CHAPTER 13 • Mercury in Marine Environments
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in the presence of dead fish tissue (Jensen and Jernelov 1969). The methyl-mercury
ion CH3Hg + forms a very stable complex CH3HgCl. Ahrland (1985) suggested that because of the very high concentrations of cr in sea water, CH3HgCI should be more
abundant than CH3HgCH3 (DMHg).
The pH has a major effect: in alkaline anoxic sediments, DMHg is produced, whereas
at lower pH values, MMHg is produced (Fagerstrom and Jenerlov 1972). At higher pH
ranges (>7) the dominant species of mercury in sediments are those bound to sulfides or oxides of iron and manganese, whereas in more acidic conditions mercury is
associated with humic acids or sulfides. Each form of mercury has a different potential for methylation. Hg(II) is bound to sulfur in sulfate-rich anoxic marine sediments,
and becomes less available for methylation (Capone and Kiene 1988), whereas Hghumic complexes are more readily methylated than is HgS (Kannan and Falandysz
1998). Thus, in fresh-water sediments, high amounts of humic material favour methylation, and concentrations of MMHg are generally higher in fresh water than in
marine sediments. There is an inverse correlation between methylation and sulfate
levels which is strange, since the principal methylators are sulfate reducing bacteria
(Compeau and Bartha 1985). This apparent anomaly was explained by Choi and Bartha
(1994), who pointed out that the H2S, formed in reducing sulfate rich environments,
precipitates the Hg(II), thus rendering it unavailable for methylation.
Topping and Davies (1981) demonstrated the production of methyl mercury in
coastal marine surface waters, thus sediment is not the exclusive natural source of
methyl mercury entering the food chain.
13.3
Mercury in Coastal Marine Sediments
Kannan and Falandysz (1998) measured total mercury, (HgT), methyl mercury and
Hg(II) in coastal marine sediments collected from the Baltic, South China and Bering
Seas (Table 13.1). They reported that in most sediments only 10% of the total mercury
(HgT) was accounted for by the sum of Hg(II) and methyl mercury. They concluded that
most of the mercury was strongly bound to sulfide and thus unavailable for methylation.
Since there are generally more humic materials in fresh-water sediments, and methylation has been positively correlated with humic content and negatively correlated with
salinity (Choi and Bartha 1994; Compeau and Bartha 1983), a larger percentage of HgT
was found to be in the methylated form in the fresh-water sediments (Table 13.1).
The percentage of the total mercury present in some marine sediments, in the methylated form, is shown in Table 13.2. In unpolluted locations the percentage is usually
<1, and some authors have suggested a maximum value of 1%. The highest values were
found in the Elbe Estuary and in the Mulde, a tributary of the Elbe (Wilken and
Hintlemann 1991). The Elbe Estuary includes Hamburg Harbour, and is one of the most
polluted locations in the world. The mercury pollution, resulting from chlor-alkaline
plants, has led not only to high percentages of methyl mercury, but also to high total
Hg in sediments and particulate matter (Wilken and Hintlemann 1991). These authors
suggest that the very high percentage of methylated mercury might be accounted for
by the presence, in the polluted Elbe environment, of specially adapted bacteria, which
can methylate Hg2+ very efficiently. The results of Kannan and Falandysz (1998) indicate that the percentage of methyl mercury does not depend on climatic zone (Table 13-1).
249
in the presence of dead fish tissue (Jensen and Jernelov 1969). The methyl-mercury
ion CH3Hg + forms a very stable complex CH3HgCl. Ahrland (1985) suggested that because of the very high concentrations of cr in sea water, CH3HgCI should be more
abundant than CH3HgCH3 (DMHg).
The pH has a major effect: in alkaline anoxic sediments, DMHg is produced, whereas
at lower pH values, MMHg is produced (Fagerstrom and Jenerlov 1972). At higher pH
ranges (>7) the dominant species of mercury in sediments are those bound to sulfides or oxides of iron and manganese, whereas in more acidic conditions mercury is
associated with humic acids or sulfides. Each form of mercury has a different potential for methylation. Hg(II) is bound to sulfur in sulfate-rich anoxic marine sediments,
and becomes less available for methylation (Capone and Kiene 1988), whereas Hghumic complexes are more readily methylated than is HgS (Kannan and Falandysz
1998). Thus, in fresh-water sediments, high amounts of humic material favour methylation, and concentrations of MMHg are generally higher in fresh water than in
marine sediments. There is an inverse correlation between methylation and sulfate
levels which is strange, since the principal methylators are sulfate reducing bacteria
(Compeau and Bartha 1985). This apparent anomaly was explained by Choi and Bartha
(1994), who pointed out that the H2S, formed in reducing sulfate rich environments,
precipitates the Hg(II), thus rendering it unavailable for methylation.
Topping and Davies (1981) demonstrated the production of methyl mercury in
coastal marine surface waters, thus sediment is not the exclusive natural source of
methyl mercury entering the food chain.
13.3
Mercury in Coastal Marine Sediments
Kannan and Falandysz (1998) measured total mercury, (HgT), methyl mercury and
Hg(II) in coastal marine sediments collected from the Baltic, South China and Bering
Seas (Table 13.1). They reported that in most sediments only 10% of the total mercury
(HgT) was accounted for by the sum of Hg(II) and methyl mercury. They concluded that
most of the mercury was strongly bound to sulfide and thus unavailable for methylation.
Since there are generally more humic materials in fresh-water sediments, and methylation has been positively correlated with humic content and negatively correlated with
salinity (Choi and Bartha 1994; Compeau and Bartha 1983), a larger percentage of HgT
was found to be in the methylated form in the fresh-water sediments (Table 13.1).
The percentage of the total mercury present in some marine sediments, in the methylated form, is shown in Table 13.2. In unpolluted locations the percentage is usually
<1, and some authors have suggested a maximum value of 1%. The highest values were
found in the Elbe Estuary and in the Mulde, a tributary of the Elbe (Wilken and
Hintlemann 1991). The Elbe Estuary includes Hamburg Harbour, and is one of the most
polluted locations in the world. The mercury pollution, resulting from chlor-alkaline
plants, has led not only to high percentages of methyl mercury, but also to high total
Hg in sediments and particulate matter (Wilken and Hintlemann 1991). These authors
suggest that the very high percentage of methylated mercury might be accounted for
by the presence, in the polluted Elbe environment, of specially adapted bacteria, which
can methylate Hg2+ very efficiently. The results of Kannan and Falandysz (1998) indicate that the percentage of methyl mercury does not depend on climatic zone (Table 13-1).