Mercury in the Norwegian Fjord Framvaren

Abstract

Depth profiles of various mercury fractions in the permanently anoxic Framvaren Fjord, southern Norway, were determined on two occasions: September 1983 and February 1985. The mercury fractions were operationally defined as ‘dissolved gaseous’ Hg (DGM), ‘reactive’ Hg, ‘reactive — non-reactive’ Hg and ‘total’ Hg. The concentrations of mercury were generally higher during fall 1983 than winter 1985. The highest mercury concentrations in the profiles, independent of fraction, were found at the redox cline in 1983. No such general trend was revealed in 1985. The DGM concentrations down to the redox boundary were between 0.3 and 6 ng l −1 (1983), and 0.01 and 0.4 ng l −1 (1985). In 1983, the concentrations of dissolved ‘reactive’ Hg were in the range of 0.3–25 ng l −1. The concentrations of the dissolved ‘ reactive + non-reactive’ Hg fraction in the depth profile for 1983 and 1985 were in the ranges 1.5–30 ng l −1 and 0.2–1.7 ng l −1, respectively. No significant difference in ‘ reactive + non-reactive’ Hg was found between filtered and unfiltered samples collected in the winter of 1985. In 1983, the ‘total’ Hg concentrations in filtered samples from the near-surface water were usually twice the ‘ reactive + non-reactive’ Hg concentrations. The same relationship was found in unfiltered samples in 1985. Release from mercury-containing particles, due to the drastic change in the redox potential (reductive dissolution of Fe and Mn oxides coatings) and a large biological activity (degradation of organic material), are suggested as being responsible for the enrichment at the redox cline in the fall. The lower Hg values in the winter may be a result of the absence of direct atmospheric deposition (due to ice cover), low input of Hg from the catchment area, low biological activity and low sediment flux in the water column. In the oxic surface water, a positive correlation between the concentration of DGM, or labile volatile precursors to DGM, and the biological productivity is indicated. The concentration of DGM, probably elemental Hg, in the surface water of Framvaren indicates an efflux to the atmosphere.