A mesocosm experiment on methyl mercury formation after capping of U-864 sediments enriched with powdered algae

Summary

A mesocosm experiment was performed on methylation of mercury in marine sediments from the wreck site at Fedje. The mercury contaminated sediment was enriched with zero, low, medium or high dose of labile organic carbon from pulverized algae and either capped with a thin layer of clay or left uncapped exposed to the seawater flowing through the mesocosm. The small fraction of methyl mercury (MeHg) of 0,006% of the total concentration (TotHg) in the batch of sediment shipped form Fedje indicated that in situ methylation at the wreck site is low. In the boxcosms, no significant methylation was observed in sediments with no addition of algae. In the low dose treatment, initial methylation rates were found to be 0.2-0,3 ng MeHg m-2 day-1, but the process ceased after the first month, presumably due to depletion of the reservoir of available algae carbon added. In medium and high dose treatments the process proceeded throughout the experimental period at apparently constant rates proportional with the amount of algae added. Methylation was observed in both capped and uncapped Fedje sediments, but the highest rates of methylation were observed in the capped sediments, in which up to 0.6 ng m-2 day-1 of methyl mercury (MeHg) accumulated during the six months experimental period. Fluxes of TotHg and MeHg to the seawater were only measured in the uncapped sediments. High fluxes were observed in all treatments. The six months median fluxes showed a four-fold increase of TotHg from zero to high algae addition, whereas the corresponding fluxes of MeHg increased 100x from 0,97 to 94.1 ng MeHg m-2 day-1. The flux of MeHg could only account for a small fraction (in general less than 10% of the total flux) indicating the formation of soluble organic complexes other than MeHg. A more long-term experiment using petrogenic hydrocarbons is recommended to improve assessment of the risks of Hg methylation at Fedje and at Hg-contaminated wreck sites in general.