CHAPTER 17 

Ecotoxicology of Selenium 

Harry M. Ohlendorf 

CONTENTS 

17.1 Introduction .......................................................................................................................... .466 

17.2 Environmental Chemistry .................................................................................................... .467 

17 .2.1 Chemical Characteristics ......................................................................................... .467 

17.2.2 Environmental Sources and Occurrence ................................................................. .468 

I 7 .2.3 Cycling ...................................................................................................................... 469 

17.3 Bioaccumulation .................................................................................................................. .470 

17.3.1 Plants ........................................................................................................................ .470 

17.3.1.1 Aquatic ..................................................................................................... .470 

17.3. I .2 Terrestrial .................................................................................................. .472 

17.3.2 lnvertebrates ............................................................................................................. .473 

17.3.2.1 Aquatic/Marine ......................................................................................... .473 

I 7.3.2.2 Terrestria1 .................................................................................................. .474 

17.3.3 Fish .......................................................................................................................... .474 

17.3.3.1 Freshwater ................................................................................................ .474 

17.3.3.2 Marine ...................................................................................................... .475 

17.3.4 Amphibians and Reptiles ........................................................................................ .475 

17.3.4.1 Amphibians ............................................................................................. ..476 

17.3.4.2 Reptiles ..................................................................................................... .476 

I 7.3.5 Birds ......................................................................................................................... .4 77 

17 .3.5.1 Eggs .......................................................................................................... .4 77 

17.3.5.2 Livers ......................................................................................................... 478 

I 7.3.5.3 Kidneys ..................................................................................................... .478 

I 7.3 .5 .4 Muscle ...................................................................................................... .478 

17.3.5.5 Blood ....................................................................................................... ..4 79 

17.3.5.6 Feathers ..................................................................................................... 479 

17.3.6 Mammals ................................................................................................................. .480 

17.3.6. I Aquatic/Marine ......................................................................................... .480 

I 7.3. 6.2 Terrestri a! .................................................................................................. .480 

17.4 Effects in Animals ............................................................................................................... .481 

17.4.1 Nutritional Requirements vs. Toxicity .................................................................... .481 

17.4.2 Toxic Effects ............................................................................................................ .481 

465 
466 HANDBOOK OF ECOTOXICOLOGY 

17 .4.2.1 Invertebrates ............................................................................................. .481 

17 .4.2.2 Fish ........................................................................................................... .482 

17.4.2.3 Amphibians and Reptiles ......................................................................... .483 

17.4.2.4 Birds ......................................................................................................... .484 

17 .4.2.5 Mammals .................................................................................................. .488 

17.4.3 Interactions .............................................................................................................. .489 

17.5 Summary and Conclusions .................................................................................................. .490 

Acknowledgments ......................................................................................................................... .491 

References .................................................................................................................................... ..491 

17.1 INTRODUCTION 

Selenium is a naturally occurring scmimctallic (also referred to as a metalloid) trace clement 

that is essential for animal nutrition in small quantities but becomes toxic at dietary conccntmtions 

that arc not much higher than the required levels for good health. Thus, dietary selenium concentrations that arc either below or above the optimal range arc of concern. This chapter summarizes 

the ccotoxicology of excessive selenium exposure for animals, especially as reported during the 

last 15 years. It focuses primarily on freshwater fish and aquatic birds because fish and birds are 

the groups of animals for which most toxic effects have been reported in the wild. However, 

information related to bioaccumulation by plants and animals as well as effects in invertebrates, 

amphibians, reptiles, and mammals also arc presented. 

Selenium was recognized long ago as a cause of toxicity in domestic poultry and livestock 

(e.g., References 1-3), although a recent reevaluation of some of the historic samples and related 

information by O'Toole aftd Raisbeck1 indicates that the condition known as "blind staggers" in 

livestock is probably not caused by selenium. For fish and wildlife, selenium became much more 

of a concern in the 1970s and 1980s with the discovery of selenium bioaccumulation and severe 

impacts in fish and aquatic birds. These problems and other general infom1ation (such as environmental occurrence, general and biochemical properties, and other characteristics) arc summarized 

in a number of recent reviews and books about selenium (sec References 5-19) and arc reviewed 

briefly in this chapter. Readers may wish to consult those references for additional information. 

As a result of new information related to food-chain bioaccumulation and effects of selenium 

in freshwater fish, the ambient water quality criteria were revised in 1987,20 and they arc being 

reviewed again.2L22 Water quality criteria for freshwater systems are based on total recoverable 

selenium (as discussed later in this chapter), whereas the saltwater criteria arc based on dissolved 

selenium. Because of differences in the toxicity of different chemical forms, the current acute 

criterion (criteria maximum concentration) for freshwater considers selenite and selenate as relative 

fractions of the total recoverable selenium concentration. 

The environmental significance of selenium is a topic of ongoing discussion and debate. A 

recent issue of the journal Aquatic Toxicology (Vol. 57, No. 1) was devoted to papers on selenium. 

In December 1999, the journal Human ami Ecological Ri.r;k Assessment devoted an issue (Vol. 5, 

No. 6) to debate and commentary on the topic, "Selenium -A potential time bomb or just another 

contaminant?" and published several papers on the topic. Needless to say, the debate continues, as 

not all the issues were resolved. In particular, there are differences of opinion concerning waterborne 

concentrations of selenium that are protective for fish and wildlifc, 23~::H the relative importance of 

sediment vs. waterborne selenium in affecting aquatic biota, <J~JI and the specific thresholds of 

selenium in the diet or eggs of birds at which reproduction is adversely affected. 32 1 These topics 

are discussed in later sections of this chapter. 

Selenium concentrations typically are reported as micrograms per liter ()..lg/L) in fluids and 

milligrams per kilogram (mglkg) or micrograms per gram ()..lg/g) in soil, sediment, plant or animal 

tissues, and diets. Concentrations in soil, sediment, tissues, and diets can be expressed either on a 
ECOTOXICOLOGY OF SELENIUM 467 

wet-weight or fresh-weight basis (which are considered to be synonymous), or on dry-weight basis. 

Conversion from one basis to the other is a function of the moisture content in the sample (which 

should be reported regardless of which basis is used), as follows: 

D . I W . I 100 ry-wctg 1t cone. = ct-wctg 1t cone. x ( 100 M . ) - oisture percentage 

For example, 10 mg/kg on a wet-weight basis in a sample having 80% moisture is equal to 50 

mg/kg on a dry-weight basis. Selenium concentrations in soil, sediment, and tissue throughout this 

chapter arc given on a dry-weight basis, unless otherwise noted. 

17.2 ENVIRONMENTAL CHEMISTRY 

From an ccotoxicological perspective, it is important to have an understanding of selenium's 

chemical characteristics, environmental sources and occurrence, and cycling in the environment. 

Those aspects of selenium chemistry can help define geographic areas and environmental settings 

in which selenium may be a chemical of ecological concern, and they are bricl1y summarized in 

this section. 

17.2.1 Chemical Characteristics 

Selenium chemistry is complex, and chemical forms vary in their environmental occurrence, 

biogeochemistry, and toxicity.7·

15·17•19•

35-Js In nature, selenium is generally recognized to occur in 

four oxidation states: selenide (Se'·), elemental selenium (Se0), selenite (Se·1

+), and selenate (Se6 '). 

although a fifth form (selenium dioxide [Se'+]) may occur as a product of combustion of elemental 

selenium present in fossil fuels or rubbish. 19 Soluble selenates are the predominant form under 

oxidizing conditions in alkaline soils that are commonly found in arid areas. Selenates are readily 

available to plants, or they can be slowly reduced to selenites, which also can be taken up by plants 

and converted into organic forms. The common organic forms in plants include sclenomethionine, 

selenocysteine, dimethylselenide, and dimethyldiselcnidc. Selenite is the more common soluble 

form of selenium under reducing conditions and in acidic soils, which occur more typically in 

higher-rainfall areas. Selenium occurs in various organic and inorganic forms in coal and other 

fossil fuels. In general, elemental selenium and metallic selcnides arc not readily bioavailablc. 

However, under oxidizing conditions they can be transformed to bioavailable forms. 

Selenium commonly occurs as a mixture of several different chemical fom1s in surface waters, 

but seicnutes and sclcnites are most common.J0· 17 ·

25 •

3

s Metal and organic selenides arc common in 

bottom sediments. Waterborne selenium can be evaluated most reliably as total recoverable concentrations, 17 and total recoverable concentrations are the basis of the ambient water quality criteria for 

freshwater ecosystems. 20·22 This is because partitioning of selenium between the water column and 

other compartments of an aquatic ecosystem greatly affects the measurement or waterborne selenium. 

Low waterborne selenium concentrations can ref-lect low mass loading of the element, but they also 

may reflect high biotic uptake 17 (sec also Kesterson Reservoir example below). Total recoverable 

selenium includes suspended detrital particulate matter (a function of biotic uptake) and thus more 

accurately reflects the total mass load of selenium in the system. Differences between dissolved and 

total recoverable measurements increase with higher levels of eutrophication of the water body. 

During the early and mid-1980s, subsurface agricultural drainage waters from the San Joaquin 

Vaiicy, California, were disposed of by discharging them to Kesterson Reservoir, which was a series 

of 12 shallow ponds totaling about 500 ha. 7 Selenium concentrations in water entering the reservoir 

during 1983 to 1985 averaged about 300 J.lg SelL, and aquatic plants and invertebrates contained 
468 HANDBOOK OF ECOTOXICOLOGY 

greatly elevated concentrations of selenium (typically 10 to 100 times those found at n nearby 

reference site). 7• 39-t:! Almost all of t11e waterborne selenium was in the selenate form. Unlike boron 

and a number of other constituents, selenium concentrations decreased as the water Hawed through 

the series of ponds and evaporated. Similarly, plants and animals as well as sediments in ponds 

nearer the int-low contained higher concentrations of selenium than those downstream. Thus, 

bioaccumulation by plants and animals removed substantial amounts of selenium from the water 

and deposited it into the sediments. Similar findings were reported for another site in Montana . ..J 3 

Selenium is essential for animals and for some plants. 19 Biochemically, it is similar to sulfur~ 

when selenium is present at elevated dietary levels, it replaces sulfur in some metabolic pathways 

and thereby causes problems23.J0·

37·.J·! (see also, later section on effects in fish). Selenium is an 

essential component of the enzyme glutathione peroxidase, which, along with vitamin E, serves as 

an antioxidant to prevent metabolic damage to tissues. 

Selenium occurs in various chemical forms in plant and animal tissues, but bioavailability is 

greater from plant selenium than from animal foods. 5·'15 Sclenomethioninc has been found to be a 

good surrogate for natural food-chain selenium in experimental studies. 17 In general, the diet is the 

most important exposure pathway for vertebrate animals and should be included when conducting 

or evaluating the effects observed in experimental studies. 

17.2.2 Environmental Sources and Occurrence 

TYpical concentrations in the Earth's crust are < 0.5 mg Se/kg, but some geologic formations 

are greatly enriched in selenium. 19·

36.39,·

16- 50 Especially when these fommtions arc modified as a 

result of human activities (e.g., mining, agricultural irrigation), the selenium may be mobilized and 

become more available to plants and animals, which bioaccumulate selenium and incorporate it 

into the food chain for other organisms. Selenium tends to be present in large amounts in areas 

where soils were derived from Cretaceous and Tertiary marine sedimentary rocks, and selenium 

from these sources is highly mobile and biologically available in arid regions having alkaline soils. 17 

Industrial sources, such as coal and oil combustion, nonferrous metal production (primarily 

copper and nickel, but also lead, zinc, and cadmium), steel and iron manufacturing, municipal 

and sewage refuse incineration, and production of phosphate fertilizers, introduce much more 

selenium into the environment than do natural sources such as volcanic activity or weathering of 

seleniferous rocks,51·5:! However, there are no ores from which selenium can be mined as a primary 

productY It is found mainly in sulfide minerals of copper, iron, and lead and is most commonly 

produced by electrolytic separation from copper during the refining process. Average annual usage 

is estimated at 1850 metric tons. Selenium is used in a wide variety of industrial applications, 

with glass (35% of demand) and electrical (30%) applications representing most of the use. Other 

uses include pigments (10%), metallurgy (10%), agricultural/biological (5%), and miscellaneous 

applications (10%). 

Fossil fuels (coal and oil), as well as associated formations from which they are extracted, may 

contain elevated concentrations of selenium. 19·:!5 Concentrations vary considerably depending on 

the source. The selenium may be mobilized through mining of coal or disposal of fly ash from 

power plants where the coal was burned. In terrestrial environments, when fly ash is deposited in 

landfills, it may be taken up in significant amounts by plants growing on the landfill.'" One wellknown example of effects associated with fly ash disposal into water bodies has been reported in 

a series of studies conducted at Belews Lake, North Carolina (see, for example, Reference 24). 

