Ecotoxicology and Environmental Safety 58 (2004) 236–245 Effects of cypermethrin on marine plankton communities: a simulated field study using mesocosms Matı´as Medina, a,b, Carlos Barata, b,c Trevor Telfer, b and Donald J. Baird b a Departamento de Ecologı´ a, Center for Advanced Studies in Ecology and Biodiversity, Pontificia Universidad Cato´lica de Chile, Alameda 340, Casilla 114-D, Santiago, Chile b Environment Group, Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland, UK c Laboratory of Environmental Toxicology, INTEXTER-UPC, Terrassa 08220, Barcelona, Spain Received 18 December 2002; received in revised form 9 July 2003; accepted 19 July 2003 Abstract In earlier single-species toxicity tests we showed the negative effects on the calanoid copepod Acartia tonsa upon exposure to cypermethrin, a pesticide used in treatment for sea lice in salmon farming. In the present study we assessed effects at a higher level of biological organization and under a more realistic exposure scenario using mesocosms. The results showed that simulated field studies (SFSs) could be conducted with the mesocosms designed here. When cypermethrin was applied inside these mesocosms, its concentration decreased exponentially following a first-order kinetics model. The pesticide immediately reduced zooplankton density and biodiversity not only directly, by killing copepods, but also indirectly, by increasing the numbers of rotifers. Zooplankton density recovered after treatment, but zooplankton biodiversity remained altered. In an open environment, however, the rapid dissipation of the pesticide, coupled with population processes of compensation, migration, and immigration, may lead to recovery of the affected zooplankton communities. r 2003 Elsevier Inc. All rights reserved. Keywords: Ecotoxicology; Aquaculture; Sea lice; Cypermethrin; Acartia tonsa; Zooplankton; Mesocosms 1. Introduction To predict environmental hazards of chemical sub- stances, risk analyses use mostly data obtained from laboratory single-species tests that include one life stage, age, and size. These provide valuable information on the duration of the exposure that produces changes in mortality, growth, reproduction, behavior, and physiol- ogy of species. However, these tests can seldom be used to assess environmental hazards at higher levels of biological organization, because natural ecosystems are more complex and variable than laboratory standar- dized systems (Burnett and Liss, 1990; Rand et al., 1995). Therefore, toxicity bioassays done in the labora- tory should be complemented with higher tier assess- ment conducted in fieldlike scenarios. There is increasing public concern about the environ- mental hazards of chemicals used in aquaculture (Redshaw, 1995). In previous studies we assessed the ecological hazard of cypermethrin using single- species toxicity tests conducted with the calanoid copepod Acartia tonsa (Barata et al., 2002; Medina et al., 2002; Medina et al., 2003). Cypermethrin is a hydrophobic pesticide, recently used as chemotherapeu- tically in salmon farms for the control of copepodic parasite infestations (sea lice) (Hart et al., 1997); thus non-target copepod populations living adjacent to treated fish cages are likely to be affected by cyperme- thrin. Our earlier results showed negative effects of cypermethrin on A. tonsa’s survival, feeding, and reproduction rates at exposure levels from one to three orders of magnitude lower than the recommended treatment regimen (5 mgL 1 during 1h). This suggests that cypermethrin treatments could impair copepod populations. Nevertheless, the ecological risks of this ARTICLE IN PRESS  Corresponding author. Departamento de Ecologı´a, Center for Advanced Studies in Ecology and Biodiversity, Pontificia Universidad Cato´lica de Chile, Alameda 340, Casilla 114-D, Santiago, Chile. Fax: +56-2-6862621. E-mail address: mmedina@bio.puc.cl (M. Medina). 0147-6513/$-see front matter r 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.ecoenv.2003.07.001