Bioaccumulation refers to the uptake of a contaminant from both water and food sources. Biomagnification, on the other hand, means progressively higher concentrations when moving through successively higher trophic levels. Table 5.01 show bioaccumulation and bioconcentration factors obtained from the literture.
In aquatic environments, organisms accumulate selenium from both water and food. The bioaccumulation of selenium through the diet, however, is usually greater than the direct uptake from water, particularly when the Se occurs in natural dietary ingredients as compared to inorganic selenite or selenate (NAS 1980). Therefore, the toxic effects of food Se may be more significant than those of waterborne Se (Lemly 1985, Ohlendorf 1989).
Ohlendorf (1989) noted that the evidence for biomagnification of selenium through the food chain is contradictory. Lemly and Smith (1987) reported that selenium concentration in lower invertebrates or fish (whole-body basis) was two to six times the concentration in producers (i.e., phytoplankton, algae, and rooted plants) and sometimes up to 2 000 times the concentration in water.
In studying boron (B), molybdenum (Mo) and selenium (Se) in the aquatic food chain of the Lower San Joaquin River, California, Saiki et al. (1993) observed that the Se concentration was lower in filamentous algae than in invertebrates and fish. However, Se levels in or on detritus were similar to or higher than in invertebrates and fish. They concluded that Se in invertebrates and fish accumulated through food-chain transfer from Se-enriched detritus rather than from filamentous algae. Baudo (1983) stated that B, Mo, and Se did not exhibit evidence of biomagnification.
Sanders and Gilmour (1994) examined the transfer of selenium between bacteria and the ciliated protozoan, Paramecium putrinum, in laboratory cultures. The selenium concentration of ciliates was similar to that of their bacterial food on a dry-weight basis. These investigators concluded that selenium uptake by the ciliate occurred primarily during feeding and that biomagnification of selenium did not occur in this simple food chain.
Dobbs et al. (1996) exposed Chlorella vulgaris, Brachionus calyciflorus, and Pimephales promelas to 0, 0.110, 0.207, and 0.396 mg Se/L for 25 days in a flow-through system. The amount of Se in whole organisms was measured throughout the experiment. In this three-trophic level food chain, these authors found that bioconcentration factors were dependent on species, Se level, and length of exposure period, and they ranged between 100 and 1 000.
McDonald and Strosher (1998) monitored selenium mobilization from surface coal mining in the Elk River Basin, British Columbia. They found 100 to 200-fold increases in waterborne Se below the coal mines. However, levels in sediments, algae, and aquatic insects and fish tissue were only 2 to 5 times greater than the reference sites. The authors concluded that there was a limited amount of selenium bioaccumulation occurring in the fast-flowing Elk River. This conclusion was supported by a review of the literature conducted by Adams et al. (2000). The investigators found significant differences in the bioaccumulation of Se by fishes and invertebrates from lotic (flowing) and lentic (standing) water bodies. Bioaccumulation in fish is a factor of 10 or more higher in lentic systems as compared to lotic systems; these differences are function of selenium's site-specific biogeochemical cycling.