Page 58 - The Toxicology of Fishes

38 The Toxicology of Fishes

Studies of MeHg accumulation in natural systems are often complicated by the fact that a given factor
may impact both bioavailability and net methylation rate. In poorly buffered freshwater systems that are
susceptible to acid deposition, MeHg concentrations at all trophic levels tend to be inversely correlated
with pH (Watras et al., 1998; Winfrey and Rudd, 1990). Decreased pH, particularly when associated
with the deposition of sulfuric acid, has been associated with increased microbial production of MeHg
(Gilmour et al., 1991); however, biogeochemical processes that release mercury from soils and sediments
2+
at low pH values may also play a role by contributing to the pool of Hg available for bacterial
methylation. In well-buffered systems and those that drain relatively large watersheds, pH and MeHg
accumulation by fish may be poorly correlated (Richardson et al., 1995).
The DOC content of water is an important determinant of MeHg accumulation and, in many systems,
a better predictor of fish mercury residues than pH. DOC levels usually correlate positively with MeHg
residues in fish, although in a study of Adirondack lakes very high levels of DOC were associated with
a decline in MeHg (Driscoll et al., 1995). DOC may contribute indirectly to increased levels of MeHg
2+
in fish by promoting the translocation of Hg and MeHg from watersheds to water bodies (Hurley et
al., 1995). The addition of DOC to a system tends to increase sediment microbial activity, hence the
2+
potential for Hg methylation. An increase in the DOC content of water may also promote the release
of MeHg from sediment to the water column, making it more available for uptake by fish and other
aquatic biota (Miskimmen, 1991).
2+
2+
At low Hg /DOC ratios, DOC binding of Hg is dominated by covalent interactions with thiol groups
(Drexel et al., 2002; Haitzer et al., 2002). These complexes are very stable, and it has been suggested
2+
2+
that high levels of DOC reduce Hg methylation in the water column by reducing uptake of Hg by
microbes (Gilmour and Henry, 1991). In laboratory studies, DOC addition to water reduced the uptake
2+
of Hg and MeHg by larvae of a benthic dipteran (Chaoborus) (Sjöblom et al., 2000). DOC addition
was also shown to reduce MeHg uptake by fish directly from water (Choi et al., 1998).
The affinity of DOC for MeHg and other metals varies with pH. In general, a decrease in pH results
in lower MeHg binding to DOC because of competition with hydrogen ion for anionic binding sites
(Hintelmann et al., 1995). The binding of MeHg to DOC may also be affected by the presence of other
cationic metals. For example, Driscoll et al. (1995) found that MeHg levels in fish were positively
correlated with concentrations of monomeric aluminum and suggested that this was because aluminum
competes with MeHg for binding sites on DOC, thereby increasing the concentration of unbound MeHg.
In many aquatic systems, benthic organisms contribute substantially to the transfer of energy and
contaminants from sediments to the food web. Uptake of Hg and MeHg from ingestion of contaminated
2+
sediment was investigated by feeding mussels a suspension of sediment particles (Gagnon and Fisher,
2+
1997). Assimilation efficiencies for Hg were uniformly low (<10%). Those for MeHg were generally
higher (all but one value > 30%) but varied with sediment type. Pretreatment of artificial sediment with
2+
organic material (fulvic acid) increased MeHg assimilation in all cases. In contrast, uptake of Hg and
MeHg from sediment by an estuarine amphipod (Letocheirus plumulosus) was negatively correlated with
sediment organic matter content, and it was suggested that binding to organic matter prevented MeHg
from becoming solubilized within the intestinal tract (Lawrence and Mason, 2001).
Methylmercury levels as a percentage of total mercury are generally very low in sediment but tend
to increase in pore water and overlying waters, suggesting that sediment-to-water binding constants for
2+
MeHg are lower than for Hg . The natural cycling of biological material may also cause MeHg to be
retained by organisms living in the water column (Watras et al., 1998). Benthic and pelagic environments
may become coupled, however, when sediment-dwelling organisms consume algae or detritus contam-
inated with MeHg (Lawrence and Mason, 2001) or fish consume benthic species.
Laboratory studies and modeling efforts both indicate that the diet is the primary route of uptake of
MeHg for large piscivorous fish (Wiener and Spry, 1996). The predominant route of uptake by smaller
fish is less well known, and it has been suggested that uptake from water may contribute substantially
to accumulated residues depending on seasonal changes in prey selection and environmental factors
(Post et al., 1996). The relative importance of food and water exposure routes in accumulation of MeHg
by herbivorous fish and fish that ingest large quantities of sediment is unknown. Depuration studies
suggest that the elimination half-life for MeHg in fish may range from weeks to years (Trudel and
Rasmussen, 1997). Indeed, elimination proceeds so slowly that MeHg may accumulate throughout a