Similarly, selenium from crude oil may be discharged with process wastewater from the refinery, 

thereby contributing to bioaccumulation by organisms in the receiving waters.:!5.5.J 

Mass-loading of selenium into aquatic environments most typically resulls from disposal of coal 

fly ash, irrigation wastewater, or oil refinery wastewater. !7,:!S,s.J Mining or smelting of sulfide ores, 

uranium, phosphate, bentonite, and coal also contributes to mobilization of selenium because of its 

presence in the mined materials or the overburden that is moved during the mining process. 17•

19••17•

55 
ECOTOXICOLOGY OF SELENIUM 469 

In freshwater systems, nonnal background levels of selenium in sediment are 0.2 to 2.0 mg/kg, 

and in water 0.1 to 0.4 11g1L. 17 

17.2.3 Cycling 

Selenium reaches the earth's surface through volcanic activity, and some authors have proposed 

that oceans were enriched in selenium from volcanic activity during Cretaceous timcs. 7.3S,·Hi.5n.sti 

This early enrichment, along with bioaccumulation of selenium in the oceans and later deposition 

in the sediments;nl would explain the high selenium concentrations found in many Cretaceous 

marine rocks. Tertiary marine sedimentary rocks contain selenium resulting from bioaccumulation 

and deposition of particulate matter derived by erosion of older (Cretaceous) sclcniferous sedimentary deposits and also are generally seleniferous. 

Irrigation of seleniferous soils can dissolve and mobilize selenium and then transport it in 

groundwater to irrigation drains:1H-so Geology, climate, and hydrology were found to be important 

factors affecting the mobilization and transport of selenium into habitats where biota arc adversely 

affected in the western United States. Although selenium can be transported naturally as a result 

of rock wca.thering and dmina.gc into water bodies, the process has been increased through irrigation 

of soils derived from sclenifcrous formations, especially Cretaceous and Tertiary marine sedimentary deposits. 

Many biogeochemical processes affect the cycling of selenium through different components of 

the environment. 7

•

10•3552·57•

5H In wetlands, selenium oxidation-reduction reactions arc the most important processes controlling speciation, precipitation/dissolution, sorption/desorption, methylation, and 

volatilization of selenium. Selenate is stable in well-oxidized environments, but it is converted slowly 

to selenite or elemental selenium under less-oxidized conditions. It can be further reduced to metal 

selenides or volatile methylated forms (primarily dimethylselcnide) through microbial processes. 

The metal selenides tend to be deposited in the wetland sediments, whereas volatile fom1s escape 

to the atmosphere. Selenides and elemental selenium lend to be very insoluble, altlmugh they can 

be converted to more soluble fonns (selenite and selenate) under oxidizing conditions. 

The changes described above can be important considerations in the wetting and drying cycles 

that occur in seasonal wetlands, as well as periodic draw-down of water levels in permanent 

wetlands. When submerged, especially where large amounts of organic material are present, selenium tends to be present in reduced (and less toxic) forms, and volatilization is favored. 10 If the 

water level is lowered, the selenium becomes more oxidized and bioavailable. Thus, the selenium 

present in the wetland sediments and organic matter would become more likely to bioaccumulate 

into aquatic organisms soon after the wetland is refiooded than when it was previously f-looded. 

Although selenium in igneous and sedimentary rocks may be insoluble and unavailable to 

terrestrial plants, chemical weathering and plant and microbial action transform much of it into 

soluble (and bioavailable) forms:16.56 Oxidation of this selenium in alkaline soils produces selenate, 

making it readily available to plants. Selenium uptake by plants is affected by both the concentration 

and its bioavailability in soil. Selenium uptake by plants is greater in soils having higher pH and 

in areas of low rainfall. In areas with acidic soils and higher rainfall, selenium is generally less 

available to plants for uptake. 

Selenium enters the atmosphere through volcanic activity, burning of fossil fuels (especially 

coal), and volatilization from terrestrial or aquatic environments.J5·

51 Although concentrations are 

low (typically around I ng/mJ air, but ranging up to 6000 times that level near a smelter59), 

atmospheric dispersion and subsequent deposition of the selenium redistributes it in the environment. For example, deposition is estimated to have contributed to a 15% increase in soil selenium 

in the United Kingdom during the last century and to contribute between 33 and 82% of plant leaf 

selenium uptake. Deposition occurs either as "wet deposition" (with precipitation), which accounts 

for most of the deposition, or "dry deposition" (exchange of particulate and gaseous material 

between the atmosphere and the global surface). 
470 HANDBOOK OF ECOTOXICOLOGY 

Numerous studies of selenium transformations, cycling, and volatilization in aquatic and terrestrial ecosystems have been reported (or reviewed) in recent years. 16·

18·.J351·60--67 Not surprisingly, 

rates for these processes vary greatly, depending on temperature, moisture, organic carbon content 

of soil/sediment, selenium concentration and chemical form, and microbiological activity. Although 

details of some of the processes arc not yet well known, it is clear that microbes arc largely 

responsible for many of the changes that occur. 

Plants vary widely in their ability to accumulate and volatilize selenium from contaminated 

wastewater, soils, and sedimcnts. 7·

66·

68 Under some circumstances, phytoremediation of these contaminated media may be a viable approach to reducing environmental exposures. However, waterborne selenium can be accumulated by plants and animals living in the wetlands to levels that are 

harmful to aquatic organisms as well as birds that feed upon them.7·

19 As seleniferous wnters enter 

a wetland, selenium is removed from the water through a combination of bioaccumulation by plants 

and animals, deposition in the sediments, and volatilization (as relatively low-toxicity dimethylselenide) to the atmosphcre.40A2·

69•

70 Because of selenium bioaccumulation in the food chain and 

deposition to sediments, wetlands should not be used for phytoremediation of selenium-contamimtted wastewater without conducting an ecological risk assessment to evaluate potential adverse 

effects to birds and other potentially exposed animals. 

The biological haltCJifc of selenium in animals is relatively short. varying from 10 days in 

pheasants to 64 days in earthworms.71,n More detail concerning uptake and loss of selenium in 

birds is presented in the section on bioaccumulation in birds. 

17.3 BIOACCUMULATION 

Selenium bioaccumulates in aquatic and terrestrial food chains and in higher trophic-level 

animals that feed on those plants and animals. Bioaccumulation is defined as the combined net 

accumulation (taking into account any concurrent loss) of a chemical from abiotic media and 

ingestion of selenium-containing biota (i.e., the food chain). In aquatic systems, bioaccumulation 

may occur by direct absorption or partitioning from water or sediment or by ingestion. The greatest 

level of bioaecumulation is typically from water to aquatic plants or invertebrates (often a !ODDfold or greater increase occurs). In terrestrial systems, bioaccumulation is primarily from ingestion, 

because uptake from air is less than that from water, and airborne concentrations generally are 

lower than U10se in water. When concentrations in animals arc considered on whole-body basis 

(which is the appropriate perspective), selenium does not biomagnify through the various trophic 

levels. (Biomagnification is defined as increasing concentrations Uuough successive trophic levels.) 

Nevertheless, significant bioaccumulation does occur (as described in this section) and leads to 

adverse effects in sensitive organisms (especially fish and birds; see next section). 

This section presents a general summary of selenium bioaccumulation by plants and animals. 

Selenium concentrations typically are higher in marine organisms than in those from freshwater 

ecosystems, and they are higher in areas having selcniferous soils or sediments and in areas receiving 

industrial, agricultural, or municipal wastes. Additional information, including more detailed tabulations of selenium concentrations in biota, are provided by Jenkins,73 Wilber/ 1 and Eisler. 19 

17.3.1 Plants 

17.3.1.1 Aquatic 

Background concentrations in freshwater algae arc 0.1 to 1.5 mg Se/kg, and in rooted plants 

they arc D. I to 2.D mg Se/kg. 17•

19 By comparison, algae and rooted aquatic plants accumulated 20 

to 390 mg Se/kg in Kesterson Reservoir, California, which received agricultural drainage waters 

containing up to 330 flg Se/L:10- 1

2 Selenium bioaccumulated in aquatic plants 28 to 5100 (mean== 
ECOTOXICOLOGY OF SELENIUM 471 

11 05) times the concentration in water:E Elevated concentrations of selenium also have been found 

in plants from ponds receiving coal fty ash.74· 7

5 

The bioaccumulation and volatilization of selenium by wetland plants (referred to as phytoremcdiation) can be used to treat waterborne selenium under certain conditions. There was at least 

a 50-fold variation in selenium accumulation and volatilization among 20 aquatic species tested 

with water containing selenite or sclcnate.66 Several of the aquatic plant species showed selenium 

volatilization and accumulation rates that were similar to that of Indian mustard (Brassica juncea), 

the best-known terrestrial plant species for phytoremediation. However, the overall implications of 

using vegetated wetlands to remove selenium from agricultural or industrial wastewaters arc not 

yet well studied. This technology may be appropriate for waters containing moderate levels of 

selenium (perhaps up to 25 or 30 J.lg/L), but only if it is designed and managed to limit access by 

aquatic birds that would feed on plants and invertebrates in the wetland. However, at higher 

waterborne concentrations of selenium, or with inadequate attention to design and management 

concerns, the wetland could present undesirable levels of exposure to birds. 

Several laboratory microcosm studies (summarized below) have measured selenium bioconcentration (i.e., direct uptake) by algae from water and subsequent transfer of selenium to invertebrates 

or fish consuming the algae. These studies show that the chemical form of the waterborne selenium 

is an important factor in affecting uptake by aquatic biota and that sulfate content of the water can 

influence uptake. These and other similar laboratory studies of aquatic systems show that selenium 

bioaccumulation is a significant mechanism of concern for higher trophic-level organisms. 

Bioconcentration factors (BCFs) from water to macrophytes (rooted plants and filamentous 

algae), periphyton (algae and associated material attached to the wall of the microcosm), and 

zooplankton (daphnids and their offspring) were significantly higher for selenomethionine than for 

selenite, which in turn had higher bioconcentration factors than selenate.76 Volatilization of selenium 

from the microcosms followed the same pattern (i.e., highest for sclenomethionine at 24% loss 

during the 28-day exposure). For ali three forms of selenium, bioaccumulation was higher in 

zooplankton than in periphyton, which had greater accumulation than macrophytes. 

Another study by Besser et al.77 with algae (Chlamydomonas reinhardtii), daphnids (Daphnia 

magna), and fish (bluegill, Lepomis macrochirus) also showed higher bioaccumulat.ion from waterborne selcnomethionine Umn from either selenite or selenate. Algae and daphnids concentrated selenium more strongly from selenite (BCFs = 220 to 3600) than selenate (BCFs = 65 to 500), whereas 

bluegills concentrated selenium about equally from both inorganic forms (estimated BCFs = 13 to 

106). Bioaccumulation of foodborne selenium by daphnids and blucgills was similar in food chains 

dosed with different forms of selenium, and they did not accumulate selenium concentrations greater 

than those in their diet (except at very low dietary selenium concentrations). In exposures based on 

selenite, bluegills accumulated greater selenium concentrations from food than from water, and 

aqueous and food-chain uptakes were approximately additive. The results indicate that most of the 

accumulation of inorganic waterborne selenium by blucgills in selenium-contaminated environments 

is via food uptake. However, organoselenium compounds, such as selenomethionine, may contribute 

significantly to selenium bioaccumulation by bluegills through both aqueous and food-chain uptake. 

Corixids (TI·iclzocorixa reticulata), which are common insects in wetlands receiving agricultural 

drainage waters and are important food-chain organisms for fish and birds, were exposed to 

waterborne and foodbome selenium.78 Algae (Oscillatoria sp. filaments and a Microcystis-type 

colonial alga), the food for the corixids, were exposed to water containing sodium selenate at 

selenium concentrations ranging from 7.3 to 870 Jlg/L for 48-h exposures. Corixids exposed to 

waterborne selenate did not accumulate selenium above control concentrations. Corixids fed algae 

exposed to ;?:87 ~Lg Se/L selenate had significantly higher selenium concentrations than controls, 

suggesting that corixids may be isolated from the water and that selenium accumulation is solely 

through dietary exposure. 

