Ecotoxicology and Environmental Safety 57 (2004) 153–161 Sensitivity of aquatic plants to the herbicide metsulfuron-methyl Nina Cedergreen, a, Jens C. Streibig, a and Niels Henrik Spliid b a Department of Agricultural Sciences, The Royal Veterinary and Agricultural University (KVL), H^jbakkega ˚rd Alle´9, Taastrup DK-2630, Denmark b Department of Crop Protection, Danish Institute of Agricultural Sciences, Research Centre Flakkebjerg, Slagelse DK-4200, Denmark Received 18 June 2002; received in revised form 20 November 2002; accepted 3 December 2002 Abstract The sensitivity of 12 aquatic plant species to the herbicide metsulfuron-methyl was tested in microcosm experiments under two growth conditions. As reference species, barley (Hordeum vulgare) and oil-seed rape (Brassica napus) were grown with their roots submerged in the microcosms. Two response variables were chosen: relative growth rate and specific leaf area (SLA). SLA was the most sensitive response variable, with 11 of the 12 aquatic species responding to the herbicide. EC 50 values varied 56-fold between species, with the commonly used aquatic test species Lemna minor being one of the most sensitive. Fast-growing species with a small exposed leaf area proved to be more sensitive to the herbicide than slow growing species with a large exposed leaf area, which was believed to be primarily due to variations in growth rates rather than to variations in exposed leaf area. The aquatic plants displayed high tolerance in growth to metsulfuron compared with the sensitive crop oil-seed rape. Hence, possible spray-drift events and leaching of the herbicide applied at agricultural rates are not considered to have a large impact on the growth of the aquatic flora tested. r 2003 Elsevier Science (USA). All rights reserved. Keywords: Ecotoxicology; Dose–response; Macrophytes; Sulfonylureas; Toxicity 1. Introduction Aquatic plants play a major role in the environmental conditions of stagnant and flowing waters. They produce organic matter and oxygen, and they provide food, shelter, and substrate for a variety of aquatic organisms. In addition, rooted macrophytes modify flow, stabilize sediments and promote retention of organic matter and nutrients (Sand-Jensen, 1997). A large number of the Danish aquatic ecosystems are situated adjacent to cultivated land or forests where pesticides are frequently applied. Despite this, few studies have been made to investigate the effect of herbicides on the aquatic flora (Peterson et al., 1994; Roshon et al., 1999). One group of herbicides that could potentially pose a problem to the aquatic flora is sulfonylureas. The sulfonylureas are a group of very potent herbicides the use of which has increased dramatically since their discovery about two decades ago. They inhibit the enzyme acetolactate synthase (ALS), which is a key enzyme in the biosynthesis of branched chain amino acids (Cobb, 1992). They are weak acids that adsorb poorly to neutral and alkaline soils (Nystro¨ m et al., 1999; Oliveira et al., 2001), and are therefore considered potentially leachable, which has been confirmed by the finding of sulfonylureas in soil layers below 50cm (Black et al., 1999; Stork, 1995). Other studies have shown that even doses under 1% of the recommended rate can have a significant effect on growth, morphology, and reproduction of a suit of nontarget terrestrial plants (Boutin et al., 2000; Fletcher et al., 1996; Bhatti et al., 1995). If sulfonylureas therefore do reach the aquatic environment, effects on the aquatic flora could be expected even at small doses. The only aquatic plants routinely tested are the floating macrophytes Lemna minor and L. gibba (Boutin et al., 1995). Knowing that the agricultural use of sulfonylureas is based on differential susceptibility of species, varying by a factor of 100–1000 among terrestrial species (Boutin and Rogers, 2000), it is expected that also within the aquatic environment, differential susceptibility occurs among species. This has been demonstrated for microalgae in microplate tests, where 40 marine and freshwater species had ED 50 values for chlorsulfuron and metsulfuron-methyl ARTICLE IN PRESS  Corresponding author. Fax: +45-35-28-21-75. E-mail address: ncf@kvl.dk (N. Cedergreen). 0147-6513/03/$-see front matter r 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0147-6513(02)00145-8