The Selenium Cycle
107
refers to the conversion of inorganic selenium in the reduced organic,
elemental, or selenite forms to the selenite or selenate forms; methylation is the conversion of inorganic or organic selenium to an organic
form containing one or more methyl groups, which usually results in
a volatile form): (1) oxidation and methylation of inorganic and organic selenium by plant roots and microorganisms; (2) biological mixing and associated oxidation of sediments that results from the burrowing
of benthic invertebrates and feeding activities of fish and wildlife; (3)
physical perturbation and chemical oxidation associated with water
circulation and mixing (current, wind, stratification, precipitation, and
upwelling); and (4) sediment oxidation by plant photosynthesis.
Two additional pathways provide for direct movement of selenium
from sediments into food chains, even when the surface water does
not contain elevated concentrations of the element. These pathways
are (1) uptake of selenium by rooted plants, and (2) uptake by bottomdwelling invertebrates and detrital-feeding fish and wildlife. These 2
pathways may be the most important factors in long-term cycling of
potentially toxic concentrations of selenium. Thus, rooted plants and
the detrital food pathway can continue to be highly contaminated and
expose fish and wildlife through dietary routes, even though concentrations of selenium in water are very low (Lemly and Smith 1987).
Role of Habitat Variability
The processes regulating selenium cycling are similar in all aquatic
habitats, but the relative contribution of each process may vary from
habitat to habitat. In fast-flowing waters, fine organic sediments, such
as those produced by the deposition and decay of particulate matter
and plant and animal tissue, may be rare because they are continually
flushed from the system. In these waters, there is little opportunity for
a contaminated surface layer of sediment to develop, and rooted plants
are often scarce. The benthic-detrital components of the system and
the associated food pathways thus playa smaller role in the selenium
cycle in flowing waters than in slow-water habitats such as wetlands
or reservoirs.
The aquatic systems that accumulate selenium most efficiently are
shallow, slow-moving waters that have low flushing rates. In these
systems, biological productivity is often high, and selenium may be
trapped through immobilization processes or through direct uptake by
organisms. Sediments tend to build up a selenium load that can be
remobilized gradually, yet continually, through detrital and planktonic
food pathways. These habitats are also some of the most important
feeding and breeding habitats for fish and wildlife, especially waterfowl and shorebirds, reservoir and lake fish, and riverine fish that utilize off-channel areas during early life.
107
refers to the conversion of inorganic selenium in the reduced organic,
elemental, or selenite forms to the selenite or selenate forms; methylation is the conversion of inorganic or organic selenium to an organic
form containing one or more methyl groups, which usually results in
a volatile form): (1) oxidation and methylation of inorganic and organic selenium by plant roots and microorganisms; (2) biological mixing and associated oxidation of sediments that results from the burrowing
of benthic invertebrates and feeding activities of fish and wildlife; (3)
physical perturbation and chemical oxidation associated with water
circulation and mixing (current, wind, stratification, precipitation, and
upwelling); and (4) sediment oxidation by plant photosynthesis.
Two additional pathways provide for direct movement of selenium
from sediments into food chains, even when the surface water does
not contain elevated concentrations of the element. These pathways
are (1) uptake of selenium by rooted plants, and (2) uptake by bottomdwelling invertebrates and detrital-feeding fish and wildlife. These 2
pathways may be the most important factors in long-term cycling of
potentially toxic concentrations of selenium. Thus, rooted plants and
the detrital food pathway can continue to be highly contaminated and
expose fish and wildlife through dietary routes, even though concentrations of selenium in water are very low (Lemly and Smith 1987).
Role of Habitat Variability
The processes regulating selenium cycling are similar in all aquatic
habitats, but the relative contribution of each process may vary from
habitat to habitat. In fast-flowing waters, fine organic sediments, such
as those produced by the deposition and decay of particulate matter
and plant and animal tissue, may be rare because they are continually
flushed from the system. In these waters, there is little opportunity for
a contaminated surface layer of sediment to develop, and rooted plants
are often scarce. The benthic-detrital components of the system and
the associated food pathways thus playa smaller role in the selenium
cycle in flowing waters than in slow-water habitats such as wetlands
or reservoirs.
The aquatic systems that accumulate selenium most efficiently are
shallow, slow-moving waters that have low flushing rates. In these
systems, biological productivity is often high, and selenium may be
trapped through immobilization processes or through direct uptake by
organisms. Sediments tend to build up a selenium load that can be
remobilized gradually, yet continually, through detrital and planktonic
food pathways. These habitats are also some of the most important
feeding and breeding habitats for fish and wildlife, especially waterfowl and shorebirds, reservoir and lake fish, and riverine fish that utilize off-channel areas during early life.