Transfer of selenium also was followed through a laboratory food chain (water-algae-rotiferslarval fish), and its effects on the larval fathead minnows (Pimepha/es prome/as) were evaluated." 
472 HANDBOOK OF ECOTOXICOLOGY 

Selenium uptake was measured in rotifers (Braclzionus calycijlorus) fed algae (Chiarella pyrenoidosa) that had been cultured in a selenate-containing medium. The rotifers were then fed to larval 

fathead minnows, and uptake and loss of selenium as well as effects in the fish were measured. 

Selenium concentrations in the fish (51.7 and 61.1 Jlg/g in two experiments) were similar to those 

in their food (68 and 55 /lg/g). Final weights of larvae fed selenium-contaminated rotifers were 

significantly lower than those of controls, although mortality did not occur. Biological half-life of 

food-derived selenium in the fish larvae was 28 days. 

Selenium concentrations in field-collected widgcongrass (Ruppia maritima) varied among samples from four evaporation pond systems in the San Joaquin Valley, CaliforniaJ9 These plants were 

used as a substrate to measure selenium bioaccumulation and effects in a laboratory benthic-detrital 

food chain with midge (Chirmwmus decorus) larvae. Following a 96-h exposure, selenium concentrations had increased significantly in the three groups of larvae exposed to plants having the 

highest selenium concentrations, but there was no consistent pattern of adverse effects on larvae 

weights. After a 14-day exposure, selenium bioaccumulation patterns in midge larvae were similar 

to that observed after 96 h. However, as midge selenium concentrations increased, mean weight 

decreased, indicating that selenium apparently had a significant effect on growth. The resulls of 

this study complement other findings that indicate sediment and the detrital pathway arc important 

in the cycling of selenium in aquatic systcms.HO-H~ 

Waterborne sulfate reduces the uptake of selenium by plants when waterborne selenium is in the 

selenate form. Increasing sulfate resulted in significantly reduced selenate uptake by algae (Selenastrum capricormttum) and increased algal growth.H3 Sulfate also reduced selenium uptake from 

waterborne selenate by widgeongrass, but selenite and selenomethionine uptake were not affected 

by sulfate-'·' As in other studies summarized above, uptake of sclcnomethioninc (BCFs up lo 2 I ,800) 

by widgeongrass was much higher than for selenite or selenate, and the highest rates of uptake (i.e., 

BCFs) occur at the lowest waterborne concentrations. Results of this and other field and laboratory 

studies show that waterborne selenium occurs under field conditions in a mixture of chemical forms, 

and BCFs therefore vary depending on the environmental conditions and sources of selenium. 

17.3.1,2 Terrestrial 

Background selenium concentrations in terrestrial plants are 0.01 to 0.6 mg/kg. 1

H 5 Bioaccumulation of selenium by plants depends on the species of plant, environmental conditions, age and 

phase of plant growth, and the chemical form of selenium present. 3·

36·•16·H5·86 Some plants, such as 

while clover (Trifolium repens), buffalograss (Buchloe dactyloides), and grama (Bollte/oua spp.), 

growing on sclcnifcrous soils accumulate surprisingly low levels of selenium. In contrast, high 

sulfur-containing plants like the Brassica species (mustard, cabbage, broccoli, and cauliflower) and 

other Brassicaceae arc relatively good accumulators of selenium. 

Three groups of plants are generally recognized on the basis of their tendency to accumulate 

selenium when grown on high-selenium soils (as originally described by Rosenfeld and Beath3). The 

first two groups of plants arc referred to as selenium accumulator or indicator plants. These plants 

grow well on soils containing high levels of available selenium, and some have been used to locate 

selcniferous soils. Plants in Group I are called primary indicators or accumulators and normally 

accumulate selenium to very high concentrations (often several thousand mg/kg). Plants in Group 2 

arc referred to as secondary accumulators and may contain a few hundred mg Se/kg in their tissues. 

Those in Group 3 include grains, grasses, and many other plants that do not nom1ally accumulate 

selenium in excess of 50 mg/kg when grown on selcniferous soils. Deep-rooted shrubs and other 

plants with long tap roots may act as pumps, bringing selenium from the deeper soil profiles to the 

surface and near-surface soils, where it is available to shallower-rooted plants such as grasses:'l7 

Selenium concentrations in plants usually decline with maturity, so the highest concentrations 

usually occur during the most active growth phase.3•

85 Because selenium is associated with protein 

in the plant, leaves usually contain higher selenium concentrations than seeds (e.g., see also Schuler 
ECOTOXICOLOGY OF SELENIUM 473 

ct al..J2). Although plant selenium concentration can generally be used as an indicator of soil 

selenium status,85 total selenium concentrations in soil do not necessarily determine whether the 

plants growing there will induce toxicity or nutritional deficiency in animals.118 

Following the discovery of significant effects of selenium in aquatic birds feeding at Kesterson 

Reservoir, California, inflow of agricultural drainage water was halted, the reservoir was dcwater~d, 

and lower-elevation portions of the site were filled with soil to prevent groundwater from rising to 

the ground surfacc.li9

·

90 Loss of this wetland habitat was mitigated through wetland enhancement 

in nearby areas. These actions converted the site to a mosaic of three terrestrial habitats, including 

areas that were filled, those higher-elevation areas where existing vegetation (mainly cattail, Typha 

sp.) was disked, and other higher-elevation grassland areas where no action was taken. Further 

descriptions of the site and its early history are provided by Ohlendorf" and Ohlendorf and Hothcm.91 

Biological monitoring of Kesterson Reservoir has been conducted annually since 1988 and has 

included extensive sampling and analyses of soil (for total and water-extractable selenium), plants, 

invertebrates, birds (mainly their blood or eggs), and marnmals.H9•

90 The chemical behavior of 

selenium in the Kesterson environment has been characterized through studies of selenium speciation and fractionation in soils, long-term monitoring of the spatial and depth distribution of 

selenium in soil, field-measured volatilization rates from several experimental plots, laboratory 

measurements of selenium reoxidation rates under controlled conditions, modeling studies of the 

above-mentioned data to determine reoxidation and leaching rates, and bioaccumulation by plants 

and animals at the site. This information was integrated to conduct ecological risk assessments in 

1993 with the limited data set available at that time and again with a much larger data set in 2000. H9•

90 

The key factor in the exposure model for the site is the water-extractable selenium concentration 

in soil, which influences bioaccumulation by plants that form the base of the food chain. Although 

there is a highly significant (P < 0.00 l) increase in plant selenium with increasing water-extractable 

soil selenium, the relation between the two matrices is highly variable.HtJ,IJo Selenium concentrations 

vary somewhat from year to year and among plant species. For example, in 1995, the selenium 

concentration ranged from 0.20 to 90 mg/kg in 286 samples, with a geometric mean of 3.4 mg/kg; 

in 1998, the range in 240 samples was 0.20 to 68 mg/kg, with a geometric mean of 3.8 mg/kg. 

Uptake and accumulation of selenium by plants growing on lly-ash landfills has been studied 

extensively and reviewed recently. 1

H Concentrations of selenium in vegetation vary by plant species, 

with legumes typically accumulating more selenium than grasses. Although some other elements 

(such as boron and molybdenum) also bioaccumulate in these plants, this docs not occur to elevated 

concentrations as consistently as the bioaccumulation of selenium. Application of sulfur in the form 

of gypsum reduced the bioaccumulation of selenium in several plant species. Plants growing on 

fly-ash landfills can transport selenium to the surface and ma!<e it available to the surrounding 

environment. Reported concentrations of selenium in plants growing on or near these landfills are 

typically less than 10 mg/kg, although concentrations exceeding that level have been found in grass 

(Piw/aris anmdinacea), trees (Populus deltoides, Salix spp.), and herbaceous vegetation growing 

at the margin of an ash-settling pond and in plants growing in t-ly ash without soil cover. 

17.3.2 Invertebrates 

17.3.2. 1 Aquatic/Marine 

Background concentrations in aquatic invertebrates are 0.4 to 4.5 mg Se/kg, although concentrations are typically less than 2 mg Se/kg. ~· 17 19 Elevated concentrations have been reported in 

freshwater and estuarine invertebrates from areas receiving subsurface agricultural drainage;10- 12.H2,92 

urban/industrial discharges,93 and in ponds or lakes that received selenium-containing fly ash.75·

9.J.-tJ(, 

Among the aquatic invertebrates sampled at Kesterson Reservoir, California, selenium concentrations were highest in benthic species, such as midge larvae (Chironomidae) and dragonfly and 

damselfly nymphs (Odonata), and lowest in water boatmen (Corixidae):",_,' Mean concentrations 
474 HANDBOOK OF ECOTOXICOLOGY 

in some of the invertebrates from Kesterson Reservoir exceeded 100 mg Se/kg. Selenium concentrations in adult damselflies were significantly higher than in nymphs:11 Selenium bioaccumulation 

factors for invertebrates ranged from 168 to 3700 (mean= 1090):"·Most aquatic insects contained 

lower concentrations than rooted aquatic plants but higher concentrations than water, sediment, 

algae, and diatoms. 

Midge larvae show small-scale spatial variability in selenium accumulation,97 which probably 

reflects variability in concentration and bioavailability of selenium in sediment and perhaps the 

ages of the larvae. Grain size and organic carbon content can influence sediment selenium concentration and bioavailability. Consequently, when sampling/moniloring is being conducted to characterize dietary exposure of fish and aquatic birds that feed on midge larvae, it is important to 

analyze randomly collected, composited samples taken throughout the potential feeding area. 

Laboratory experiments have indicated that 96% of selenium in mussels (Mytilus edulis) was 

obtained from ingested food, with much less accumulation directly from water than for several 

other elcments.'1

H Several other laboratory studies of the relative bioaccumulation in freshwater 

aquatic organisms from waterborne selenite, selenate, and selenornethionine arc discussed above 

in the section on plants. At selenate concentrations greater than 5 ~g Se/L, selenium uptake by 

daphnids (Daphnia magna) was inversely related to waterborne sulfate concentrationY~J Similar 

effects were reported for aquatic plants, as described abovc.83·

8

·

1 

17.3.2.2 Terrestrial 

Background concentrations in terrestrial invertebrates are O.l to 2.5 mglkg. 17 Earthworms were 

found to bioaccumulate elevated concentrations of selenium from selenite-enriched soil (up to 7.5 

mg Sclkg, fresh weight HKl), and earthworms from soil amended with sewage and those from a 

control field contained 15 to 22 mg Se/kg (dry weight). 101 

Extensive sampling of terrestrial invertebrates (various kinds of insects as well as spiders) has 

been conducted at Kesterson Reservoir, California since its conversion to terrestrial habitats in 1988 

(described above and in Ohlendorf and Santolo"'). From 1989 to 1995, samples were collected 

annually, and sampling was conducted again in 1998.9° Generally, selenium concentrations in 

carnivorous invertebrates (such as ambush bugs, scarab beetles, and spiders) were significantly 

higher than those in noncarnivorous insects (such as herbivorous beetles, crickets, and grasshoppers). For example, the geometric mean for 35 samples of carnivorous invertebrates in 1995 was 

18 mg Se/kg (range 4.5 to 48 mg Se/kg), and the geometric mean for 110 samples of noncamivorous 

insects was 7.4mg Se/kg (range 0.60 to 40 mg Se/kg). In 1998, U1e geometric mean for 35 samples 

of camivorous invertebrates was I 7. I mg Se/kg (range 8. I to 36 mg Se/kg), and the geometric 

mean for 104 samples of noncarnivorous insects was 12.8 mg Se/kg (range 1.8 to 61 mg Se/kg). 

17.3.3 Fish 

17.3.3. 1 Freshwater 

Background concentrations in livers of freshwater fish are 2 to 8 mg Se/kg, and in other tissues 

they are 1.0 to 4.0 mg Se/kg. 17 In the National Contaminant Biomonitoring Program, sampling of 

freshwater fish was conducted five times throughout the United States during the period I 976 to 

1986."" Geometric means for whole-body fish were between 0.4 and 0.5 mg Se/kg wet weight (or 

about 1.6 to 2.0 mg/kg dry weight) in all years, with the highest melm in samples from 1976 to 

77. The 85th percentile values were 0.66 to 0.82 mg Se/kg wet weight (about 2.6 to 3.3 mg/kg dry 

weight). These values were typically exceeded at several stations, most of which are in arid areas 

of the western United States in basins containing substantial amounts of irrigated agriculture. One 

exception to that pattern is the Waikele Stream site at Waipahu, Hawaii, where the 85th percentile 

values were regularly exceeded. 
ECOTOXICOLOGY OF SELENIUM 475 

Elevated levels of selenium have been reported in fish primarily from seleniferous areas of the 

western United States and from reservoirs contaminated by fty ash from combustion of 

coal. H 19•

25•·10•·11 •

96·Hl3

- 108 Typical patterns found in these fish (and those sampled elsewhere) are that 

selenium concentrations in muscle tissue are considerably lower than in livers, gonads, or wholebody fish. Selenium concentrations in gonads (and especially the eggs) as well as whole-body fish 

provide the best basis for evaluating exposure of the fish to selenium and the potential for adverse 

effects in the fish or their reproduction (as discussed in a later section). However, it is possible, 

especially if the fish of concern is an endangered species, to evaluate exposure through nonlethal 

sampling of muscle plugs or eggs so the fish is not sacrificed for analysis. 

