitsch and Gosselink, 2000; Lamers et al., 2002). Although sulphur pollution of minerotrophic freshwater wetlands has received less interna- tional attention than that of other ecosystem types, it can lead to severe deterioration of these wetlands too (Holmer and Storkholm, 2001). Sulphur pollution has various origins, the main sources being agriculture (fertilizers), atmospheric deposition as a result of combustion of fossil fuels and industrial waste, and acid mine drainage (Schlesinger, 1997). In the present study the latter (acid mine drainage) is not involved. Although the atmospheric deposition of sulphur com- pounds in Europe has strongly decreased over the last decades, sulphate concentrations in freshwater wetlands have increased from 200 to 500 μmol/L and even higher as a result of input of sulphate-enriched river water and mobilization from recent or geological pyrite deposits due to drainage (Lamers et al., 2002; Zak et al., 2006). There are three major effects of sulphur pollution: acidification as a result of atmo- spheric SOx deposition and the combined effect of desiccation and FeSx oxidation (Schlesinger, 1997; Lamers et al., 2002), increased mobilization of phosphate from within the system, without external input (Patrick and Khalid, 1974; Boström et al., 1982; Roelofs, 1991; Lamers et al., 1998; Smolders et al., 2006), and sulphide toxicity. In addition to the above sources of sulphur pollution, there is another possible source, which has often been overlooked. When nitrate-polluted groundwater passes through soil layers SCIENCE OF THE TOTAL ENVIRONMENT 406 (2008) 426 – 429 ⁎ Corresponding author. Royal Haskoning, P.O. Box 8520 3009 AM Rotterdam, The Netherlands. Tel.: +31 10 286 53 79. E-mail address: m.vanderwelle@royalhaskoning.com (M.E.W. van der Welle). 0048-9697/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2008.05.056 available at www.sciencedirect.com www.elsevier.com/locate/scitotenv