An excessive usage of herbicides has been reported in the past years due to an intensive practice in agriculture fields in the Mediterranean region. The presence of chemicals led to the contamination of the surrounding aquatic environments, as it is the case of the Mondego estuary, situated near the city of Figueira da Foz (Portugal). Primextra Gold TZ is the most used herbicide in the study area in corn crops fields. The herbicide is composed by two main active ingredients, terbuthylazine (TBA) and S-metolachlor (SMOC), that in previous studies have shown to be toxic to non-target aquatic species. The benthic bivalve species Scrobicularia plana was selected to carry out bioassays due to its capacity to filter and bioaccumulate pollutants and its key position in the trophic food web, greatly influencing the functioning and structure of estuarine communities. Two size classes of Scrobicularia plana were exposed to different concentrations of the commercial formulation of Primextra Gold TZ (C1: 0.5 mg.L -1 , C2: 2.5 mg.L - 1 , C3: 5 mg.L -1 , C4: 10 mg.L -1 , C5: 20 mg.L -1 ). The highest concentration caused mortality to the big size class organisms. Sublethal effects were assessed by a battery of biomarkers related with the antioxidant defense system (the enzymes glutathione-S-transferase (GST), glutathione reductase (GRed), glutathione peroxidase (tGPx), superoxide dismutase (SOD) and catalase (CAT) and lipid peroxidation (TBARs)) and the profile in fatty acids (FA) and carbohydrates (CH). Results indicate that the effects were dose dependent, with worst effects in the highest concentrations and that FA, CH, and big size organisms of Scrobicularia plana were the best biomarkers to detect the presence of Primextra Gold TZ in estuarine systems.
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