Mosquitofish ( Gambusia affinis) from the selenium-contaminated Kesterson Reservoir contained exceptionally high concentrations of selenium.7

••10•

41 Fish were analyzed as composite samples 

of many individuals taken from different portions of the site, and concentrations were compared 

to those in mosquitofish from a nearby reference site. Mosquitofish from Kesterson typically 

contained geometric mean concentrations greater than 100 mg Se/kg, in comparison to 1 to 2 mg 

Selkg at the reference site. In 1984, one composite sample from Kesterson contained 430 mg Selkg, 

and the means for fish from two of the ponds exceeded 300 mg/kg. 

Laboratory bioaccumulation studies discussed above describe bioaccumulation of selenium by 

fish in relation to different waterborne forms of selenium and illustrate the importance of dietary 

exposure as a route of selenium uptake by fish. Studies report more frequently the rates of uptake 

of different forms of selenium than depuration after exposure to uncontaminated media (food and 

water). In one study,'10 cycling and fate of selenite and selcnornethionine were compared in a 318-

day experiment in which the selenium was introduced as an acute release into two experimental 

ponds. Biotic components sampled included periphyton, rooted plants (Elodea canadensis), snails 

(Helisoma sp.), and mosquitofish. Uptake rates for selenomethionine in the biota were at least an 

order of magnitude greater than for selenite, but elimination rates were similar (different by less 

than a factor of two). This study also demonstrated ll1e importance of sediment and organic detritus 

in the recycling/remobilization of selenium within the ponds. 

17.3.3.2 Marine 

Estuarine and marine fish tend to have higher selenium concentrations than do freshwater 

species, but the differences for most species are generally less than an order of magnitude and 

sometimes are very similar. 19 Higher concentrations were found in species such as tunas and marlins. 

Selenium has been shown to accumulate with age/length in several studies of marine fish, and a 

positive correlation between selenium and mercury has been demonstrated in several species. HJIJ A 

positive correlation of selenium with mercury in fish may be a significant factor influencing 

bioaccumulation. One study has shown that a lower portion of selenium is present as selenate in 

marine fish (24%) than in freshwater fish (36%). 110 

Kasegalik Lake (on the Belcher Islands, Hudson Bay, in the southern Canadian Arctic) is known 

to have bol11 salt- and freshwater populations of Arctic char (Salve/inus a/pinns). 111 Fish of both 

types were analyzed for mercury and selenium. There was an apparent difference in the two 

populations for mercury concentrations, but there was no difference between salt- and freshwater 

groups for selenium (which varied from 0.7 to 1.9 mg Se/kg, wet weight). Thus, some of the 

apparent differences in selenium levels between marine and freshwater fish may be attributed to 

species differences as well as relationships between selenium and mercury in larger, long-lived 

marine species. 

17.3.4 Amphibians and Reptiles 

Background concentrations in livers of amphibians and reptiles are 2.9 to 3.6 mg/kg and in 

other tissues 1 to 3 mg/kg. 17 Many fewer studies of selenium bioaccumulation in amphibians and 
476 HANDBOOK OF ECOTOXICOLOGY 

reptiles have been conducted than those of other animals/· 17·

19 but information on bioaccumulation 

is summarized below. 

17.3.4.1 Amphibians 

Selenium concentrations in adult southern toads (Bufo terrestris) from coal-ash-settling basins 

(17.4 mg/kg, whole-body analysis) were elevated by comparison to toads from a reference site 

(2.10 mg/kg). 11 ~ When other toads were transferred from the reference site to the settling basin for 

a period of 7 weeks, their concentrations increased significantly - from 2.10 mg Sc/kg to 5.46 

mg/kg. Sediments in the ash basin contained 4.4 mg Sc/kg, whereas those at the reference site had 

0.10 mg Selkg. Frog (Rana clamitans) tadpoles from a fly-ash-contaminated pond contained 4.7 

mg Se/kg (wet weight), whereas those from a reference site had 1.5 mg/kg. 75 Red-spotled newts 

(Notophthalmus \'iridescens) from those same sites contained 4.2 and 1.8 mg Se/kg (wet weight), 

respectively. Similarly, bullfrog tadpoles (Rana catesbeiana) from an ash-waste site had elevated 

selenium (25.7 mg/kg) by comparison to a reference sit~ (3.4 mg/kg).IJJ 

Because tadpoles apparently accumulate metals readily from sediments and vegetation they 

consume, some authors (e.g., References 114, 115) have suggested that they may be good indicators 

of contaminated environments. To evaluate the effects of gut contents (especially ingested sediment) 

on analyses of bullfrog tadpoles, wild-caught tadpoles were maintained in clean water and analyzed 

after 0, 24, 48, and 72 h and then analyzed for metals to test the effects of "clearing" of the gut 

contents. 115 Metals concentrations in whole bodies and digestive tracts also were evaluated separately. Selenium concentrations in the bodies and tails of the tadpoles were unaffected by clearing, 

although selenium concentration was higher in the digestive tract than in the body without digestive 

tract, in the tail, or in the whole body. The tadpoles had higher selenium concentrations than those 

found in the sediment from the sampling location. 

Bullfrogs collected from reference sites in the San Joaquin Valley, California contained 1.0 to 

1.9 mg Sc/kg when analyzed as whole-body samples. 1 Hi Livers of three bullfrogs from other 

reference sites in the San Joaquin Valley contained 3.6 to 9.3 mg Sc/kg (mean 6.2 mg/kg). m The 

lower end of that range is probably representative of background, whereas the upper value may 

reflect some elevation above background. Few frogs were observed in nearby Kesterson Reservoir, 

probably because of the highly saline water it contained. However, ten bullfrogs were collected 

from the San Luis Drain (which conveyed agricultural subsurface drainage waters to Kesterson); 

livers of these bullfrogs contained 25 to 88 mg Se/kg (mean 45 mg/kg). These liver concentrations 

were similar to the selenium concentrations in aquatic insects and mosquitofish in the Drain. 

17.3.4.2 Reptiles 

Livers of banded water snakes (Nerodia fasciata) from ash-settling basins associated with a 

coal-burning electric power plant were analyzed for selenium and several other elements that were 

found at elevated levels in sediment within the basins. 118 Selenium concentrations in the livers of 

snakes from these basins averaged about 140 mg/kg, compared to 3.62 mg Sc/kg in livers of snakes 

from a reference site. Water snakes were found to be feeding on smaii bullfrogs and bullfrog 

tadpoles, green treefrogs (Hy/a cinerea), southern toads, bluegill sunfish (Lepomis macrochirus), 

mosquitofish, red-fin pickerel (Esox americanus), and largemouth bass (Microptems salmoides). 

Samples of those potential prey items were collected at the polluted ash-basin site and at the 

reference site. Mean selenium concentrations in the amphibians and fish from the polluted site were 

about 10 to 27 mg/kg, whereas those in amphibians and fish from the reference site were typically 

less than 2 mg/kg. Thus, the water snakes from the ash-basin site bioaccumulated selenium in their 

livers to concentrations about tenfold the prey diet concentrations, and those at the reference site 

had selenium concentrations that were about equal to the prey item levels. Selenium concentrations 

in livers showed a significant linear relationship with snake body mass. 
ECOTOXJCOLOGY OF SELENIUM 477 

Water snakes (Nerodia sp.) from Florida contained 0.3 to 0.5 mg Se/kg (whole body, wet 

wcight). 119 Whole-body selenium concentrations in lizards and snakes from reference areas in the 

San Joaquin Vailcy averaged 0.7 to 2.0 mg/kg.ll6 Livers of gopher snakes (Pituophis me/anoleucus) 

from other reference sites in the San Joaquin Valley contained mean selenium concentrations of 

2.05 and 2.14 mg/kg. 117 Livers of gopher snakes from the nearby Kesterson Reservoir had a mean 

selenium concentration of II. I mg/kg. Selenium bioaccumulation in these snakes at Kesterson and 

associated reference sites reflected selenium levels in prey species for the snakes at those sites. 

Selenium levels in the plasma of wild and l~mn-reared American alligators (Alligator mississippiensis) reflected selenium concentrations in their diet. 0 Plasma selenium levels in captive 

alligators fed fish (11-licropogon undulatus) were significantly higher (monthly means of 0.23 to 

0.27 mg/kg) than in captive animals fed nutria (Aiyocastor coypus) (0.16 to 0.23 mg/kg) or in wild 

females (0.15 to 0.20 mg/kg); plasma selenium levels in the alligators fed nutria and in the wild 

females were not significantly different from each other. Fish contained 2.8 mg Sc/kg (dry weight), 

compared to only 0.04 mg/kg (wet weight) in the nutria. Diet of the wild alligators was composed 

primarily (-70%) of nutria. 

17.3.5 Birds 

Background concentrations in whole-body birds arc < 2 mg/kg. 17 However, bioaccumulation 

in birds is more frequently (and appropriately) evaluated on the basis of various tissues, as discussed 

below. 

17.3.5.1 Eggs 

Mean background concentrations in eggs of freshwater and terrestrial species arc < 3 mg Se/kg 

(typically 1.5 to 2.5 mg/kg), and maximums are < 5 mg Sc/kg. 17·

19 In a wide variety of species, 

selenium concentrations (on a dry-weight basis) in bird eggs range from roughly equivalent to 

about three or four times the concentrations in the diet of the female at the time of egg-laying. -1 27 

When birds feed on selenium-contaminated diets during the laying season, the exposure is 

quickly reflected by elevated levels of selenium in cggs. 12H Similarly, when the birds are switched 

to a clean diet, selenium concentrations in eggs decline quickly. When mallard (Anas platyrhy11chos) 

hens were fed a diet containing I 5 mg Se/kg (as selenomethioninc), levels peaked in eggs (to about 

13 to 20 mg Se/kg, wet weight) in about 2 weeks on the treated diet and leveled off at a relatively 

low level ( <5 ·mg Sc/kg, wet weight) after about I 0 days back on the untreated diet. The findings 

of this study have important implications for evaluation of field exposures, such as how quickly 

and for what duration selenium exposure may adversely affect bird reproduction. Concentrations 

of selenium in eggs arc especially important because they provide the best samples for evaluation 

of potential adverse reproductive effects (sec also later scction). 12lJ 

Selenium concentrations in the eggs of marine species arc variable but may be higher, even in 

remote areas. 7 For example, eggs of three species (wedge-tailed shearwatcr [Pl{{finus pac(/icus], 

red-footed booby [Sula sula], and sooty tern [Sterna }itscata]) were sampled at four locations 

throughout the Hawaiian archipelago, from Oahu to Midway. 130 Mean concentrations (converted 

from wet weight) varied only slightly by location- from 4.4 to 5.3 mg Se/kg for shcarwaters, 5.0 

to 6.1 mg Se/kg for boobies, and 4.1 to 5.1 mg Se/kg for terns- but all were higher than typical 

of freshwater species. Surprisingly low selenium concentrations were found in eggs of white-winged 

seaters (lv/elcmitta jitsca) when compared to the concentrations of selenium found in their livers. 131 

Elevated selenium concentrations have been found in eggs of birds nesting in areas affected by 

agricultural drainage, coal-fly-ash disposal, and discharge from an oil refinery.7·

25,()J,t:!<J,m.D3 At 

Kesterson Reservoir, California, mean selenium concentrations in bird eggs during 1983 to 1985 

were as high as 69.7 mg/kg (in eared grebes [Podiceps nigricollis]), with many of the species means 

exceeding 20 mg Se/kg.'H Some success in reducing selenium exposures of aquatic birds at evap-
478 HANDBOOK OF ECOTOXJCOLOGY 

oration ponds for agricultural drainage water was achieved by placing freshwater mitigation ponds 

ncar the contaminated ponds.tH 

17.3.5.2 Livers 

Background concentrations in bird livers are < I 0 mg Se!k:g. 17 In a manner similar to that for 

eggs, Heinz et al.m found that selenium concentrations in the liver respond quickly when birds are 

placed on or taken off a selenium-contaminated diet (see further description of this study below 

under Muscle). Thus, selenium concentrations measured in the livers of birds sampled outside the 

breeding season are not good predictors of potential reproductive effects. In laboratory studies of 

reproductive effects, livers of male mallards had higher concentrations of selenium than those of 

females, probably because females excreted part of the selenium they had accumulated tl1rough 

cgg-laying. 121 •

122 Nevertheless, analysis of livers of either male or female field-collected birds can 

provide a useful indication of the relative level of exposure experienced by the population. 

