Predictability of combined effects of eight chloroacetanilide herbicides on algal reproduction

Abstract

Chloroacetanilides are pre-emergence herbicides for the control of annual grasses and broadleaf weeds. As a result of their extensive use, residues are often found in surface waters. Observed simultaneous occurrence of different chloroacetanilide herbicides gives reason for concern about potential combination effects on aquatic non-target organisms. This study aimed to clarify whether joint effects of various chloroacetanilide herbicides may be predictable from knowledge of concentration-response relationships of single substances. Whether the chloroacetanilides all share the same mode of action is unclear. Therefore we investigated the predictive value of two alternative concepts for the prediction of combined effects: concentration addition, which assumes a similar mode of action, and independent action, which is based on the idea of a dissimilar mode of action of the mixture components. Eight chloroacetanilides (acetochlor, alachlor, butachlor, dimethachlor, metazachlor, metolachlor, pretilachlor and propachlor) were experimentally tested for their individual as well as for their combined effects in mixtures on the reproduction of the green alga Scenedesmus vacuolatus. Individual chloroacetanilides impaired algal reproduction, with EC50 values ranging from 3 to 232 microg litre(-1). The differences in EC50 values were strongly correlated with the lipophilicities of the compounds. Effects of chloroacetanilide mixtures were considerably higher than those of the individual components: a complete inhibition of algal reproduction was observed when every mixture component was present in a concentration that would cause only 5% effect if applied singly. However, the combined effects proved to be predictable by using the concept of concentration addition. The alternative concept, independent action, distinctly underestimated the mixture toxicity. These findings (1) indicate a similar mechanism of action of chloroacetanilides in algae and (2) reinforce the view that concentration addition is a reasonable assumption for the predictive hazard assessment of groups of similarly acting herbicides.

Predicting the aquatic toxicity of commercial pesticide mixtures

“…For algae and Lemna, the mixture toxicity predictions met the reported product toxicity with less than tenfold deviation. This finding is in accordance with toxicity experiments conducted in algae with mixtures of herbicides from one homogenous mode-ofaction group such as chloroacetanilide [30] or phenylurea [31] and from diverse mode-of-action groups [32]. In contrast, the acute joint toxicity towards Daphnia and fish of all (assessable) products with iodosulfuron was at least tenfold underestimated.…”

Assessment of the potential toxicity of herbicides and their degradation products to nontarget cells using two microorganisms, the bacteria Vibrio fischeri and the ciliate Tetrahymena pyriformis

“…EC 50 values in green algae are speciesdependent: 6 g/L for Pseudokirchneriella subcapitata (Fairchild et al, 1997), 37.8 g/L for Scenedesmus vacuolatus (Junghans et al, 2003), 198 g/L for Lemna minor (Fairchild et al, 1997) and 5.54 mg/L for Chorella vulgaris (Yang et al, 2004). On the aquatic invertebrate Daphnia Table II. magna, 96 h-EC 50 was 0.05 mg/L (Agritox, 2006).…”

Histopathological and haematological response of male rainbow trout (Oncorhynchus mykiss) subjected to butachlor

“…0.39 mg/l, to induce acute butachlor toxicity (Purity 60%, H.P.C., Iran). To ensure consistency, we validated the concentration of butachlor using a HPLC system (Junghans et al 2003;Del Buono et al 2005).…”