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6. Hydrological Units and Selenium Criteria
Several habitat types often occur together in One aquatic system.
For example, rivers may have fast-flowing waters, slow-moving pools,
and standing backwater areas, all within a few hundred meters. The
degree of fish and wildlife exposure to selenium varies among habitats
according to intensity of use, type of use, and the relative contributions of the various processes that regulate selenium cycling.
Water quality criteria for selenium should be based upon an assessment of the degree of contamination and risk of toxic effects, which in
turn, will depend upon the spatial and temporal variation of the selenium cycle at the site under consideration. In addition to protecting resident biota at the "site", it is important that criteria protect against possible
impacts to down-gradient aquatic habitats as well. For example, a criterion that is appropriate for a stream or river where low bioaccumulation
occurs may result in seemingly harmless concentrations of selenium becoming a problem in downstream reservoirs or in off-channel bays and
wetlands where bioaccumulation is greater (Fig. 6.4).
Importance of Bioaccumulation
In developing water quality criteria for selenium, it is critical to incorporate the bioaccumulation phenomenon into the derivation process.
The major principle to remember is that reproductive effects in fish
and aquatic birds are the most sensitive biological indicators of aquatic
ecosystem-level impacts of selenium. Selenium in water can be concentrated from 100 to over 30,000 times in the food organisms eaten
by fish and wildlife, exposing them to a highly concentrated dietary
source of contamination. Biomagnification may also occur, resulting
in a 2- to 6-fold increase in selenium from primary producers to forage
fish. Moreover, if the ecosystem is allowed to reach an equilibrium
that recycles selenium from sediment, the detrital food pathway can
deliver toxic doses of selenium for many years even if waterborne
sources are eliminated (Lemly 1982, 1985a, 1997).
A significant portion of the selenium consumed by fish and wildlife
is passed to their offspring in eggs, where it can kill developing embryos outright or induce a variety of lethal or sublethal teratogenic
deformities (Lemly 1993). However, parents can COnSume a seleniumladen diet and experience partial or complete reproductive failure without exhibiting symptoms of selenium toxicosis themselves. Moreover,
aquatic food organisms of wildlife strongly bioaccumulate seleniumto a level that is hundreds to thousands of times the waterborne concentration-but are unaffected by tissue residues that are high enough
to cause reproductive failure when consumed by fish and aquatic birds.
Thus, bioaccumulation in aquatic food chains and dietary transfer to
eggs cause otherwise innocuous concentrations of waterborne sele-
6. Hydrological Units and Selenium Criteria
Several habitat types often occur together in One aquatic system.
For example, rivers may have fast-flowing waters, slow-moving pools,
and standing backwater areas, all within a few hundred meters. The
degree of fish and wildlife exposure to selenium varies among habitats
according to intensity of use, type of use, and the relative contributions of the various processes that regulate selenium cycling.
Water quality criteria for selenium should be based upon an assessment of the degree of contamination and risk of toxic effects, which in
turn, will depend upon the spatial and temporal variation of the selenium cycle at the site under consideration. In addition to protecting resident biota at the "site", it is important that criteria protect against possible
impacts to down-gradient aquatic habitats as well. For example, a criterion that is appropriate for a stream or river where low bioaccumulation
occurs may result in seemingly harmless concentrations of selenium becoming a problem in downstream reservoirs or in off-channel bays and
wetlands where bioaccumulation is greater (Fig. 6.4).
Importance of Bioaccumulation
In developing water quality criteria for selenium, it is critical to incorporate the bioaccumulation phenomenon into the derivation process.
The major principle to remember is that reproductive effects in fish
and aquatic birds are the most sensitive biological indicators of aquatic
ecosystem-level impacts of selenium. Selenium in water can be concentrated from 100 to over 30,000 times in the food organisms eaten
by fish and wildlife, exposing them to a highly concentrated dietary
source of contamination. Biomagnification may also occur, resulting
in a 2- to 6-fold increase in selenium from primary producers to forage
fish. Moreover, if the ecosystem is allowed to reach an equilibrium
that recycles selenium from sediment, the detrital food pathway can
deliver toxic doses of selenium for many years even if waterborne
sources are eliminated (Lemly 1982, 1985a, 1997).
A significant portion of the selenium consumed by fish and wildlife
is passed to their offspring in eggs, where it can kill developing embryos outright or induce a variety of lethal or sublethal teratogenic
deformities (Lemly 1993). However, parents can COnSume a seleniumladen diet and experience partial or complete reproductive failure without exhibiting symptoms of selenium toxicosis themselves. Moreover,
aquatic food organisms of wildlife strongly bioaccumulate seleniumto a level that is hundreds to thousands of times the waterborne concentration-but are unaffected by tissue residues that are high enough
to cause reproductive failure when consumed by fish and aquatic birds.
Thus, bioaccumulation in aquatic food chains and dietary transfer to
eggs cause otherwise innocuous concentrations of waterborne sele-