Bioavailability of sediment-associated mercury to Hexagenia mayflies in a contaminated floodplain river

We examined the bioavailability of mercury in sediments from the contaminat ed Sudbury River (Massachusetts, U.S.A.). Mayfly nymphs (Hexagenia) were ex posed in four 21-day bioaccumulation tests to contaminated and reference se diments (treatments) from reservoirs, flowing reaches, palustrine wetlands, and a riverine lake. Mean total mercury (Sigma Hg) ranged from 880 to 22 0 59 ng.g dry weight(-1) in contaminated sediments and from 90 to 272 ng.g(-1 ) in reference sediments. Mean final concentrations of methyl mercury (MeHg ) in test water were greatest (8-47 ng Hg.L-1) in treatments with contamina ted wetland sediments, which had mean Sigma Hg ranging from 1200 to 2562 ng .g(-1). In mayflies, final mean concentrations of MeHg were highest in trea tments with contaminated wetland sediments (122-183 ng Hg.g(-1)), intermedi ate in treatments with contaminated sediments from reservoirs, flowing reac hes, and a riverine lake (75-127 ng Hg.g(-1)), and lowest in treatments wit h reference sediments (32-41 ng Hg.g(-1)). We conclude that the potential e ntry of MeHg into the benthic food chain was greater in contaminated palust rine wetlands than in the contaminated reservoirs, which had the most conta minated sediments.