### 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

..........................................................................................................................477 17.3.5.1 Eggs

...........................................................................................................477 17.3.5.2 Livers

.........................................................................................................478 I7.3.5.3 Kidneys

......................................................................................................478 I7.3

### .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 I7.3.6.2 Terrestria!

...................................................................................................480 17.4 Effects in Animals

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

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

.............................................................................................................481

465466 HANDBOOKOF 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 ofconcern. 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 ofthe 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 ofnew information related to food-chain bioaccumulation and effects ofselenium 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 freshwaterconsiders selenite and selenate as relative fractions ofthe total recoverable selenium concentration. The environmental significance of selenium is a topic of ongoing discussion and debate. A recent issue ofthejournal

### Aquatic Toxicology (Vol. 57, No. 1) was devoted to papers on selenium. In December 1999, thejournal 

### Human ami Ecological Ri.r;kAssessment devoted an issue (Vol. 5, No. 6) to debate and commentary on the topic, "Selenium -Apotential time bomb orjust another contaminant?" and published several papers on the topic. Needless to say, the debate continues, as notall theissues wereresolved. In particular, thereare differencesofopinionconcerningwaterborne concentrations ofselenium 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 ofbirds at which reproduction is adversely**affected. 32** **1** These topics

are discussed in later sections ofthis chapter. Selenium concentrations typically are reported as micrograms per liter ()..lg/L) in fluids and milligrams per kilogram (mglkg) ormicrograms pergram ()..lg/g) in soil, sediment, plant oranimal tissues, and diets. Concentrations in soil, sediment, tissues, and diets can be expressed eitheron aECOTOXICOLOGY OF SELENIUM 467



### wet-weightorfresh-weightbasis (which are considered to be synonymous),oron dry-weightbasis. Conversion from one basis to the other is a function ofthe moisture content in the sample (which should be reported regardless ofwhich basis is used), as follows:

D . I W . I 100

ry-wctg 1t 

### cone. = 

### ct-wctg 1t

### cone. x ( 100 M . )

- oisture percentage

Forexample, 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 ofselenium chemistry can help define geographic areas and environmental settings in which selenium may be a chemical ofecological 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 occuras a productofcombustion ofelemental 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, orthey can be slowly reduced to selenites,which also can be taken upby 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 ofseveral 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 seleniumcan be evaluated mostreliably as total recoverable concentrations, 17 

### and total recoverable concentrations are the basis ofthe ambient water quality criteria for freshwater ecosystems. 20·22 

### This is because partitioning ofselenium between the water column and othercompartmentsofan aquaticecosystemgreatlyaffects themeasurementorwaterborneselenium. Low waterborne selenium concentrations can ref-lect low mass loading ofthe 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 ofbiotic uptake) and thus more accurately reflects the total mass load ofselenium in the system. Differences between dissolved and total recoverable measurements increase with higher levels ofeutrophication ofthe water body. During the early and mid-1980s, subsurface agricultural drainage waters from the San Joaquin Vaiicy, California, were disposed ofby discharging themto Kesterson Reservoir, which was a series of 12 shallow ponds totaling about 500ha.7 

### Seleniumconcentrations in waterentering thereservoir during 1983 to 1985 averaged about 300 J.lg 

### SelL, and aquatic plants and invertebrates contained468 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. Unlikeboron and a numberofotherconstituents, seleniumconcentrations

### decreasedas 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

. ..J3 

### 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 ofthe 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 importantexposure pathway for vertebrate animals and should be included when conducting

orevaluating 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 ofhuman 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 sourcesis highly mobile and biologically available in arid regionshavingalkaline 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 seleniumcan be mined as a primary productY It is found mainly in sulfide minerals ofcopper, iron, and lead and is most commonly producedby electrolytic separation from copperduring 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% ofdemand) and electrical (30%) applications representing most ofthe 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 offly 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 ofeffects 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 wastewaterfrom the refinery, thereby contributing to bioaccumulation by organisms in the receiving waters.:!5

### .5.J Mass-loadingofseleniuminto aquatic environmentsmosttypically resulls from disposal ofcoal fly ash, irrigation wastewater, or 

