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

Abstract

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 (60-80tons, 1947-1987) was evident through high Hg concentrations (491mgTot-Hgkg, 268μgMeHgkg) in intermediate sediment depths (10-20cm) outside of the meadow, with reduced concentrations within the meadow. Natural recovery of the fjord was revealed by lower sediment surface concentrations (1.9-15.5mgTot-Hgkg, 1.3-3.2μgMeHgkg). Within the meadow, vertical gradients of sediment hydrogen sulfide (HS) E and pH suggested microbial sulfate reduction in 2-5cm 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. O) within the rhizosphere. Following this, % MeHg in sediment (0-5cm) 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 (0.1-0.6ngmd) compared to sediment without macrophytes (0.02-0.2ngmd). 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.

Formation and availability of methylmercury in mercury-contaminated sediment: effects of activated carbon and biochar amendments

“…There is also a small outlet in the other end of the fjord through a small channel, built as an exit from the Gunneklev fjord into the Inner Grenlandsfjord for small recreational boats from the marina situated in the inner part of the Gunneklev fjord. The Gunneklev fjord is a brackish fjord (salinity 5-20 PSU in the bottom water (Olsen et al (2018) and references therein) with an average depth of 4.6 m and water volume of 3.5 × 10 6 m 3 . It is one of the most polluted fjord systems in Norway due to historic release of Hg-containing sludge from a chlor-alkali plant (Olsen et al 2018).…”

“…The Gunneklev fjord is a brackish fjord (salinity 5-20 PSU in the bottom water (Olsen et al (2018) and references therein) with an average depth of 4.6 m and water volume of 3.5 × 10 6 m 3 . It is one of the most polluted fjord systems in Norway due to historic release of Hg-containing sludge from a chlor-alkali plant (Olsen et al 2018). Previous studies have shown sediment concentration ranges of total mercury (THg) ranging from 2 to 5 mg kg −1 at the 0-5 cm surface layer to 500 mg kg −1 at 10-20 cm depth, and MeHg of 1-3 µg kg −1 at the surface to 300 µg kg −1 at 10-20 cm depth (Olsen et al 2018).…”

“…It is one of the most polluted fjord systems in Norway due to historic release of Hg-containing sludge from a chlor-alkali plant (Olsen et al 2018). Previous studies have shown sediment concentration ranges of total mercury (THg) ranging from 2 to 5 mg kg −1 at the 0-5 cm surface layer to 500 mg kg −1 at 10-20 cm depth, and MeHg of 1-3 µg kg −1 at the surface to 300 µg kg −1 at 10-20 cm depth (Olsen et al 2018). It has been estimated that the fjord contains about 20 to 30 tonnes of Hg, of a total of 80 tonnes released from Herøya Industrial Complex between 1947 and 1987 (Olsen et al 2019).…”

Production of methylmercury by sulphate-reducing bacteria in sediments from the orbetello lagoon in presence of high macroalgal loads

“…This fact confirms possible problems related to the presence of a high algal biomass in this ecosystem that could increase the process of Hg-methylation. MeHg production in macrophyte habitats in presence of Hg contaminated sites request more attention, with regards to the factors controlling the methylation processes (Olsen et al, 2018).…”

An In Situ Experimental Study of Effects on Submerged Vegetation After Activated Carbon Amendment of Legacy Contaminated Sediments

“…Seagrass meadows are known to trap particles from the water column 19,20 , thus enhancing sediment deposition and reducing resuspension 21 and are therefore suspect to high concentrations of contaminants within polluted areas. Accordingly, submerged meadows may be important exposure sites for contaminants to inhabiting organisms, and recent studies have shown enhanced bioavailability of sediment Hg within vegetated areas [22][23][24] , which may initiate a transfer of contaminants through food webs, with a potential to biomagnify at each trophic level. Thus, ecologically important submerged meadows within polluted areas potentially face the duality of being suspect to both remediation and conservation, which actualises the need to develop low-impact risk-reducing remediation strategies.…”