The influence of a submerged meadow on uptake and trophic transfer of legacy mercury from contaminated sediment in the food web in a brackish Norwegian fjord

Sammendrag

There are contradicting results on the importance of legacy mercury (Hg) contaminated sediments to Hg fish tissue concentrations. Still, sediment remediation actions often aim at minimizing ecosystem exposure and human risk caused by the consumption of fish and seafood. The aim of this study was to investigate the possible influence of a permanently submerged meadow on the availability and transfer of Hg from sediment to biota, three decades after the Hg discharges was halted and the previous biota survey was carried out, in the severely contaminated brackish fjord Gunneklevfjorden in southern Norway. We examined total Hg (Tot-Hg) and methyl-Hg (MeHg) concentrations and stable isotopes of nitrogen (N) and carbon (C) in zooplankton, benthic invertebrates, and fish to map the food web and to investigate the trophic transfer of Hg. Sediment and water data were available from a previous study. Overlap in δ13C in benthos and fish reveals that benthos is a preferred prey to fish, though despite elevated Tot-Hg concentrations in benthos, fish predator (perch, pike and eel) Hg concentrations are comparable to concentrations reported in nearby lakes without contaminated sediments (mean 1.6 ± 1.3 mg Tot-Hg kg−1 dw). We propose that habitat reliance is an important factor controlling the uptake of Hg from sediments, as both benthos and fish prefer to forage within the meadow where sediment concentrations of Tot-Hg are lower than outside the macrophyte meadow, though %MeHg is higher than outside. Further, we propose that sediment remediation actions performed outside the meadow may have limited effect on the Hg concentrations in fish.