### oil refinery wastewater.!7,:!S,s.J Mining or smelting ofsulfide ores, uranium, phosphate, bentonite, and coal also contributes to mobilization ofselenium because ofits presence in the mined materials orthe overburden that is moved during the mining process. 17•19••17• 55ECOTOXICOLOGY OF SELENIUM 469



### In freshwater systems, nonnal background levels ofselenium 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 ofselenium 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 ofparticulate matterderived 

### by erosion ofolder (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:1 H-so 

### Geology, climate, and hydrology were found to be important factors affecting the mobilization and transport ofselenium into habitats where biota arc adversely affected in the western United States. Although selenium can be transported naturally as a result ofrockwca.thering and dmina.gc into waterbodies, theprocess has been increasedthrough irrigation ofsoils derived from sclenifcrous formations, especially Cretaceous and Tertiary marine sedimentary deposits. Many biogeochemical processes affect the cycling ofselenium through different components of the environment. 7 •10•

3552

·57• 5H

### In wetlands, selenium oxidation-reductionreactions arc the most importantprocesses controlling speciation, precipitation/dissolution, sorption/desorption, methylation, and volatilization ofselenium. Selenateis stablein well-oxidized environments, butitisconverted slowly to selenite orelemental 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 importantconsiderations 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 oforganic 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 ofthis selenium in alkaline soils produces selenate, making itreadily available to plants. Seleniumuptakeby plants is affectedby both theconcentration 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 smelter 59), 

### 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% ofplant leaf selenium uptake. Deposition occurs either as "wetdeposition" (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 ofsoil/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 ofthe 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 ofthese 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 waterthrough acombination ofbioaccumulation 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 ofselenium-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.7 1,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 ofselenium-containing biota (i.e., the food chain). In aquatic systems, bioaccumulation may occurby direct absorption orpartitioning from water orsedimentorby ingestion. The greatest level ofbioaecumulation is typically from water to aquatic plants or invertebrates (often a !ODD

### fold orgreaterincrease 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 successivetrophic 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 ofselenium bioaccumulation by plants and animals. Selenium concentrations typically are higher in marine organisms than in those from freshwater ecosystems, and they are higherin areashavingselcniferoussoilsorsediments andin areasreceiving industrial, agricultural, or municipal wastes. Additional information, including more detailed tabulations ofselenium 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

- 12 

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



### 1105) times the concentration in water:EElevated concentrationsofselenium also have been found in plants from ponds receiving coal fty 

### ash.74
· 75 

### The bioaccumulation and volatilization ofselenium 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 watercontaining selenite or sclcnate.66 

### Several ofthe aquatic plant species showed selenium volatilization and accumulation rates that were similarto that ofIndian mustard 

### (Brassicajuncea), 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 ifit 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 ofexposure to birds. Several laboratory microcosm studies (summarized below) have measured selenium bioconcentration (i.e., directuptake) by algae from water and subsequenttransferofselenium to invertebrates

orfish consuming the algae. These studies show that the chemical form ofthe waterborne selenium is an important factor in affecting uptake by aquatic biota and that sulfate content ofthe water can influence uptake. These and other similarlaboratory studies ofaquatic systems show that selenium bioaccumulation is a significant mechanism ofconcern 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 turnhad higherbioconcentrationfactors than selenate.76

### Volatilization ofselenium 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 offoodborne selenium by daphnids and blucgills was similar in food chains dosed with different forms ofselenium, 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 ofinorganic 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 arecommoninsects 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 ofselenium also was followed through a laboratory food chain (water-algae-rotiferslarval fish), and its effects on the larval fathead minnows 

### (Pimepha/esprome/as) were evaluated."472 HANDBOOK OF ECOTOXICOLOGY

Selenium uptake was measured in rotifers 

### (Braclzionus calycijlorus) fed algae 

### (Chiarellapyrenoidosa) 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 ofcontrols, 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 SanJoaquin Valley, CaliforniaJ9 Theseplants were used as a substrate to measure seleniumbioaccumulation and effects in a laboratorybenthic-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 ofselenium in aquatic**systcms.HO-H~** Waterborne sulfate reduces the uptake ofselenium 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 otherstudies summarized above, uptake ofsclcnomethioninc (BCFs up lo 2 I,800) by widgeongrass was much higherthan for selenite orselenate, and the highestrates ofuptake (i.e., BCFs) occur at the lowest waterborne concentrations. Results ofthis and other field and laboratory studies show that waterborne selenium occurs under field conditions in a mixture ofchemical 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.1H 5 Bioaccumulation ofselenium by plants depends on the species ofplant, environmental conditions, age and phase ofplant growth, and the chemical form ofselenium 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-containingplants like the 

