OCCURRENCE OF SEDIMENT-BOUND PYRETHROIDS IN DANISH STREAMS AND THEIR IMPACT ON ECOSYSTEM FUNCTION RASMUS B. LAURIDSEN 1,2,* , BRIAN KRONVANG 1 and NIKOLAI FRIBERG 1 1 Department of Freshwater Ecology, National Environmental Research Institute, Silkeborg, Denmark 2 CEH Dorset, Winfrith Technology Centre, Winfrith Newburgh, Dorchester, Dorset DT2 8ZD, UK ( * author for correspondence, e-mail: rasl@ceh.ac.uk; phone: +44-1305-213558; fax: +44-1305- 213600) Abstract. In a total of 189 water samples collected from Danish streams no traces of the pyrethroid esfenvalerate were detected. However, pyrethroids have previously been found in sediments in 9 out of 30 streams investigated. We found that the shredding activity of the Trichopteran Sericostoma personatum and the amphipod Gammarus pulex was significantly reduced with increased concentration of the pyrethroid lambda-cyhalohtrin adsorbed to the leaves on which they fed. Predation rate on the Plecopteran Leuctra nigra by the leech Erpobdella octoculata increased significantly with increasing concentration of lambda-cyhalothrin on the leaves on which L. nigra was fed. Our results clearly indicate that the ongoing monitoring of pesticides is likely to underestimate pyrethroid occurrence and that sediment-bound pyrethroids have a potential negative impact on ecosystem function and biotic interactions in streams. Keywords: ecosystem impact, pesticides, pyrethroid, stream bed sediment, water 1. Introduction Pesticides are an integral part of modern agriculture that can potentially harm natural ecosystems. One example is the loss of pesticides to stream eco- systems and numerous studies report findings of pesticides in stream water (e.g., Kreuger, 1998; Mogensen & Spliid, 1995) and impacts on the biota (e.g., Hatakeyama, Fukushima, Kasai, & Shiraishi, 1994; Schultz & Liess, 1999). Pyrethroids are currently widely used insecticides and in 2003 they accounted for 83% of the area sprayed with insecticides in Denmark (Danish EPA, 2004). Dissolved pyrethroids are known to induce increased macroinverte- brate drift (e.g., Hechmann & Friberg, 2005), increased mortality and life cycle changes (Schulz & Liess, 2000). Pyrethroids are hydrophobic and have extremely high binding capacities to sediments (Wauchope, Buttler, Hornsby, Augutijnbeckers, & Burt, 1992). It is therefore likely that the primary route of pyrethroids into stream ecosystems will be through the sediments, but this has received little attention (Schulz & Liess, 1999). Water, Air, and Soil Pollution: Focus (2006) 6: 423–432 DOI: 10.1007/s11267-006-9056-z © Springer Science + Business Media B.V. 2006