Toxic Thresholds
19
nium in water, sediments, food-chain organisms, and fish and aquatic
bird tissues.
Toxic Thresholds
All values for tissue selenium residues in this chapter are given on a dryweight basis. Data from references that only reported wet weights were
converted to dry weight assuming 75% moisture, that is, by multiplying
the wet-weight concentration by 4. It is important to understand that the
values given as thresholds are levels at which toxic effects begin to occur
in sensitive species of fish and aquatic birds; for example, centrarchid and
salmonid fishes (eg, genus Lepomis, Micropterus, Oncorhynchus, Salvelinus),
ducks (genus Anas), and wading birds (genus Recurvirostra). They are not
levels that signify the point at which everything dies from selenium poisoning, that is, the point at which total reproductive failure or massive
mortality of juveniles and adults occurs. At the threshold level, seleniumtolerant species will be unaffected.
Water
Selenium is strongly bioaccumulated in aquatic habitats; this results
in a marked elevation of residues in food-chain organisms as compared to waterborne concentrations (Lemly 1985b; Maier et al. 1988;
Ogle et al. 1988; Ohlendorf 1989). It is critical to know how much
bioaccumulation can be expected for a given aqueous level of selenium in order to evaluate the potential for dietary toxicity and reproductive effects in predatory species of fish and wildlife. Laboratory studies
show that organoselenium compounds (seleno-L-methionine) can be
bioconcentrated over 200 000 times by zooplankton when water concentrations are in the 0.5 to 0.8 f.1g SelL (parts per billion) range (Besser
et al. 1989, 1993). Resultant selenium residues were over 100 f.1g Selg,
a concentration that far exceeds the dietary toxicity threshold for fish
(3 f.1g Se/g). Organoselenium compounds can comprise a substantial
portion of the total waterborne selenium concentration in aquatic environments (Chau et al. 1976; Cutter 1982, 1986, 1991; Cooke and
Bruland 1987), although the complete range of chemical species is
poorly described. The potential for bioaccumulation and toxicity due
to organic selenium is very high.
Ino,rganic selenium (selenate, selenite) bioaccumulates more readily
in phytoplankton than zooplankton, and residues of up to 18 f.1g Selg
can occur when waterborne concentrations are in the 7 to 10 f.1g SelL
range (Besser et al. 1993), resulting in bioaccumulation factors of about
3000. It is at the primary producer and primary consumer levels of the
food chain (phytoplankton and zooplankton) that most of the
19
nium in water, sediments, food-chain organisms, and fish and aquatic
bird tissues.
Toxic Thresholds
All values for tissue selenium residues in this chapter are given on a dryweight basis. Data from references that only reported wet weights were
converted to dry weight assuming 75% moisture, that is, by multiplying
the wet-weight concentration by 4. It is important to understand that the
values given as thresholds are levels at which toxic effects begin to occur
in sensitive species of fish and aquatic birds; for example, centrarchid and
salmonid fishes (eg, genus Lepomis, Micropterus, Oncorhynchus, Salvelinus),
ducks (genus Anas), and wading birds (genus Recurvirostra). They are not
levels that signify the point at which everything dies from selenium poisoning, that is, the point at which total reproductive failure or massive
mortality of juveniles and adults occurs. At the threshold level, seleniumtolerant species will be unaffected.
Water
Selenium is strongly bioaccumulated in aquatic habitats; this results
in a marked elevation of residues in food-chain organisms as compared to waterborne concentrations (Lemly 1985b; Maier et al. 1988;
Ogle et al. 1988; Ohlendorf 1989). It is critical to know how much
bioaccumulation can be expected for a given aqueous level of selenium in order to evaluate the potential for dietary toxicity and reproductive effects in predatory species of fish and wildlife. Laboratory studies
show that organoselenium compounds (seleno-L-methionine) can be
bioconcentrated over 200 000 times by zooplankton when water concentrations are in the 0.5 to 0.8 f.1g SelL (parts per billion) range (Besser
et al. 1989, 1993). Resultant selenium residues were over 100 f.1g Selg,
a concentration that far exceeds the dietary toxicity threshold for fish
(3 f.1g Se/g). Organoselenium compounds can comprise a substantial
portion of the total waterborne selenium concentration in aquatic environments (Chau et al. 1976; Cutter 1982, 1986, 1991; Cooke and
Bruland 1987), although the complete range of chemical species is
poorly described. The potential for bioaccumulation and toxicity due
to organic selenium is very high.
Ino,rganic selenium (selenate, selenite) bioaccumulates more readily
in phytoplankton than zooplankton, and residues of up to 18 f.1g Selg
can occur when waterborne concentrations are in the 7 to 10 f.1g SelL
range (Besser et al. 1993), resulting in bioaccumulation factors of about
3000. It is at the primary producer and primary consumer levels of the
food chain (phytoplankton and zooplankton) that most of the