### Brassica species (mustard, cabbage, broccoli, andcauliflower) and otherBrassicaceae arc relatively good accumulators ofselenium. Three groups of plants are generally recognized on the basis of their tendency to accumulate selenium when grown on high-selenium soils (as originallydescribedby Rosenfeld and Beath3). The first two groups ofplants arc referred to as 

### selenium accumulator or 

### indicator plants. These plants grow well on soils containing high levels ofavailable 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 occurduring the most active growth phase.3• 85 Because selenium is associated with protein in the plant, leaves usuallycontain higherseleniumconcentrations than seeds (e.g., see also SchulerECOTOXICOLOGY OF SELENIUM 473



### ctal..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 ofsignificant effects ofselenium in aquatic birds feeding at Kesterson Reservoir, California, inflow ofagricultural drainage waterwas halted, the reservoirwas**dcwater~d,** 

### and lower-elevation portions ofthe site were filled with soil to prevent groundwaterfrom 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 ofthree 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 descriptionsofthe siteand itsearlyhistory areprovidedby Ohlendorf"andOhlendorfandHothcm.91 

### Biological monitoring ofKesterson Reservoir has been conducted annually since 1988 and has included extensive sampling and analyses ofsoil (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 ofselenium 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 thelimited datasetavailableatthattimeand again with amuch largerdatasetin 2000.H 9•

90 

### Thekey 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 

### < 0.00l) 

### 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 of3.4 mg/kg; in 1998, the range in 240 samples was 0.20 to 68 mg/kg, with a geometric mean of3.8 mg/kg. Uptake and accumulation ofselenium by plants growing on lly-ash landfills has been studied extensively and reviewed recently. 1H

### Concentrationsofseleniumin vegetation vary by plantspecies, with legumes typically accumulating more selenium than grasses. Although some other elements (such as boron and molybdenum) also bioaccumulatein these plants, this docs notoccurto elevated concentrations as consistently as the bioaccumulationofselenium.Application ofsulfurin 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 ofselenium 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 freshwaterandestuarineinvertebratesfrom areas receiving subsurface agricultural drainage;10

- 1 2.H2,92 

### urban/industrial discharges,93 

### and in ponds or lakes thatreceived selenium-containing fly ash.75

·9

### .J.-tJ(, Among the aquatic invertebrates sampled at Kesterson Reservoir, California, seleniumconcentrations were highest in benthic species, such as midge larvae (Chironomidae) and dragonfly and damselfly nymphs (Odonata), and lowest in water boatmen (Corixidae):",_,' Mean concentrations474 HANDBOOKOF ECOTOXICOLOGY



### in some ofthe 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 ofthe 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.'1H 

### 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
Backgroundconcentrations 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 weightHKl), 

### 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 ofterrestrial invertebrates (various kinds ofinsects as well as spiders) has beenconducted at Kesterson Reservoir, California sinceits 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 ofcarnivorous invertebrates in 1995 was 18 mg Se/kg (range4.5 to 48 mg Se/kg), and the geometricmeanfor 110 samplesofnoncamivorous 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 I7.I mg Se/kg (range 8.I to 36 mg Se/kg), and the geometric

mean for 104 samples ofnoncarnivorous 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 offreshwater fish are 2 to 8 mg Se/kg, and in othertissues 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 I976 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 ofwhich are in arid areas ofthe western United States in basins containing substantial amounts ofirrigated agriculture. One exception to that pattern is the Waikele Stream site atWaipahu, Hawaii, where the 85th percentile values were regularly exceeded.ECOTOXICOLOGY OF SELENIUM 475



### Elevated levels ofselenium have been reported in fish primarily from seleniferous areas ofthe 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 ofthe 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 

