Natural and Anthropogenic Mercury Sources
29
Lake, Massachusetts (Mason et al. 1995a), which found that both the concentration of Hg( 0) and its production rate are highest in the surface waters and
then decrease sharply down the water column. These authors were able to
identify heterotrophic bacteria as the primary source of Hg(II)-reduction, while
minor contributions were made by abiotic processes and possibly by phytoplankton and cyanobacteria. On the other hand, there is strong evidence that Hg
volatilization from lakes is induced by sunlight (Amyot et al. 1994), but this could
be due either to photochemical reactions or light-induced biochemical processes,
so it does not necessarily contradict findings that favor biotic reduction of
reactive Hg(II).
Another way of obtaining information on the processes that form Hg( 0) in
the water column is by looking at its depth concentration profiles and seasonal
variations in the water columns of lakes. During studies in Onondaga Lake, an
industrially polluted lake in near Syracuse, New York, USA, maximum
concentrations were observed in April and August near the surface, while
the lowest concentrations were found in the hypolimnion during periods of
anoxia (Jacobs et al. 1995). This matches with observations in Pallette Lake,
Wisconsin, where it was also found that Hg(o) concentrations are highest in
the epilimnion and decrease with depth through the thermocline and the
hypolimnion (Vandal et al. 1995); this observation was taken as an indication
that Hg reduction occurs only in the mixed layer (Porcella 1994). Finally, in
wind-sheltered parts of the lake, the Hg( 0) concentrations in the epilimnion
were elevated compared to wind-exposed regions. This finding strongly
suggests that wind is an important driving force behind gas exchange at the
lake surface; thus, Hg fluxes to the atmosphere are temporarily increased by
windy conditions and waves.
Reactive Hg(II), the most likely precursor for Hg( 0), showed variable seasonal
concentration depth profiles. Its concentration was highest in the epilimnion in
the early summer with low concentrations in the thermocline, but exactly the
opposite in late summer. Since this parameter seems to be influenced by
numerous and more complex processes, it is not surprising that two relevant
sources of reactive Hg(II) were identified in these studies, one being atmospheric
deposition and the other being remobilization from the sediments. Rain events
increase reactive Hg(II) in the epilimnion, either by direct wet deposition of
reactive Hg(II) or by deposition of particulate Hg that becomes transformed to
reactive Hg(II) in the epilimnion. Since it has been shown that both reactive
Hg(II) and particulate Hg are major fractions in wet deposition (Fitzgerald et al.
1994), a mixture of both processes seems to be the most likely source of reactive
Hg(II) in the water column. Although Hg(o) concentrations in sediment
porewaters are comparable to those in the epilimnion, they are lower than in
the overlying lake water and it was concluded that Hg( 0) export from the
sediments is not an important source of Hg( 0) in the water column. On the other
hand, Hg(II) concentrations are higher in porewaters than in lake water and it
also exceeds Hg(o) concentrations in porewater by at least 1 order of magnitude.
Thus, it was concluded that export of Hg(II) from the sediments is a significant
source of Hg(II) in the water column and, consequently, of Hg(o) evasion to the
atmosphere (Fitzgerald et al. 1994).
29
Lake, Massachusetts (Mason et al. 1995a), which found that both the concentration of Hg( 0) and its production rate are highest in the surface waters and
then decrease sharply down the water column. These authors were able to
identify heterotrophic bacteria as the primary source of Hg(II)-reduction, while
minor contributions were made by abiotic processes and possibly by phytoplankton and cyanobacteria. On the other hand, there is strong evidence that Hg
volatilization from lakes is induced by sunlight (Amyot et al. 1994), but this could
be due either to photochemical reactions or light-induced biochemical processes,
so it does not necessarily contradict findings that favor biotic reduction of
reactive Hg(II).
Another way of obtaining information on the processes that form Hg( 0) in
the water column is by looking at its depth concentration profiles and seasonal
variations in the water columns of lakes. During studies in Onondaga Lake, an
industrially polluted lake in near Syracuse, New York, USA, maximum
concentrations were observed in April and August near the surface, while
the lowest concentrations were found in the hypolimnion during periods of
anoxia (Jacobs et al. 1995). This matches with observations in Pallette Lake,
Wisconsin, where it was also found that Hg(o) concentrations are highest in
the epilimnion and decrease with depth through the thermocline and the
hypolimnion (Vandal et al. 1995); this observation was taken as an indication
that Hg reduction occurs only in the mixed layer (Porcella 1994). Finally, in
wind-sheltered parts of the lake, the Hg( 0) concentrations in the epilimnion
were elevated compared to wind-exposed regions. This finding strongly
suggests that wind is an important driving force behind gas exchange at the
lake surface; thus, Hg fluxes to the atmosphere are temporarily increased by
windy conditions and waves.
Reactive Hg(II), the most likely precursor for Hg( 0), showed variable seasonal
concentration depth profiles. Its concentration was highest in the epilimnion in
the early summer with low concentrations in the thermocline, but exactly the
opposite in late summer. Since this parameter seems to be influenced by
numerous and more complex processes, it is not surprising that two relevant
sources of reactive Hg(II) were identified in these studies, one being atmospheric
deposition and the other being remobilization from the sediments. Rain events
increase reactive Hg(II) in the epilimnion, either by direct wet deposition of
reactive Hg(II) or by deposition of particulate Hg that becomes transformed to
reactive Hg(II) in the epilimnion. Since it has been shown that both reactive
Hg(II) and particulate Hg are major fractions in wet deposition (Fitzgerald et al.
1994), a mixture of both processes seems to be the most likely source of reactive
Hg(II) in the water column. Although Hg(o) concentrations in sediment
porewaters are comparable to those in the epilimnion, they are lower than in
the overlying lake water and it was concluded that Hg( 0) export from the
sediments is not an important source of Hg( 0) in the water column. On the other
hand, Hg(II) concentrations are higher in porewaters than in lake water and it
also exceeds Hg(o) concentrations in porewater by at least 1 order of magnitude.
Thus, it was concluded that export of Hg(II) from the sediments is a significant
source of Hg(II) in the water column and, consequently, of Hg(o) evasion to the
atmosphere (Fitzgerald et al. 1994).