In laboratory studies with birds fed diets containing selenomethionine, selenium concentrations in 

livers of mallards, black-crowned night-herons (Nycticorax nycticorax), and eastern screech-owls (Otus 

asia) ranged from roughly equal to the dietary concentration to about three times the dietary levcl,l21-126 

At Kesterson Reservoir and a nearby reference site, several species of aquatic birds were 

sampled early and late in the nesting season as adults, and juveniles were sampled at about the 

same time as the late-season adults. 136 Selenium concentrations in livers of these birds were 

consistently much higher at Kesterson than at the reference site for both early and late samplings 

and seemed to reflect the period of exposure as well as the foraging range of the birds at the two 

sites. For example, adults of the more mobile species (such as dabbling ducks, Anas spp.) showed 

only three- to fivefold differences in mean selenium concentrations between sites, whereas more 

sedentary species (such as American coot [Fulica americana] and black-necked stilt [Himantopus 

mexicanus]) typically showed 10- to 20-fold differences. Selenium concentrations in juvenile birds 

were generally similar to those in late-season adults. 

Selenium concentrations in the 1 ivers of aquatic birds from a number of other areas receiving 

subsurface agricultural drainage also were elevated and have reflected periods of residence in the 

contaminated areas. 25·

137- 139 When use of subsurface agricultural drainage waters for wetland management was curtailed and replaced with better-quality water, selenium concentrations in aquatic 

birds declined. 

Livers of diving ducks (such as seaters [Me/anitta spp.] and scaups [Aythya spp.]) from estuarine 

habitats have been found to contain higher concentrations of selenium than other aqualic birds in 

the same habitats. 1

·JO-J . .JJ The apparent reason for the higher concentrations of selenium in the diving 

ducks is that they forage on benthic organisms, which bioaccumulate selenium to a higher degree 

than foods of some of the other aquatic birds. 

17.3.5.3 Kidneys 

Selenium concentrations in kidneys of birds from selenium-normal areas were somewhat higher 

than those in the Jiver (liver/kidney ratios of less than L), but concentrations in the two tissues were 

similar in birds from selenium-contaminated Kesterson Reservoir and in the Imperial Valley of 

California.IJ ·'"~· 15 Selenium concentrations in American coots from Kesterson Reservoir and the 

reference site were significantly correlated (r = 0.98). Significant ratios and significant positive 

correlations also have been found in other field studies (e.g., Reference 146). 

17.3.5.4 Muscle 

Background selenium concentrations in muscle tissues of birds are 1 to 3 mg/kg.l 7 Concentrations increase and decrease in response to changes in dietary exposure, but the changes occur more 
ECOTOXICOLOGY OF SELENIUM 479 

slowly than those in eggs or liver. 135 However, for muscle as well as other tissues, when tissue 

levels of selenium are made very high by elevated dietary exposures and the birds are then placed 

on a lower-selenium diet, the loss rate for all tissues is fast at first but then slows down. 

Mallards were placed on a diet containing I 0 mg Selk:g for 6 weeks; predicted equilibrium time 

for concentrations in the breast muscle of females (81 days) was more than ten times that for time 

to equilibrium in the liver (7.8 days).'" The ducks were then provided an untreated diet for 6 

weeks, and loss of selenium from liver and muscle was monitored; half-times were 18.7 days for 

liver and 30.1 days for muscle. Males reached similar levels of selenium in the liver and breast 

muscle as females and declined to similar levels when treatment ended. In another experiment with 

mallards, females were fed increasing levels of selenium until some died; survivors were then 

switched to an untreated diet and selenium was measured in blood, liver, and breast muscle over 

64 days. Half-times were 9.8 days for blood and 23.9 days for muscle. Selenium initially decreased 

in liver by one-half in 3.3 days, with subsequent half-times of 3.9, 6.0. and 45.1 days. 

Selenium concentrations in breast muscle from juvenile ducks (An as sp.) at Kesterson Reservoir 

and a reference site were measured because of concern about human consumption of ducks 

harvested in the vicinity of Kesterson. JJh Mean selenium concentrations were higher at Kesterson 

than the reference site and were only slightly lower than those in livers of these birds. However, 

the relationships between muscle and liver (r' = 0.69) of the ducks were considerably more variable 

than those between kidneys and livers of American coots from the two sites (r' = 0.97). 

17.3.5.5 Blood 

Background concentrations in whole blood are 0.1 to 0.4 mg Se/kg on a wet-weight basis. 17 In 

experimental studies, selenium concentrations in blood of mallards and American kestrels (Falco 

sparverius) reflected dietary exposure levels. 127·IJ5,I.J7

- 1·

1

!J In kestrels, maximal blood concentrations 

were about the same as those in the selenomethionine-supplemented diet. Mallards receiving 

selenium (as selcnomethionine) at dietary concentrations of 10, 25, or 60 mg/kg had blood-selenium 

concentrations of 4.5, 8.9, or 16 mg/L (wet-weight basis).'" The concentration of selenium in blood 

increased in a time- and dose-dependent manner and reached a plateau after 40 days. (The study 

by Heinz et al.us is described in the previous section.) 

Selenium concentrations were measured in terresLrial birds of several species from Kesterson 

Reservoir, the area surrounding that site, and several reference areas in California from 1994 to 

1998. 150 Except for loggerhead shrikes (Lanius /udovicianus), blood-selenium was higher in birds 

from within Kesterson than in birds from other areas. For shrikes, the mean concentrations for 

birds lrom Kesterson (13 mg Sc/kg [dry weight]) were not significantly different than those from 

nearby surrounding areas (8.5 mg Selkg), although the maximum selenium concentration at Kesterson (38 mg/kg) was more than twice the maximum for the surrounding area (16 mg/kg). Among 

species at Kesterson Reservoir, blood-selenium was higher in loggerhead shrikes and northern 

hmTiers (Circus cyaneus) than in the other species (hawks and owls) sampled. 

17.3.5.6 Feathers 

Background concentrations of selenium in feathers are I to 4 mg/kg and are typically less than 

2 mg/kg. 17 Selenium concentrations may be higher in the feathers of birds from areas with elevated 

levels of mercury because of the interactions between these two elements. 

Analysis of feathers may provide useful information concerning exposures of birds to selenium 

if they are considered carefully. It is important to recognize that the selenium may have been 

deposited into the feathers at the time they were formed (which may have been months earlier and 

thousands of miles away from the sampling time and location), or the selenium may be the result 

of external contamination. ISI-15·! Concentrations also may have been reduced through leaching. 

Different kinds of feathers from the same bird may contain different concentrations, depending 

partly on when and where the feathers were grown during the moult cycle. 
480 HANDBOOK OF ECOTOXICOLOGY 

Feathers of five species of seabirds were sampled at Johnston Atoll (about 1300 km southwest 

of Hawaii) and three species at Manana Island, Oahu, Hawaii, and analyzed for selenium and 

several other elements. 155 Selenium concentrations were generally higher in brown noddies (Anous 

stolidus) than in other species at both locations, and noddies had significantly higher concentrations 

at Johnston Atoll (1 I.l mg Se/kg) than Hawaii (7.59 mg Sen<g). 

17.3.6 Mammals 

Background whole-body selenium concentrations in terrestrial and freshwater mammals are< l 

to 4 mg/kg. 17 In muscle, concentrations arc typically< I mg Se/kg~ in liver, they range from about 

I to 10 mg Sc/kg and arc typically $.5 mg/kg; in hair, normal concentrations are <3 mg/kg in 

individual samples, with population averages ranging from 0.5 to 1.5 mg Sc/kg; selenium in whole 

blood is typically 0.1 to 0.5 mg/L, with concentrations below 0.1 mg/L indicating a deficiency in 

domestic livestock. Both hair and blood arc considered good media for sampling/monitoring 

selenium status of live mammals. 

17.3.6. 1 Aquatic/Marine 

Selenium concentrations in raccoons (Procyon /otor) collected at Kesterson Reservoir, California averaged 19.9 mg/kg in liver, 28.3 mg/kg in hair, 21.6 mg/kg in feces, and 2.61 mg/L in blood 

(wet weight in blood, others in dry weight). 156 These concentrations were 12, 30, 21, and lO times 

higher, respectively, than those found in raccoons from a nearby reference site. Selenium concentrations in hair provided the strongest statistical separation between the two study areas. Selenium 

concentrations in raccoons and river alters (Lmltra canadensis) at sites in Canada were similar to 

each other, but they were higher than those found in beavers (Castor canadensis). 157 

Selenium concentrations in the tissues of marine mammals are highly variable, with some 

animals having concentrations in the liver or other tissues that are typical of mammals in freshwater 

or terrestrial environments and others (even of the same species in the same areas) having greatly 

elevated levels. 19·

109 Part of this variation may be due to interactions between selenium and mercury, 

with much greater bioaccumulation (especially as reHected by concentrations in the liver) when 

the animals are exposed to elevated levels of mercury. 

Selenium concentrations greater than 100 mg/kg have been reported for livers of pilot whales 

( G/obicephala macrorhync/ms), 158 beluga whales (De/pltinaptems /eucus), 159 striped dolphins 

(Stene/la coemleoa/ba), 111n bottle-nosed dolphins (Tursiops tnmcatus), 11'

1 ringed seals (Pltoca !lispida),1c1'! harbor seals (P. vitulina), 1

w gray seals (Halichoerus g1)'pus), 1M and California sea lions 

(Zaloplws califomianus). 165 

17.3.6.2 Terrestrial 

Mean selenium concentrations in several species of small mammals at Kesterson Reservoir, 

California, were greatly elevated by comparison to a nearby reference arca.J(f, For example, means 

for California voles (lvficrotus californicus) from Kesterson were as much as 522 times those from 

the reference area (means up to 119 mg Se/kg vs. 0.228 mg Se/kg in livers). There were speciesto-species differences at Kesterson; higher selenium concentrations occurred in carnivorous species 

and species that f'ed on foods closely linked to the pond water. There were also pond-to-pond 

differences at Kesterson, with mammals ref-lecting a pattern similar to that for aquatic organisms 

(described above). 

Following the conversion of Kesterson Reservoir to a terrestrial habitat in 1988 (as described 

above), selenium concentrations in small mammals have been monitored periodically. 89·lJ0 Mean 

whole-body selenium concentrations in mice on a site-wide basis are typically 5 to 7 mg Se/kg 

and tend to be lower in voles than in mice. However, the selenium levels tend to vary spatially 
ECOTOXICOLOGY OF SELENIUM 481 

within the site, especially as related to habitat type and history of management activities that have 

altered the site since it served as a reservoir for agricultural drainage waters. 

Elevated levels of selenium were found in woodchucks (!vlarmota monax) from the vicinity of 

a fly-ash landfill."'' and in American bison (Bison bison), elk (Cel1'/ts elaplws), and mule deer 

(Odocoileus hemiomts) that ate forage with elevated levels of selenium in Wyoming. 168 In contrast, 

analysis of blood from free-ranging mule deer sampled from a number of herds in California 

indicated the deer were selenium-dcficient. 16lJ-l 73 Several methods of selenium supplemcntalion to 

the deer were tested; some were more practical and effective than others and resulted in increased 

reproductive performance of the herds. 

17.4 EFFECTS IN ANIMALS 

17.4.1 Nutritional Requirements vs. Toxicity 

Selenium is essential for animal nutrition at sub-mg/kg dietary concentrations, but it is toxic 

to sensitive species when concentrations arc only a few mg/kg. For most animals, dietary requirements appear to fall in the range between about 0.05 and 0.5 mg/kg.5·

19••111 ·

5fi,I 7.J-1 79 Thresholds for 

toxicity are usually not much more than an order of magnitude higher than the concentrations that 

cause deficiency (as described below). Selenium deficiency is a more widespread problem for 

animal health than is excess selenium, so the issue of selenium ecotoxicology is one of balance 

between meeting the dietary requirements without exceeding toxicity thresholds. Signs of selenium 

deficiency have been reported in many domesticated species (including chickens, turkeys, pheasants, 

quail, ducks, cattle, horses, goats, sheep, and swine) as well as wild species (including species such 

as fish, bighorn sheep [Ovis canadensis], pronghorns [Antilocapra americana], elk, and deer). In 

poultry, selenium deficiency causes severe pancreatic atrophy as well as reduced egg production 

and hatchability; in mammals it causes poor health and reproduction. One of the more common 

myopathies resulting from selenium deficiency (referred to as "white muscle disease") has been 

reported in alleasl 30 of the 50 U.S. stales. 

