CHAPTER 13 • Mercury in Marine Environments
253
and Fitzgerald 1985; Bloom and Fitzgerald 1988). Mason and Fitzgerald (1990, 1991, 1993,
1994) reported the presence of both MMHg and DMHg in the low oxygen waters of
the equatorial Pacific Ocean and thus demonstrated that a pathway exists for the accumulation of methylated mercury in marine pelagic fish. Results from sampling and
analysis during a cruise in January/February 1990 between the Panama Canal and
American Samoa lead to a number of conclusions:
• The substrate for methylation is labile inorganic mercury, which is composed of labile inorganic and organic complexes of Hg(II) and oflabile particulate associations.
• Labile inorganic mercury is supplied primarily by atmospheric deposition; it then
reaches the subthermocline waters by vertical mixing and dissolution of particles.
• DMHg and MMHg are produced in the O2 minimum regions and the concentrations
of methylated species decrease as the minimum oxygen concentration rises.
• Biological methylation in anoxic environments is the dominant methylating process.
• DMHg is the dominant methylated mercury compound, whereas MMHg dominates
in fresh waters.
• Demethylation is the principal source of HgO in low oxygen waters and direct reduction of Hg(II) is the main source of HgO in the mixed layer.
• The main processes consuming Hg(II) in natural waters are methylation, reduction
and particulate scavenging.
From these conclusions Mason and Fitzgerald (1990, 1991, 1993, 1994) proposed a
model showing the principal cycles and processes that affect the distributions of various mercury species in the equatorial Pacific Ocean (Fig. 13.4)
Mason et al. (1995, 1998) investigated species of mercury in the deep ocean waters
of the North Atlantic Ocean. Their investigations confirmed the processes that had been
found in the equatorial Pacific Ocean. The results also suggested the presence of organic complexes of mercury in the surface water. Some concentrations of the mercury
species are shown in Table 13.3. The speciation of mercury in the open ocean is dominated by HgO and interconversion between the species accounts for the complex profiles. It was suggested (Mason et al. 1998) that HgO formation is constrained more by
rate of supply than by rate of conversion. There is a general increase in the concentrations of reactive mercury, and of DMHg in the colder, deeper waters of the North Atlantic, suggesting that the latter compound is relatively stable under these conditions.
Winter mixing releases DMHg to the atmosphere. There is also evidence of MMHg
throughout the water column.
13.5
Toxicity of Mercury to Marine Life
Mance (1987) has collected data on toxicity studies on marine biota. Exposure tests
did not reveal any differences between mercury chemical species: all compounds tested
(organic or inorganic Hg(II)) had adverse effects at "low" concentrations. For the
embryos of the fish Fundulus heteroclitus, the 4-day LC so was 0.067 mg rl. However,
exposure for one day followed by 3 days in clean water gave and LC so of 0.09 mg rl,
indicating that the effects were rapid and irreversible (Sharp and Neff 1980). Crusta-
253
and Fitzgerald 1985; Bloom and Fitzgerald 1988). Mason and Fitzgerald (1990, 1991, 1993,
1994) reported the presence of both MMHg and DMHg in the low oxygen waters of
the equatorial Pacific Ocean and thus demonstrated that a pathway exists for the accumulation of methylated mercury in marine pelagic fish. Results from sampling and
analysis during a cruise in January/February 1990 between the Panama Canal and
American Samoa lead to a number of conclusions:
• The substrate for methylation is labile inorganic mercury, which is composed of labile inorganic and organic complexes of Hg(II) and oflabile particulate associations.
• Labile inorganic mercury is supplied primarily by atmospheric deposition; it then
reaches the subthermocline waters by vertical mixing and dissolution of particles.
• DMHg and MMHg are produced in the O2 minimum regions and the concentrations
of methylated species decrease as the minimum oxygen concentration rises.
• Biological methylation in anoxic environments is the dominant methylating process.
• DMHg is the dominant methylated mercury compound, whereas MMHg dominates
in fresh waters.
• Demethylation is the principal source of HgO in low oxygen waters and direct reduction of Hg(II) is the main source of HgO in the mixed layer.
• The main processes consuming Hg(II) in natural waters are methylation, reduction
and particulate scavenging.
From these conclusions Mason and Fitzgerald (1990, 1991, 1993, 1994) proposed a
model showing the principal cycles and processes that affect the distributions of various mercury species in the equatorial Pacific Ocean (Fig. 13.4)
Mason et al. (1995, 1998) investigated species of mercury in the deep ocean waters
of the North Atlantic Ocean. Their investigations confirmed the processes that had been
found in the equatorial Pacific Ocean. The results also suggested the presence of organic complexes of mercury in the surface water. Some concentrations of the mercury
species are shown in Table 13.3. The speciation of mercury in the open ocean is dominated by HgO and interconversion between the species accounts for the complex profiles. It was suggested (Mason et al. 1998) that HgO formation is constrained more by
rate of supply than by rate of conversion. There is a general increase in the concentrations of reactive mercury, and of DMHg in the colder, deeper waters of the North Atlantic, suggesting that the latter compound is relatively stable under these conditions.
Winter mixing releases DMHg to the atmosphere. There is also evidence of MMHg
throughout the water column.
13.5
Toxicity of Mercury to Marine Life
Mance (1987) has collected data on toxicity studies on marine biota. Exposure tests
did not reveal any differences between mercury chemical species: all compounds tested
(organic or inorganic Hg(II)) had adverse effects at "low" concentrations. For the
embryos of the fish Fundulus heteroclitus, the 4-day LC so was 0.067 mg rl. However,
exposure for one day followed by 3 days in clean water gave and LC so of 0.09 mg rl,
indicating that the effects were rapid and irreversible (Sharp and Neff 1980). Crusta-