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The annual total and organic mercury bioaccumulation pattern of Scrobicularia plana and Hediste diversicolor was assessed to evaluate the potential mercury transfer from contaminated sediments to estuarine food webs. S. plana was found to accumulate more total and organic mercury than H. diversicolor, up to 0.79 mg kg À1 and 0.15 mg kg À1 (wet weight) respectively, with a maximum annual uptake of 0.21 mg kg À1 y À1 , while for methylmercury the annual accumulation was similar between species and never exceeded 0.045 mg kg À1 y À1 . The higher organic mercury fraction in H. diversicolor is related to the omnivorous diet of this species. Both species increase methylmercury exposure by burrowing activities and uptake in anoxic, methylmercury rich sediment layers. Integration with the annual biological production of each species revealed mercury incorporation rates that reached 28 mg m À2 y À1 , and to extract as much as 11.5 g Hg y À1 (of which 95% associated with S. plana) in the 0.4 km 2 of the most contaminated area, that can be transferred to higher trophic levels. S. plana is therefore an essential vector in the mercury biomagnification processes, through uptake from contaminated sediments and, by predation, to transfer it to economically important and exploited estuarine species.

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h i g h l i g h t sSulfathiazole shows high stability in seawater with 18% degradation over 5 d. Ulva lactuca reduced the concentrations of STZ in the water by efficiently taking up the drug. U. lactuca kept the internal values constant most likely through a detoxification mechanism. Sulfathiazole slightly inhibited the growth of U. lactuca after 96 h of exposure. Macroalgae should be considered as potential bioindicator species in risk assessment of STZ. Sulfonamides (SA) are a class of antibiotics routinely found in environmental matrices and therefore their role as contaminants should be investigated in non-target organisms. With this purpose the present experimental work has evaluated the exposure of the chlorophycean Ulva lactuca L. to sulfathiazole (STZ), a SA drug commonly used in aquaculture, at two concentrations representing prophylactic (25 lg mL À1 ) and therapeutic (50 lg mL À1 ) administrations. Results showed that STZ exhibits high stability in seawater with only 18% degradation over the 5 d assay at both dosages tested. Also, macroalgae demonstrated an efficient uptake capacity with constant internal concentrations after 24 h regardless of the external solutions and thus should be considered as a bioindicator species in risk assessment. Both STZ concentrations induced a slight inhibition of the macroalgae growth after 96 h. a r t i c l e i n f o

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