Growth rate of Pseudokirchneriella subcapitata exposed to herbicides found in surface waters in the Alqueva reservoir (Portugal): a bottom-up approach using binary mixtures

Previous work showed the existence of ecotoxicity of water samples from the Alqueva reservoir due to the presence of the herbicides atrazine, simazine, terbuthylazine and metolachlor. In the present study we examine the effects of these herbicides singly and as binary mixtures on the growth rate of the microalgae Pseudokirchneriella subcapitata. Usually, the toxicity of mixtures is evaluated in relation to the reference models CA (concentration addiction) and IA (independent action). In this study CA model was selected to evaluate the joint effects of s-triazine herbicides on the growth of algae due to their similar mode of action. Moreover, IA reference model was chosen to evaluate the joint toxicity of the chloroacetanilide metolachlor and the s-triazine herbicides due to their different mode of action. In this study dose ratio was the common deviation obtained on both reference models. In the binary mixtures between atrazine/simazine and atrazine/terbuthylazine the increase of the mixtures toxicity (synergism) was mainly due to atrazine. Also, in the binary mixture between atrazine and metolachlor, atrazine was responsible for the increase (synergism) of the mixture toxicity. In the cases of the binary mixtures between simazine/metolachlor and terbuthylazine/metolachlor, the increase of the mixtures toxicity (synergism) was detected when metolachlor was the herbicide dominant, and antagonism was detected when simazine and terbuthylazine were dominant in both mixtures. This study represents an important step to understand the interactions among herbicides detected previously in the waters of the Alqueva reservoir.