Modelling long-term ecotoxicological effects on an algal population under dynamic nutrient stress D. Bontje a, *, B.W. Kooi a , M. Liebig b , S.A.L.M. Kooijman a a Vrije Universiteit, Faculty of Earth & Life Sciences, Department of Theoretical Biology, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands b ECT Oekotoxikologie GmbH, Bo ¨ttgerstrasse 2-14, D-65439 Flo ¨rsheim/Main, Germany article info Article history: Received 9 September 2008 Received in revised form 11 February 2009 Accepted 20 April 2009 Published online - Keywords: Bio-variability Dynamic energy budget Population extinction threshold Process-based model Extinction probability abstract We study the effects of toxicants on the functioning of phototrophic unicellular organism (an algae) in a simple aquatic microcosm by applying a parameter-sparse model. The model allows us to study the interaction between ecological and toxicological effects. Nutrient stress and toxicant stress, together or alone, can cause extinction of the algal population. The modelled algae consume dissolved inorganic nitrogen (DIN) under surplus light and use it for growth and maintenance. Dead algal biomass is mineralized by bacterial activity, leading to nutrient recycling. The ecological model is coupled with a toxicity- module that describes the dependency of the algal growth and death rate on the toxicant concentration. Model parameter fitting is performed on experimental data from Liebig, M., Schmidt, G., Bontje, D., Kooi, B.W., Streck, G., Traunspurger, W., Knacker, T. [2008. Direct and indirect effects of pollutants on algae and algivorous ciliates in an aquatic indoor microcosm. Aquatic Toxicology 88, 102–110]. These experiments were especially designed to include nutrient limitation, nutrient recycling and long-term exposure to toxicants. The flagellate species Cryptomonas sp. was exposed to the herbicide prometryn and insecticide methyl parathion in semi-closed Erlenmeyers. Given the total limiting amount of nitrogen in the system, the estimated toxicant concentration at which a long-term steady pop- ulation of algae goes extinct will be derived. We intend to use the results of this study to investigate the effects of ecological (environmental) and toxicological stresses on more realistic ecosystem structure and functioning. ª 2009 Elsevier Ltd. All rights reserved. 1. Introduction When assessing the ecological status of a river, the effects of both toxicant and environmental stresses on multiple species have to be accounted for (Brack et al., 2005). Laboratory toxicity tests, on the other hand, generally concern a single stress and a single species. The use of population models for extrapolation from single species ecotoxicological observations to the relevant effects on an ecosystem is discussed in Forbes and Callow (2002) and in Forbes et al. (2008). Here for modelling purposes, an aquatic ecosystem contains a limiting nutrient and functional groups: producers, predators and decomposers to ensure nutrient recycling. In order to study direct and indirect effect of toxicants, the bottom trophic levels of this system have been exposed and studied by Liebig et al. (2008) in these microcosm experiments. In these Erlenmeyer experiments the flagellate Cryptomonas sp. represents the producers, the ciliate Urotricha furcata represents the predators and undetermined bacteria are the decomposers. The system is exposed to either a herbicide or * Corresponding author. Tel.: þ31 205987133. E-mail address: daniel.bontje@falw.vu.nl (D. Bontje). Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/watres ARTICLE IN PRESS 0043-1354/$ – see front matter ª 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2009.04.036 water research xxx (2009) 1–9 Please cite this article in press as: Bontje, D. et al., Modelling long-term ecotoxicological effects on an algal population under dynamic nutrient stress, Water Research (2009), doi:10.1016/j.watres.2009.04.036