Selenium deficiency in some species can be ameliorated through a variety of means including 

addition of selenium to diets of domestic species, use of intrarumcnal boluses that release selenium 

slowly, and application of selenium-containing fertilizers to grazing 1ands.5-176-l 79 Selenium fertilization and usc of intrarumenal boluses have been tested for improvement of reproduction in freeranging mule deer, sometimes with significant benefit. 170·

1

7:!. 173 However, selenium deficiencies in 

other kinds of wildlife may not be corrected as easily as those in species such as deer. 

17.4.2 Toxic Effects 

Selenosis and selenium toxicosis are used interchangeably to refer to the pathological conditions 

resulting from ingestion of high-selenium diets. Those conditions include impaired health and 

reproduction, as discussed below in relation to vertebrate animals. When water is the only exposure 

route, selenium is not very toxic to fish or wildlife. 17·

20.57 Eggs and larvae of fish and amphibians 

may be the most sensitive life stages of vertebrate animals to direct e!Tccts of waterborne selenium. 

Eggs and larvae of fish and birds are very sensitive to the lethal or teratogenic effects of selenium 

transferred lo the eggs by the female parent (as discussed below). 

17.4.2.1 Invertebrates 

Invertebrates are not particularly sensitive to the toxic effects of selenium, and there is limited 

information concerning adverse effects to invertebrates under field conditions. 17 In laboratory 

studies, selenite was found to be more toxic than selenate, regardless of whether the effects were 
482 HANDBOOK OF ECOTOXICOLOGY 

measured as direct toxicity or impairment of reproduction. 20·

1

Ho.un Duration of exposure is also an 

important factor, with toxicity increasing as exposure time increases, at least up to 14 days. For 

example, ti1e 96-hour LC50 for the scud (Hyalella az.teca) was 340 fig Se/L or 760 fig Se/L in 

different studies,'"·'"' whereas the 14-day LC50 was only 70 fig SelL."·' Similarly, lor the Daphnia 

magna ti1e 96-hour LC50 was 710 fig Se/L and the 14-day LC50 was 430 ~tg Se/L. "·' 

In a comparison involving the cladoceran Daphnia magna and larvae of the midge Chironomus 

riparius, the daphnids were more acutely sensitive than midges to the toxic effects of inorganic 

selenium. JHS An organic form of selenium (selenorncthioninc) was toxic to daphnids but relatively 

nontoxic to midges. Tissue concentrations of 14.7 and 31.7 mg Se/kg in daphnids were associated 

with reduced growth and reproduction, respectively. Nevertheless, these invertebrates were not 

adversely affected at waterborne concentrations that would affect fish or would bioaccumulatc in 

the food chain to levels that are harmful to fish or aquatic birds. 

In evaluating the potential for selenium to cause toxicity to aquatic invertebrates, it is important 

to note that other elements can affect the toxicity of selenium. For example, sulfate can reduce the 

bioavailability and toxicity of selenate selenium (the predominant form in many water bodies) to 

invertebrates. 99 

Biologically incorporated selenium (presumably in organic forms) in widgeongrass from agricultural drainwater evaporation ponds in the San Joaquin Valley, California, caused significant 

reductions in growth of midge (Chirmwmus decants) larvae that were cultured on detrital material 

from these plants.79 Significant reductions in growth were observed in midges exposed to plant 

substrate having more than about 10 mg Sc/kg, and relative bioaccumulation (as BAF) was inversely 

related to selenium concentrations in the plant material (i.e., a relatively smaller amount was taken 

up when concentrations were greater than 20 mg Se/kg). 

17.4.2.2 Fish 

Several recent publications provide reviews of the effects of selenium in fish and describe case 

studies of such effects_L!.I.J.I 7•

19•23•

25•

1

m;,tH7 Those references provide more detailed descriptions of 

tissue or exposure media concentrations that arc associated with no effects or with adverse effects 

in fish. Some also emphasize that a combination of biological (e.g., condition of larvae/fry, reproductive success, etc.) and chemical (e.g., analysis of ovaries/eggs, whole-body fish, or food-chain 

organisms) indicators should be used in evaluating risks to fish populations. The mortality of 

larvae/fry that is associated with excess selenium can have important effects on populations, 

resulting in lack of recruitment and subsequent declines or disappearance of afTccted species in a 

contaminated water body without evidence of "die-olTs." Given enough time, once the levels of 

selenium input are reduced, populations may recover; however, contaminated sediments will serve 

as a continuing source for selenium cycling. 111H 

Excess selenium in the diet of fish leads to substitution of selenium for sulfur during protein 

synthesis, among other effects.23 Substitution of selenium for sulfur disrupts normal chemical 

bonding, resulting in improperly formed and dysfunctional proteins or enzymes. This causes a 

variety of toxic effects at subcellular, cellular, organ, and system levcls. 103•

1

ll\l These eftCcts are 

exhibited through effects on reproduction (especially in the form of teratogenesis) and reduced 

survival of young fish as well as effects on health, physiology, and survival of older fish. 

There is general agreement that early life stages (i.e., eggs, larvae, and fry) arc more sensitive 

to the adverse effects of excess selenium than arc adult fish, and that some species of fish are more 

sensitive than others. 1

•

1

•

17•23•

1ll6•

1

lJ0 Selenium concentrations in the ovary/eggs as well as those in 

whole-body fish and the diet are most useful in evaluating risks of adverse effects (i.e., more useful 

than selenium concentrations in water or sediment, or in livers or other tissues). However, there is 

no uniform agreement on the selenium concentrations that represent thresholds for adverse effects. 

Selenium concentrations in ovaries/eggs are considered to represent a threat of adverse effects 

when they are greater than 10 mg/kg, 1

-1·23•

190 7 to 13 mg/kg, 17 or 17 mg/kg. 1H6 At concentrations 
ECOTOXICOLOGY OF SELENIUM 483 

exceeding the threshold, there is a large increase in the occmTcncc of teratogenic deformities and 

a concomitant reduction in survival of the larvae/fry. Although many of the larvae/fry die as a result 

of the dcfonnities, some fish may survive and have noticeable deformities as adults. Seleniumrelated deformities observed in fish larvae and fry arc due to their absorbing/using the yolk from 

the egg, which contains elevated levels of selenium deposited there by the female parent for their 

nourishment and development. Feeding excess but sublethal levels of selenium to fry, juvenile fish, 

or adult fish docs not cause dcfom1itics such as those caused through maternal transfer of selenium 

to the cgg. 1'JH'IJ 

Examples of the kinds of teratogenic deformities that are caused by selenium arc provided by 

Lcmly.23 The typical defom1ities include lordosis (concave curvature of the lumbar region of the 

spine), scoliosis (lateral curvature of the spine), k-yphosis (convex curvature of the thoracic region of 

the spine, resulting in a "humpback" condition), missing or defom1cd fins, missing or dcfom1ed gills 

and gill covers (opercle), abnormally shaped head, missing or deformed eyes, and defonned mouth. 

Selenium concentrations in whole-body fish arc considered to represent a threat of adverse 

effects to health or reproductive success when they are greater than 4 mg/kg 1

·

1

• 190 or greater than 

4 to 6 mglkg; 17 however, DeForest et aJ. 1

Hfl evaluated results of available studies and considered the 

threshold to be 6 mg/kg for cold-water and anadromous species and 9 mg/kg for wam1-water fish. 

These differences in values result from different interpretations of the available infonnation from 

the various studies. Despite the differences, however, it is important to note that whole-body 

selenium concentrations in the range of 4 to 9 mg/kg arc recognized to reflect risk of adverse effects 

in fish. 

Threshold selenium concentrations in the diet of fish that arc associated with adverse effects 

have been estimated as 3 mg/kg, 1

•

1

• 1

''11 as 3-8 mg/kg, 17 or separately as I I mg/kg for cold-water 

and anadromous species and I 0 mg/kg for warm-water fish. l!!li As described in the previous section 

(Bioaccumulation), different forms of selenium bioaccumulate at very different rates in food-chain 

organisms, with sclcnomcthioninc > selenite> selenate. These difTcrences in bioaccumulation can 

greatly affect the degree of risk associated with particular waterborne selenium concentrations. 

However, studies indicate that once the selenium is biologically incorporated in the food organisms, 

the risk to fish is similar at equivalent dietary concentrations. 

In waterborne exposures, organic selenium (selenomethionine) also is much more toxic than 

selenite, which is more toxic than selenate. For example, the 96-hour LC50 values for bluegill (13 

11g Se/L) and striped bass (!vforone saxatilis; 4 11g Se/L) based on selenomcthionine were only a 

small fraction of the LC50s for selenite (7800 to I 3,000 and 1000 ~lg Se/L, respectively) and 

selenate (98,000 and 39,000 Jlg Se/L). 1H1 However, as duration of exposure increases, the toxicity 

of selenium increases. For example, when selenium was introduced to outdoor experimental streams 

at I 0 or 30 !lg!L, all adult bluegill sunfish at the higher exposure level died in Jess than a year, 

and there also was significantly reduced survival of those at I 0 Jlg Se/L when compared to 

controls. 19·

1 Exposure of adults at both lO and 30 11g Se/L for 40 weeks before spawning rcsulled 

in reduced embryo and larval survival and produced larvae with a high incidence of edema, lordosis, 

and internal hemorrhaging. 

17.4.2.3 Amphibians and Reptiles 

There is only limited information about the toxic effects of selenium in amphibians, and there 

is even less information about its effects in rcptiles, 17•

19·195- 19H 

In laboratory exposures, amphibian embryos and tadpoles were about as sensitive as aquatic 

invertebrates and fish larvae/fry to the effects of waterborne inorganic selenium. 199- 201 In short-term 

toxicity tests (typically 96-h exposures, but up to 3 days) with the South African clawed frog 

(Xenopus laevis), LC50 values for both selenite and selenate were 1500 11g SelL or greater. Not 

surprisingly, the LC50 values decrease as exposure duration increases. The 7-day LC50 for this species 

was 456 Jlg Se/L, but in a 7-day exposure with narrow-mouthed toad (Gastropluyne carolinensis) 
484 HANDBOOK OF ECOTOXICOLOGY 

embryos, the LC50 values was only 90 Jlg Se/L.101• ::.oJ The low value for the narrow-mouthed toad 

apparently reflects species differences in sensitivity or perhaps differences in exposure conditions. 

Adult and larval amphibians exposed to trace elements in coal combustion waste ponds showed 

physiological and morphological changes as well as bioaccumulation of some elcments. 1 12•

113 For 

example, tadpoles of the bullfrog collected in a coal-ash basin and a downstream drainage swamp 

had a reduced number oflabial teeth and deformities of labial papillae when compared with tadpoles 

from reference areas. Tadpoles with defom1itics were less able to graze periphyton than were normal 

tadpoles, and they had lower growth rates. These tadpoles had significantly elevated concentrations 

of selenium (25.7 mg/kg) by comparison to a reference site (3.37 mg/kg). They also had significantly 

greater concentrations of arsenic, barium, cadmium, and chromium than those from the reference 

site, but they did not significantly bioaccumulate copper or lead in comparison to the reference 

site. Thus, it is not possible to determine which elements caused the ef'fects. 

Banded water snakes from a site contaminated by coal combustion wastes had high concentrations of trace clements, especially selenium and arsenic but also cadmium, chromium, and copper, 

by comparison to snakes from a nearby reference site. 1 Jll (Results of analyses of livers and potential 

prey items were summarized in a previous section.) Snakes from the ash-basin site exhibited mean 

standard metabolic rates 32% higher than snakes from the reference site. The authors concluded 

that snakes from the contaminated site probably allocate more of their energy to maintenance and 

theoretically should have less energy available for growth, reproduction, and storage. 

17.4.2.4 Birds 

Selenium hns been identified as the agent responsible for mortality and reproductive failure in 

birds at a number of sites in the United States (see USDI' 7 and Skorupa" for a partial list). Much 

of the research evaluating the effects of selenium in birds has been related primarily to discharges 

of subsurface agricultural drainage watcr,7·

129 but effect levels in bird diets and eggs probably are 

similar once the selenium has been accumulated by food-chain organisms. 

