Effects of a mixture of tetracyclines to Lemna gibba and Myriophyllum sibiricum evaluated in aquatic microcosms Richard A. Brain * , Christian J. Wilson, David J. Johnson, Hans Sanderson, Ketut (Jim) Bestari, Mark L. Hanson, Paul K. Sibley, Keith R. Solomon Centre for Toxicology, Department of Environmental Biology, University of Guelph, Guelph, Ontario, Canada N1G 2W1 Received 17 December 2004; accepted 22 April 2005 Mixtures of tetracycline antibiotics pose a risk to submerged but not floating aquatic plants. Abstract The impact of a mixture of oxytetracycline, chlortetracycline, tetracycline and doxycycline on Myriophyllum sibiricum and Lemna gibba was investigated using fifteen 12,000-L microcosms (k Z 5, n Z 3). Significant concentrationeresponse relationships were only found for M. sibiricum, where dry mass was 69, 47, 30, and 7% of controls at respective treatment concentrations of 0.080, 0.218, 0.668, and 2.289 mmol/L. Somatic endpoints were strongly and negatively correlated with percent light transmission, except plant length, which was positively correlated. Treated microcosms experienced a reduction in the percent of surface irradiance penetrating the water column as high as 99.8% at a depth of 70 cm, relative to controls. Position relative to the water column was likely responsible for the differential effects observed between floating (L. gibba) and submerged (M. sibiricum) species of macrophytes. A hazard quotient assessment of the lowest EC 10 value indicated significant risk, exceeding the critical HQ value, but not the lowest EC 25 value. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Tetracycline; Myriophyllum; Lemna; Risk assessment; Microcosm; Light attenuation 1. Introduction Antimicrobial use in agriculture is a growing concern, with greatest focus centred on the issue of bacterial resistance, which is considered to be a public health priority (CEC, 2001). This has lead to a substantial allocation of resources and political will in terms of research and mitigating antimicrobial use. Compara- tively, ecosystem health has not received the same attention, and the impacts of these compounds on organisms such as higher aquatic plants are still poorly characterized and understood. Antimicrobials eventually reach the environment via several well-characterized routes (Halling-Sørensen et al., 1998), and several therapeutic classes of antibiotics have been detected in surface waters of the U.S. and Europe at concentrations in the ng/L to mg/L range (Halling-Sørensen et al., 1998; Kolpin et al., 2002). Generally, pharmaceuticals have relatively short half-lives, however these compounds can mimic persistent compounds due to their continual introduction into the environment, allowing for chronic exposures (Daughton and Ternes, 1999). Antibiotics are used largely as growth promoters, coccidiostatics, therapeutic treatment in livestock, and microbial control in aquaculture (Halling-Sørensen et al., 1998). In the United States, roughly 40% of the * Corresponding author. Tel.: C1 519 821 4120x58627; fax: C1 519 837 3861. E-mail address: rbrain01@uoguelph.ca (R.A. Brain). 0269-7491/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2005.04.021 Environmental Pollution 138 (2005) 425e442 www.elsevier.com/locate/envpol