Assessment of toxicity thresholds in aquatic environments: Does benthic growth of diatoms affect their exposure and sensitivity to herbicides? Floriane Larras ⁎, Bernard Montuelle, Agnès Bouchez INRA, UMR Carrtel, F-74203 Thonon, France Université de savoie, UMR Carrtel, F-73011 Chambéry, France HIGHLIGHTS • We assessed influence of diatom growth mode on their sensitivity to herbicides. • Growth mode and herbicide hydrophobicity modify diatoms sensitivity. • Most hydrophobic herbicides were more toxic under benthic than planktonic growth. • Planktonic dataset was adapted for hydrophilic herbicide risk assessment. • For hydrophobic herbicides, benthic thresholds tended to be more protective. abstract article info Article history: Received 22 January 2013 Received in revised form 14 June 2013 Accepted 14 June 2013 Available online xxxx Editor: Mark Hanson Keywords: Benthic diatoms Herbicides Biofilm Bioassays Risk assessment Benthic diatoms evolved in a biofilm structure, at the interface between water and substrata. Biofilms can adsorb toxicants, such as herbicides, but little is known about the exposure of biofilm organisms, such as benthic diatoms, to these adsorbed herbicides. We assessed the sensitivity of 11 benthic diatoms species to 6 herbicides under both planktonic and benthic conditions using single-species bioassays. The concentration that reduced the growth rate of the population by 10% (EC 10 ) and 50% (EC 50 ), respectively, varied depending on the species, the herbicides, and the growth forms involved. As a general trend, the more hydrophobic the herbicide, the more species were found to be sensitive under benthic growth conditions. Statistical differences (alpha b 5%) were observed between the sensitivities under planktonic and benthic growth conditions for many hydrophobic herbicides. A protective effect of the biofilm against herbicides was observed, and this tended to decrease (at both the EC 10 and EC 50 levels) with increasing hydrophobicity. The biofilm matrix appeared to control exposure to herbicides, and consequently their toxicity towards benthic diatoms. For metolachlor, terbutryn and irgarol, benthic thresholds derived from species sensitivity distributions were more protective than planktonic thresholds. For hydrophobic herbicides, deriving sensitivity thresholds from data obtained under benthic growth seems to offer a promising alternative. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Benthic diatoms play key roles in the environment, for instance constituting the primary production in functioning aquatic ecosys- tems. With their great diversity and wide-ranging ecological niches, they are found in a variety of habitats and provide reliable indicators of the ecological status of freshwater ecosystems (Lenoir and Coste, 1996; Rimet and Bouchez, 2012; Van Dam et al., 1994). Benthic dia- toms in biofilms live embedded in a matrix consisting of an extracel- lular polymeric substance (EPS), that is made up mainly of proteins, saccharides and nucleic acids (Sutherland, 2001). On the one hand, the biofilm may protect diatom communities against physico-chemical changes and environmental factors. On the other hand, it can also inter- act with the whole environment, catching and remobilizing dissolved substances, such as nutrients or toxicants, from the water column (Flemming and Leis, 2002). Many authors have studied the biofilm sorp- tion capabilities of herbicides, such as atrazine (Headley et al., 1998; Lawrence et al., 2001; Bohuss et al., 2005), diuron (Tlili et al., 2008), diclofop methyl (Wolfaardt et al., 1995; Lawrence et al., 2001), and var- ious other pesticides such as DDT (Headley et al., 1998; Dong et al., 2011), lindane, chlorpyrifos and carbofuran (Lundqvist et al., 2012). Sorption processes depend partly on the chemical structure and proper- ties of the biofilm involved (thickness, EPS composition, architecture) (Headley et al., 1998; Lawrence et al., 2001). Flemming (1995) identified various sorption sites, with specific affinities, including the EPS, cell Science of the Total Environment 463–464 (2013) 469–477 ⁎ Corresponding author. Tel.: +33 4 50 26 78 32; fax: +33 4 50 26 07 60. E-mail address: floriane.larras@thonon.inra.fr (F. Larras). 0048-9697/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.scitotenv.2013.06.063 Contents lists available at SciVerse ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv