The influence of permanently submerged macrophytes on sediment mercury distribution, mobility and methylation potential in a brackish Norwegian fjord

Macrophytes are shown to affect the microbial activity in different aqueous environments, with an altering of the sediment cycling of mercury (Hg) as a potential effect. Here, we investigated how a meadow with permanently submerged macrophytes in a contaminated brackish fjord in southern Norway influenced the conditions for sulfate reducing microbial activity, the methyl-Hg (MeHg) production and the availability of MeHg. Historically discharged Hg from a chlor-alkali plant (60–80 tons, 1947–1987) was evident through high Hg concentrations (491 mg Tot-Hg kg− 1, 268 μg MeHg kg− 1) in intermediate sediment depths (10–20 cm) outside of the meadow, with reduced concentrations within the meadow. Natural recovery of the fjord was revealed by lower sediment surface concentrations (1.9–15.5 mg Tot-Hg kg− 1, 1.3–3.2 μg MeHg kg− 1). Within the meadow, vertical gradients of sediment hydrogen sulfide (H2S) Eh and pH suggested microbial sulfate reduction in 2–5 cm depths, coinciding with peak values of relative MeHg levels (0.5% MeHg). We assume that MeHg production rates was stimulated by the supply and availability of organic carbon, microbial activity and a sulfide oxidizing agent (e.g. O2) within the rhizosphere. Following this, % MeHg in sediment (0–5 cm) within the meadow was approximately 10 × higher compared to outside the meadow. Further, enhanced availability of MeHg within the meadow was demonstrated by significantly higher fluxes (p < 0.01) from sediment to overlying water (0.1–0.6 ng m− 2 d− 1) compared to sediment without macrophytes (0.02–0.2 ng m− 2 d− 1). Considering the productivity and species richness typical for such habitats, submerged macrophyte meadows located within legacy Hg contaminated sediment sites may constitute important entry points for MeHg into food webs