Selenium became recognized as a significant environmental contaminant for wildlife in 1983, 

with the discovery of developmental abnormalities and excessive embryonic and adult mortality in 

aquatic birds at Kesterson Reservoir, Califomia.7·

91 ·u::..J.t.J,.2n.J . .205 Observations in the field were corroborated through a series of laboratory studies that documented the association between excessive 

dietary selenium and effects on reproduction and young birds as well as adult health, physiology, 

and survival. Those effects are described in several other reviews (e.g., References 4, 7, 8, 13, 17, 

19, 25) and arc summarized below. 

17.4.2.4. 1 Reproduction and Young Birds 

Prior to the 1980s, most studies of avian reproductive effects in relation to selenium were 

conducted with domestic poultry:1.7· 13 Studies of wild birds to determine the effects of selenium on 

reproduction and other aspects of avian biology have been conducted in the field and in the 

laboratory since 1983. Reproductive effects of selenium reported in wild aquatic birds and domestic 

poultry include embryo mortality and teratogenesis as well as the failure of adult birds to nest. 

Excess selenium in the diet of female birds during the period just before egg-laying results in 

transfer of selenium to the eggs at harmful levels, although the sensitivity varies among specics.7·11·13·25·128·129·10fi Characteristic effects observed in the field and laboratory studies include reduced 

hatchability of eggs (due to embryo mortality) and high incidence of developmental abnormalities 

(due to teratogenesis). Selenium-induced abnormalities are often multiple and include defects of 

the eyes (microphthalmia and possible anophthalmia [i.e., abnormally small or missing eyes]), feet 

or legs (amelia and ectrodactylia [absence of legs or toes]), beak (incomplete development of the 

lower beak, spatulate narrowing of the upper beak), brain (hydrocephaly and exencephaly [fluid 

accumulation in the brain and exposure of the brain]), and abdomen (gastroschisis [an open fissure 
ECOTOXICOLOGY OF SELENIUM 485 

of the abdomen]). Most of these abnonnalitics are illustrated through photographs that have been 

published elsewhere (e.g., References 4, 7, 8, 132, 144). 

Reproductive effects have been produced in laboratory dietary studies using both organic and 

inorganic forms of selenium (e.g., in mallards and domestic chickens [Gallus gallus]) . .J·131.!32·1:.'A,J:.:s.!o7-210 Selcnomcthioninc, the most bioavailablc and toxic of the different selenium 

compounds studied, is associated with reduced reproductive effects in mallards at dietary concentrations as low as 7 mg Se/kg. 135 This form of dietary selenium has been found to be the best 

surrogate for use in feeding studies to parallel the dose-effect levels found in field studies with birds. 

Studies at Kesterson Reservoir showed there was poor hatching success of fertile eggs of a 

number of aquatic bird species7·

20" and that there was also poor survival of those young that did 

hatch.7·

21 1 This was attributed to high levels of selenium in the diet of the breeding females as well 

as the diet of newly hatched birds. Most aquatic insects at Kesterson had mean selenium concentrations greater than lOO mg/kg, and some composite samples exceeded 300 mg/kg:10- 12 Mallard 

ducklings were fed diets containing either sodium selenite or selenomethionine at concentrations 

of 0, I 0, 20, 40, or 80 mg Se/kg from hatching to 6 weeks of age.212 Among ducklings that received 

selenomethionine, 100% of the group receiving 80 mg Se/kg died in less than 6 weeks, and 12.5% 

of those that received 40 mg Sc/kg died during the exposure period. 

Predictive criteria for avian selenosis were developed through a broad-scale program that 

collected response data for avian teratogenesis at selenium-impacted and reference aquatic sites in 

the San Joaquin Valley, Califomia.l1·206 The largest cumulative sampling effort occurred within the 

Tulare Basin area in the soulhem San Joaquin Valley. In the Tulare Basin, evaporative disposal of 

subsurface irrigation drainage water was accomplished by letting it evaporate from 25 shallow 

impoundments. Even though the impoundments were not constructed to attract wildlife and were 

devoid of emergent vegetation (i.e., cattails, bulrush, etc.), large populations of nesting waterbirds 

used the sites. Two of the most abundant waterbirds that nested at the evaporation basins were 

American avocets (Recurvimstra americana) and black-necked stilts (Himantopus mexicanus). 

Water discharged to the evaporation basins contained from < 1 to > 1000 ).lg SelL; consequently, 

selenium concentrations in eggs from this region spanned four orders of magnitude(< l to> 100 

mg!k:g egg selenium). 

Using data on selenium in eggs from the Tulare Basin, combined with data from several other 

western sites where elevated selenium was found, a detailed exposure-response relationship was 

documented. 17•

25•206 Statistically distinct teratogenesis response functions were delineated for ducks, 

stilts, and avocets using the Tulare Basin data. The Tulare curves were used to estimate expected 

frequencies of teratogenesis for ducks, stilts, and avocets using other sites, and the predicted levels 

were tested against the observed frequencies from the sites. The predicted and observed frequencies 

of teratogenesis were not significantly different, and therefore the data were combined to generate 

final response curves. Using this data set, Skorupa2t)(; developed species-specific response curves 

for stilts and avocets and a composite duck curve (using combined data from gadwalls [A11as 

strepera], mallards, pintails [A. acuta], and redheads [Aythya americana]). 

Based on the response coefficients and their standard errors, the teratogenesis function for 

ducks, stilts, and avocets were significantly difTercnt.206 Within this data set, these responses 

represent "sensitive" (duck), "average" (still), and "tolerant" (avocet) species. The probability of 

overt teratogenesis in stilts increases markedly when selenium concentrations are greater than 40 

mg/kg, with an EC10 for teratogenic effects of 37 mg/kg. In contrast, the threshold for teratogenesis 

(expressed as an EC10) in mallards is 23 mg/kg, and in avocets it is 74 mg/kg. 

A more sensitive measure of avian selenosis is reduced egg hatchability due to embryo inviability.17·25·206 Egg selenium concentrations that cause embryo inviability are usually below the levels 

that cause embryo deformities. The threshold for mean egg selenium associated with impaired egg 

hatchability at the population level for black-necked stills (and, therefore, the embryoloxicity 

threshold) was estimated to be 6 to 7 mg/kg. This threshold is approximately equivalent to the EC10 

on a clutchwise (or henwise) basis and the EC03 on an eggwise basis.33 Skorupa206 used more than 
486 HANDBOOK OF ECOTOXICOLOGY 

400 sample eggs from black-necked stilt nests monitored at Tulare Basin, Kesterson Reservoir, and 

Salton Sea, California to identify the background rate of inviable stilt eggs (8.9%) and developed 

the following equation to determine the number of clutches containing at least one inviable embryo: 

Y = exp(-2.327 + 0.0503X)/(l + exp(-2.327 + 0.0503X)); r' = 0.18 (17.1) 

where Y is the probability of;:: I inviable egg in a sampled clutch based on selenium concentration 

in a random sample egg (X). In contrast to the stilt, with a threshold of 6 to 7 mg Se/kg for effects 

on egg hatchability, the avocet is much less sensitive, with no effects on hatchability at concentrations below 60 mg Sc/kg. 

In his review of the available information concerning reproductive effects of selenium in birds, 

Heinz 13 suggested 10 mg Se/kg (converted from 3 mg Sc/kg wet-weight basis he reported) in avian 

eggs as a threshold for c!Tccts on hatchability. Conclusions concerning egg-selenium threshold 

levels for effects in waterfowl depend on which data are included in the analyses, how the effects 

arc expressed, and statistical approaches used in the analyses. For example, Fairbrother et al.32·

3

·

1 

used some of the data available from experimental studies with mallards and concluded that the 

threshold for reduced hatchability (ECwl is 16 mg Sc/kg in the egg. However, Skorupa" disagreed 

wilh this conclusion. 

The data available from six studies in which mallards were fed sclenomethionine 12 1. 122·

12.J.l:!5,207·

208 

are reanalyzed here using a two-parameter logistic regression model to calculate various effect 

levels for selenium, as follows: ECw = 12.5 mg/kg; EC20 = 16.3 mg/kg; EC50 = 25.7 mg/kg in the 

eggs; in the diet, ECw = 4.87 mg/kg; EC20 = 5.86 mg/kg; and EC10 = 8.05 mg/kg. Table 17.1 

provides a brief summmy of the relevant studies, including the dietary treatment level of selenium, 

reported egg hatchability, and mean selenium concentrations in the eggs. In addition, the table 

provides the egg hatchability results for the treatment groups as a proportion of the controls (to 

Table 17.1 Effects of Dietary Selenium Exposure (as selenomethlonlne) on Egg Hatchability in 

Mallards and Associated Selenium Concentrations (mg/kg, dry weight) in Eggs 

Dietary Selenium Egg Hatchability as Egg Selenium 

(mg/kg)" N Hatchability (%)' Proportion of Control (mg/kg)' Reference 

Control 11 65.7 1.0 0.16 121 

10 5 30.9 0.470 15.2 

Control 32 59.6 1.0 0.59 122 

1 15 70.7 1.19 2.74 

2 15 60.0 1.01 5.28 

4 15 53.4 0.896 11.2 

8 t5 36.9 0.619 36.3 

16 9 2.2· 0.037 59.4 

Control 10 41.3 1.0 1.35 207 

10 Qld 15 7.6· 0.184 30.4 

10 L0 15 6.4. 0.155 29.4 

Control 12 44.2 1.0 1.16 208 

10 11 24.0 0.543 25.1 

Control 37 88.2 1.0 1.4 124 

10 35 2o.o· 0.227 37 

Control 33 62.0 1.0 0.89 125 

3.5 29 61.0 0.984 11.6 

7.0 34 41.0. 0.661 23.4 

n Presented as nominal concentration in diet. In some (but not all) studies, control and treated diets were 

analyzed. Control diets typically contained 0.4 mg/l<g selenium. 

b Asterisks indicate hatchability significantly different than controls. 

c When mean selenium concentrations in eggs were reported on wet-weight basis, concentrations were 

converted to approximate dry-weight basis, assuming 70% moisture. 

d Seleno~DL-methionine. 

~ Seleno-L-methionine. 
ECOTOXICOLOGY OF SELENIUM 

1.2 

:g 1.0 

8 

ui 0,8 > 

"' c 

~ 0,6 

• J: 

c 

g 0.4 

8. 

e 

0.. 0.2 

• 

• 

- Fined do~e-re~pon~e 

- 5% LCL on lolled model 

- 95'.:. UCL on filled model 

0 Clb~Nvcd doge.te~ponne 

EC10 = 4.07 mg/kg 

5% LCL = J,51i mg/kg 

115% UCL = 5.74 mglkg 

• 

• 

0.0 ·!------~-·----~~..;::,=-~ 

0.1 1.0 10.0 

Se In D!et (mglkg, dry wt.) 

100.0 

Figure 17.1 Mallard egg hatchability vs. control as a function of selenium concentration in diet. 

487 

take into account the variable hatching success for controls in the various studies). The results of 

these studies arc illustrated graphically in Figures 17.1 and 17.2. 

Fertility has been measured in only a small number of selenium studies. In American kestrels 

fed selenomethionine at 12 mg Se/kg, egg fertility was significantly reduced (by over 14%) 

compared to kestrels fed 6 mglkg selenium."' Lack of reporting on fertility effects may be due in 

part to a general practice of simply including infertile eggs as inviable eggs in studies of selenium 

effects in birds (i.e., "infertility" effects may not be separated from "embryotoxic" effects in the 

overall measurement of hatchability). Failure to measure infertility as a separate endpoint may be 

due to the difficulty often associated with distinguishing infertile eggs from those containing 

embryos that have died very early in development. Nevertheless, decreased fertility is a distinct 

effect from embryotoxicity, particularly in that it indicates a mechanism acting on adult, rather than 

embryonic, physiology. Results obtained in kestrels suggest infertility may be a potentially important factor contributing to the overall reproductive impairment in some species. However, in mallards 

and black-crowned night-herons fed 10 mg Se/kg as selenomethionine, egg fertility was not reduced 

compared with controls. 121 •

123•207•

10H Similarly, fertility was not affected in mallards fed diets containing selenium at 7 mglkg'" or 16 mglkg1" as selenomethionine, but hatchability of fertile eggs 

1.2 

0 1,0. • 

b 

c 

0 

u 

ui O.B > 

~ 

c 

~ 0.6 

0 

J: 

~ 0.4 

• 

• • •• 

il. - F!tl~d dose-response 

e - 5% LCL on filled model 

0... 0.2 - 95% UCL on fitted mode! 

