Environmental Controls on Changes in Mercury Accumulation in Bivalves

초록

Bivalves have long been used as a bioindicator to assess environmental mercury (Hg) pollution and its potential impacts on ecosystems and human health. However, the mechanisms underlying sediment-water Hg accumulation in bivalves and how they vary with environmental conditions are not yet fully understood. This study investigated the effect of environmental variation on sediment-water Hg accumulation in bivalves, using Hg stable isotope analysis (g202Hg, Delta 199Hg) and sediment sequential extraction methods (weak acid-soluble, organic-bound, and residual fractions). Asian clams, collected from an uncontaminated area of the Hyeongsan River in Pohang city (sediment THg: 0.025 f 0.004 mg/kg), were transplanted to the upstream region of the Nakdong River in Bonghwa city, Korea (sediment THg: 0.41 f 0.05 mg/kg) for a two-month period (clam 0-day g202Hg:-0.63 f 0.01%o, Delta 199Hg: 0.35 f 0.02%o). During the early phase of in situ experiment, when rainfall and suspended particle concentrations were high, the clam Hg isotopes (14-day g202Hg:-0.61 f 0.06%o, Delta 199Hg: 0.10 f 0.04%o) turned over toward the values of sediment (Delta 199Hg: 0.02 f 0.01%o). The THg concentration in the clams increased by a factor of 2.4 compared to day 0. Weak acid-soluble Hg (g202Hg:-0.62 f 0.01%o) or organic-bound Hg (g202Hg:-0.64 f 0.03%o) in sediment may have been preferentially accumulated in the clams. During the later phase of in situ experiment, when rainfall and suspended particle concentrations were lower, the clam Hg isotopes (58-day Delta 199Hg: 0.16 f 0.07%o) were more similar to those of the water column (Delta 199Hg: 0.13 f 0.02%o). At this point, the clam THg concentration decreased moderately and remained 1.1-fold higher than day 0. This study demonstrates that bivalves are sensitive to environmental changes, which modulate sources, cycling, and fate of Hg bioaccumulation, and suggests that Hg stable isotopes are an effective tool for monitoring these processes.

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