Linking
Selenium Sources to Ecosystems
Food
web from particulates through prey to predators
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| Linking
Selenium Sources to Ecosystems |
Food
web from particulates through prey to predators |
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GUIDELINESSelenium BiochemistrySelenium
is an essential micronutrient in bacteria and animals. Beneficial effects
in humans stem mainly from Se’s role as an antioxidant. However,
Se is the most toxic of all biologically essential elements in mammals.
Toxic effects occur via biochemical pathways unable to distinguish Se
from sulfur, thus substituting Se-containing amino acids in structural
and functional proteins during critical stages of development and growth.
Hence, dramatic effects such as congenital anomalies (monstrosities) occur
in embryos of aquatic birds.
Nutritional guidelines and national guidelines for risk have been developed. The guidelines for food webs show the narrow difference between concentrations considered safe and those considered harmful. Nutritional guidelines may not be directly comparable across classes or within species because selenium toxicity is dependent on the sensitivity of the animal, the chemical form of the Se, and the dose and duration of exposure. For example, the threshold for chronic toxicity in humans (dry weight, whole diet) is based on a longer exposure time than that for horses and sheep. Despite these complexities, five western states at 11 sites have human health advisories for fish consumption because of Se. Children (< age 15) and pregnant women are advised not to consume any fish or game from a posted area, whereas adult males and non-pregnant women should consume no more than one meal each two-week period of approximately 120 grams of fish or game flesh (i.e., muscle). Ecological risk threshold ranges are indicative of the endpoints used to measure adverse biological effects. Thresholds for marginal and substantive risk are well established for water, sediment, diet, fish tissue, and bird eggs. Because the difference between essential and toxic levels for Se is narrow, concern or marginal risk levels, which are between levels considered safe (no effect) and those considered harmful (substantive risk or the toxicity threshold), are intended to provide protection for the environment. The equality of the criterion for the protection of aquatic life and the ecological threshold at which substantive risk occurs (i.e., 5 µg/L) demonstrates a need to establish a set of criteria that fully encompasses both aquatic and semi-aquatic food web components.
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URL
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