D Otlserved dose-response 

1.0 

• 

10.0 

EC10 = 12.5 mglkg 

6% LCL = li.4 mg/kg 

!Hi% UCL"' 16.5 mg/kg 

100.0 1000.0 

s~ In Eggs (mg/kg, dry wt.) 

Figure 17.2 Mallard egg hatchability vs. control as a function of selenium concentration in eggs. 
488 HANDBOOK OF ECOTOXICOLOGY 

was significantly reduced. Thus, effects on egg fertility in mallards and night-herons arc not likely 

to be as ecologically significant as reduced hatchability. 

17.4.2.4.2 Adult Health, Physiology, and Survival 

O'Toole and Raisbeck' have recently provided a comprehensive review of the various kinds of 

lesions associated with acutc-subchronic and chronic selenosis in avian species. Other reviews of 

effects in adult birds also have been published rcccntly.H.u,t!J 

Laboratory studies have been conducted with mallards to determine the kinds of lesions and 

other measurements that can be used for diagnosis of selenium toxicosis in birds. 1·Hl,:::IJ.:::t.J In general, 

ducks that received diets containing more than 20 mg Se/kg developed a number of lesions of the 

liver and integument. Those receiving 40 mg/kg or more selenium in their diets lost weight and 

had abnormal changes in the integument that involved structures containing hard keratin, such as 

feathers (alopecia/depterylation [i.e., feather Joss]), beaks (necrosis), and nails (onychoptosis 

[soughed or broken}). When corroborated by elevated selenium concentrations in tissues (especially 

the liver), these integumentary and hepatic lesions, as weil as weight loss, can serve for diagnosis 

of selenium toxicosis in birds. It should be noted, however, that some birds died without exhibiting 

any significant morphological lesions, even though they were emaciated. 

Selenium toxicosis effects had been described previously in several species of aquatic birds 

found at Kesterson Reservoir from 1984 to 19867 •

13"· 1

·1·1 Those birds exhibited many of the same 

signs of selenosis as those later found in mallards (as described above) including hepatic lesions, 

alopecia, necrosis of the beak, and weight loss. As described in a previous section, most kinds of 

aquatic invertebrates at Kesterson had mean selenium concentrations greater than I 00 mg/kg. Thus, 

the laboratory studies with mallards provided strong evidence that selenosis was the main factor 

responsible for morbidities and mortalities among birds at Kesterson Reservoir.Z1

·

1 

In a I 6-week exposure to evaluate susceptibility of mallards to over-winter mortality as a result 

of selenium exposure, ducks that received dietary concentrations of 20, 40, or SO mg Se/kg (as 

selenomethionine) had higher mortality rates than ducks receiving the control or 10 mg Se/kg 

diets. 1

•

17 After 1 week of treatment, body weights were significantly depressed by the 20, 40, or 80 

mg Se/kg diet. Ducks that survived for 16 weeks on the 20 mg/kg treatment were returned to a 

control diet; after 4 weeks, their body weight was similar to that of the control birds. Concentrations 

of selenium in blood were related to dietary treatment levels, but mortality was not clearly related 

to a threshold selenium concentration in the blood. 

A number of studies (e.g., References 144, 215-218) have described physiological changes that 

arc associated with selenium exposure in field-collected or laboratory-exposed birds. These generally involve changes in measurements associated with liver pathology and glutathione metabolism 

(e.g., glycogen, protein, total sulfhydryl and protein-bound su!lbydryl concentrations; glutathione 

peroxidase activity). 

A few studies have suggested that effects of selenium on avian immune function are a sensitive 

endpoint and may afTect birds at relatively low exposures.z 19 -

111 However, immunotoxicity of 

selenium has not been studied sufficiently to provide a threshold-effect level in wild birds.:!06 

17A,2.5 Mammals 

Most studies of the effects of excess selenium have focused on livestock and laboratory animals, 

and there have been no well-documented cases of significant selenosis problems among wild 

mammals. 17 The most common cause of chronic selenosis is consumption of forage containing more 

than 5 mg Se/kg. Sublethal effects were observed in dogs fed diets containing about 7 mg Se/kg.2 

O'Toole and Raisbeck1 have recently provided a comprehensive review of the various kinds of 

lesions associated with acute-subchronic and chronic selenosis in mammalian species, and they 

stress the importance of using multiple lines of evidence (e.g., selenium concentrations in tissues, 
i 

I 

I 

I 

I 

ECOTOXICOLOGY OF SELENIUM 489 

presence and types of lesions) in diagnosing selenosis. Subchronic and chronic selenosis (sometimes 

called "alkali disease") commonly involves changes in the integument, especially the hooves, horns, 

and hair. Loss of hair is a common sign of selenosis in a number of mammalian species and is 

similar to the bilateral loss of feathers observed in selenium-poisoned birds (described above). 

Lesions of the liver occur in some species of mammals (sheep, dogs, cats, and rats), but they 

arc not particularly significant in most livcstock:1 Unlike birds, congenital abnormalities arc not 

commonly associated with selenium toxicosis in mammals. When congenital abnormalities occur, 

the female parent usually was exposed to near-lethal dietary concentrations of selenium. 

At Kesterson Reservoir, which contained seleniferous agricultural drainage waters/ small mammals and raccoons were trapped and examined to determine whether selenium adversely affected 

the health of adult animals or their reproduction. 15h,IM' Despite very high concentrations of selenium 

in livers or whole-body small mammals, or in livers, hair, feces, or blood of raccoons, there was 

no evidence of adverse effects in the mammals. Parameters evaluated for the small mammals 

included body size, body condition, liver size, and reproduction (including examination for malformations). Those measures as well as histopathology of selected tissues (liver, kidney, and spleen) 

and blood chemistry were evaluated in the raccoons. 

17.4.3 Interactions 

Many studies have shown interactions between selenium and other inorganic elements (especially arsenic, sulfur, and various metals), vitamins A, C, and E, as well as sulfur-containing amino 

acids. 7-

19-·1

·

1

··

16-56-

222 The interactions may be synergistic or antagonistic in terms of effects on uptake 

and metabolic effects, and the degree of interaction is affected by numerous factors. Thus, the topic 

of interactions is too complex to be addressed in detail in this review, and only a few examples of 

recent studies are discussed. Readers are encouraged to consult some of the references cited above 

for more details and a further introduction to this topic. Nevertheless, some of these interactions 

can be important factors in the design of field or laboratory studies and in the evaluation of results, 

and they should be taken into consideration. 

Interactions between mercury and selenium have probably been studied more intensively in 

more kinds of animals than most of the others listed above. Mercury and selenium concentrations 

in the livers of various free-living carnivorous mammals often are highly correlated in a molar ratio 

of I: 1. 19•

109•

223·

22·

1 However, there is no consistent pattern for such a correlation in the livers of birds. 

For example, hepatic mercury and selenium were correlated, with an overall mercury:sclenium 

molar ratio of l :6, in diving ducks (surf seaters [!vlelanitta perspicillata] and greater scaups [Aythya 

marila]) from San Francisco Bay, California, during 1982. 1

•

1

n In a subsequent study of scalers 

sampled twice at six locations in the Bay during 1985, mercury and selenium in the livers were 

not correlated; mercury:selenium ratios were typically between 1:7 and 1:15 for most locations 

and collection times, but the mean ratio at one site was 1:45.225 

Elsewhere, mercury and selenium concentrations were positively correlated in some bird livers, 

but nol in others, or they were negatively correlated (see review by Ohlendorf-'16 ). These relationships 

may change as birds remain at the sampling location (due to differential accumulation and loss 

rates for mercury and selenium), they may vary because of differing relative concentrations of the 

two clements, and other factors (such as the chemical forms present) also may complicate the 

patterns of bioaccumulation. 

In the selenium-mercury studies discussed above, concentrations were reported for total selenium and total mercury. However, recent studies by Benny et aJ.227 and Spalding et alY11 have shown 

high correlations of selenium with inorganic mercury on a molar basis in fish-eating birds. Those 

authors suggested that selenium may contribute to the sequestration of inorganic mercury, thereby 

reducing its toxicity. This conclusion would be consistent with the results of a selenium-mercury 

interaction study with mallards by Heinz and Hoffman20B discussed below. 
490 HANDBOOK OF ECOTOXICOLOGY 

Interactive effects of selenium with arsenic, boron, or mercury have been evaluated in reproductive studies with mallards.L2·1.1 25,

208 Each of the studies involved varying levels of dietary exposures of breeding mallards to selenium alone, one of the other clements alone, and also selenium 

in combination with the other chemical. In each study, selenium and the other chemical caused 

significant adverse effects on reproduction when present alone in the diet at higher treatment levels, 

but the interactions varied by chemical. Antagonistic interactions between arsenic and selenium 

occurred whereby arsenic reduced selenium accumulation in duck livers and eggs and alleviated 

the effects of selenium on hatching success and embryo defonnitics. There wns little evidence of 

interaction between boron and selenium when ducks were fed the two chemicals in combination. 

Selenium provided a protective effect that reduced ti1c toxicity of mercury to adult male ducks. 

However, when ti1e diet contained 10 mg Se/kg plus 10 mg Hg/kg, the effects on reproduction 

were worse than for either selenium or mercury alone. The number of young produced per female 

and the frequency of teratogenic effects were significantly affected by the combination of mercury 

and selenium in the diet, and mercury also enhanced the storage of selenium in duck tissues. 

17.5 SUMMARY AND CONCLUSIONS 

Selenium is a naturally occurring trace element that is essential for animal nutrition, but it 

becomes toxic to sensitive species of fish or birds at dietary concentrations about two or three times 

the background levels found in many food-chain organisms. The biogeochemistry of selenium is 

complex, and it is essential to understand selenium chemistry to enable competent predictions of 

its ecotoxicology. Selenium concentrations generally are elevated in areas where soils were derived 

from Cretaceous and Tertiary marine sedimentary rocks in the western United States. Irrigation of 

such soils has led to contamination of wetlands receiving drainage from the agricultural lands. 

Industrial sources (such as coal and oil production or combustion and phosphate mining) have 

contributed selenium to habitats where fish and wildlife can be exposed. Once released to the 

surface environment, selenium cycling is affected by many biogeochemical processes, especially 

in aquatic habitats. 

Selenium bioaccumulates in aquatic and terrestrial food chains and in higher trophic-level 

animals that feed on those plants and animals. The greatest level of' bioaccumulation is typically 

from water to aquatic plants or invertebrates (often a 1000-fold or greater increase occurs). Background concentrations have been fairly well established for plants and animals in freshwater and 

terrestrial ecosystems, and these values can be used in evaluating the selenium status of an area if 

field-collected samples are analyzed and compared to background values. Such studies have been 

conducted in a number of areas. Multitrophic-levcl laboratory studies also have been conducted 

and help interpret the significance of bioaccumulation of selenium. 

Studies of the effects of selenium have focused mainly on domestic livestock and on fish and 

aquatic birds because those animals have been found to be sensitive to the adverse cll'ects of 

excess selenium. In fish and birds, the early life stages (fish larvae/fry, bird embryos) are most 

sensitive, although lethal and sublethal adverse effects also occur in adult animals under field 

conditions in some areas. Teratogenic effects in fish larvae/fry and in bird embryos may lead to 

death of young that hatch, but a significant amount of mortality also may occur without visible 

developmental abnormalities. 

Many studies have shown interactions between selenium and other inorganic elements (especially arsenic, sulfur, and various metals), vitamins A, C, and E, as well as sulfur-containing amino 

acids. These interactions may be synergistic or antagonistic in tem1s of effects on uptake and 

metabolic effects. The interactions of selenium and mercury often are of particular interest, and 

there is no universal rule of thumb as to the net result of these interactions. For example, in mallards, 

selenium provided a protective effect that reduced the toxicity of' mercury in adult ducks, but the 
ECOTOXJCOLOGY OF SELENIUM 491 

effects of mercury plus selenium in the diet caused greater impairment of reproduction than either 

chemical alone. 

Overall, there is enough information to thoroughly evaluate the significance of selenium as an 

environmental contaminant if it is included among the chemicals of potential concern when ecological risk assessments are planned and conducted.:20·

1 The main difference between selenium and 

most other contaminants is that it is essential to have a good understanding of selenium's occurrence, 

complex biogeochemistry, and ccotoxicology to avoid serious errors. 

ACKNOWLEDGMENTS 

I appreciate the assistance of Gary M. Santolo in providing some of the material for the section 

on effects in birds and Bradley E. Sample in performing the statistical analyses ol' relationships 

between effects in mallards and the concentrations of selenium in the eggs or diet. Gary H. Heinz 

reviewed the manuscript and provided useful comments. 

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ECOTOXICOLOGY OF SELENIUM 495 

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