Effects of Atrazine and Nicosulfuron on Phytoplankton in Systems of Increasing Complexity F. Seguin, 1 C. Leboulanger, 2 F. Rimet, 3 J. C. Druart, 2 A. Bérard 2 1 INRA, Ecobiologie et Qualité des Hydrosystèmes Continentaux, 65 rue de St Brieuc, 35042 Rennes, France 2 INRA, Station d’Hydrobiologie Lacustre, BP 511, 74203 Thonon les Bains Cedex, France 3 CRP-Gabriel Lippmann, CREBS, 162a av. de la Faiencerie, 1511 Luxembourg Received: 30 March 2000/Accepted: 28 August 2000 Abstract. We have tested the sensitivity of phytoplankton to the herbicides atrazine and nicosulfuron in experiments con- duced in increasingly complex systems, from single strain phytoplankton cultures (microplates) to mesocosms mimicking whole ecosystems. The endpoints used to assess sensitivity to atrazine and nicosulfuron were total biomass increase, photo- synthetic efficiency, and community diversity, depending on the system considered. Nicosulfuron appeared to be very much less toxic to phytoplankton than atrazine, in accord with the planned changes in agricultural practices to reduce the effects of surface water contamination on aquatic biota. Nevertheless, nicosulfuron had significant effects in some systems (princi- pally microcosms), whereas the single monocultures were al- most insensitive to it. This points out the inaccuracy of stan- dardized toxicity test on phytoplanktonic algae alone for predicting the effects of xenobiotics on natural communities and the need for tests in microcosms and mesocosms to obtain reliable evidence about the toxicity of a given chemical on freshwater aquatic ecosystems. Intensive crop production has used more and more herbicides over the past several decades. Increased production crops, such as cereals, involves massive herbicide consumption that leads to the significant contamination of surface water. Some uses of atrazine can result in triazine content of such water exceeding 10 g/L (Caux and Kent 1995; Solomon et al. 1996). As atrazine (chloro-2 ethylamino-4 isopropylamino-6 triazine- 1,3,5) inhibits the photosystem II, it may damage nontarget aquatic primary producers (phytoplankton, periphyton, and macrophytes). Microalgae are potential targets of herbicides in aquatic ecosystems. Herbicides can alter the ecophysiology and algal population dynamics, so that the most sensitive species are killed, allowing more tolerant species to develop; contam- ination may thus cause a shift in community structure (Kasai et al. 1993). Atrazine may indirectly influence the whole trophic food web of aquatic ecosystems, with damage to phytoplank- ton, periphyton, and macrophytes that may reduce the capacity of the aquatic habitat to sustain invertebrates and fish (Solomon et al. 1996). An agrienvironmental program was launched in France in 1993. This program promoted environmentally friendly agri- cultural practices (Dabène and Larguier 1994). One of these measures involves replacing atrazine with nicosulfuron [2-(4,6- dimethoxypyrimidin-2-ylcarbamoylsulphanoyl)-N,N-dimethyl nicotinamide], which is considered to be less harmful. This sulfonylurea herbicide prevents the synthesis of ILE, LEU, and VAL by inhibiting the enzyme ALS (Simpson et al. 1995). Unfortunately, there are few published data on the ecotoxico- logical effects of this compound. It is thus necessary to detect and compare the effects of atrazine and nicosulfuron on the aquatic biota in order to assess the contamination risk produced by changes in agricultural practices. This can be done in several ways. The most widely used experiments are monospe- cific toxicity tests because of their low cost, satisfactory repro- ducibility, and ease of execution. They are often standardized (e.g., AFNOR 1998; EPA 1989). Though they are essential for understanding how toxic compounds act, they cannot yield reliable risk assessment (Pratt et al. 1988; Bérard and Pelte 1999). The most complex experiments involve aquatic meso- cosms that have structural and functional characteristics similar to the ecosystems they are intended to model. The bioavail- ability and toxicity of pesticides can be assessed using a rep- resentative set of natural aquatic ecosystems (Crossland 1994; Touart 1994), and environmental realism is the main reason for using mesocosm instead of less complex experiments (Kraufvelin 1998). However, increased ecological realism of- ten implies less accurate measurements and statistical analyses (Giesy and Allred 1985). This method is also costly and hard to implement. Indoor and outdoor microcosms, in which natural communities are isolated from the whole ecosystem, are inter- mediate between monospecific tests and a holistic approach (mesocosm). Toxicity can be assessed in a multispecies com- munity, but not necessarily on an entire trophic web. These three experimental systems are thus complementary, and using them all provides a better understanding of the effects of toxic chemicals on aquatic species and their environment. Correspondence to: C. Leboulanger; email: leboulan@thonon.intra.fr Arch. Environ. Contam. Toxicol. 40, 198 –208 (2001) DOI: 10.1007/s002440010164 ARCHIVES OF Environmental Contamination and T oxicology © 2001 Springer-Verlag New York Inc.