### ifthe fish ofconcern 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 containedexceptionallyhighconcentrationsofselenium.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 atthereference site. In 1984, onecomposite samplefrom Kesterson contained430mg Selkg, and the means for fish from two ofthe ponds exceeded 300 mg/kg. Laboratory bioaccumulation studies discussed above describe bioaccumulation ofselenium by fish in relation to different waterborne forms ofselenium and illustrate the importance ofdietary exposure as a route ofselenium uptake by fish. Studies report more frequently the rates ofuptake ofdifferent forms ofselenium than depuration after exposure to uncontaminated media (food and water). In one study,'10 

### cycling and fate ofselenite 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 ofmagnitude greater than for selenite, but elimination rates were similar (different by less than a factor oftwo). This study also demonstrated ll1e importance ofsediment and organic detritus in the recycling/remobilization ofselenium 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 arevery similar. 19

### Higherconcentrationswere found in speciessuchas tunas andmarlins. 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 

### KasegalikLake (on the BelcherIslands, Hudson Bay, in the southern CanadianArctic) is known to have bol11 salt- and freshwater populations ofArctic char 

### (Salve/inus a/pinns). 111 

### Fish ofboth 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 ofselenium bioaccumulation in amphibians and476 HANDBOOK OF ECOTOXICOLOGY



### reptiles have been conducted than those ofotheranimals/· 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 suggestedthat they may be good indicators ofcontaminatedenvironments.To evaluate theeffects ofgutcontents (especiallyingested sediment)

on analyses ofbullfrog tadpoles, wild-caughttadpoles were maintainedin clean waterand analyzed after 0, 24, 48, and 72 h and then analyzed for metals to test the effects of "clearing" ofthe gut contents. 115 

### Metals concentrations in whole bodies and digestive tracts also were evaluated separately. Selenium concentrations in the bodies and tails ofthe tadpoles were unaffected by clearing, although seleniumconcentrationwas higherin the digestive tract than in thebodywithoutdigestive 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. 1Hi 

### 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 ofbackground, 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 ofthese 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 

### (Nerodiafasciata) from ash-settling basins associated with a coal-burning electric powerplant were analyzed for selenium and several otherelements 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 ofsnakes 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 seleniumconcentrations 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 aboutequal to the prey item levels. Seleniumconcentrations 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 SanJoaquin Vailcy averaged 0.7 to 2.0mg/kg.ll6

### Livers ofgopher snakes (Pituophisme/anoleucus) 

### from other reference sites in the San Joaquin Valley contained mean selenium concentrations of 2.05 and 2.14 mg/kg. 117 

### Livers ofgopher 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 ofthe wild alligators was composed primarily (-70%) 

### ofnutria.



### 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 ofvarious tissues, as discussed below.



### 17.3.5.1 Eggs

Mean background concentrations in eggs offreshwater 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 threeorfour times theconcentrations in the dietofthe female atthe 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 ofselenium in cggs. 12H

### Similarly, when the birds are switched to aclean diet, seleniumconcentrationsin eggs decline quickly. When mallard (Anasplatyrhy11chos) 

### 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 offat a relatively low level (<5 ·mg Sc/kg, wet weight) after about I0 days back on the untreated diet. The findings ofthis 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 ofselenium in eggs arc especially important because they provide the best samples for evaluation ofpotential adverse reproductive effects (sec also later scction). 12lJ 

### Selenium concentrations in the eggs ofmarine 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 offreshwater species. Surprisinglylow seleniumconcentrationswere found in eggsofwhite-winged seaters (lv/elcmittajitsca) 

### when compared to the concentrations ofselenium found in their livers. 131 

### Elevated selenium concentrations have been found in eggs ofbirds nesting in areas affected by agricultural drainage, coal-fly-ash disposal, and discharge from an oil refinery.7

·2 5,()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 [Podicepsnigricollis]), 

### with many ofthe species means exceeding 20 mg Se/kg.'H Some success in reducing selenium exposures ofaquatic 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 

### < I0 mg Se!k:g. 17 In a manner similar to that for eggs, Heinzetal.m found that seleniumconcentrations in the liverrespond quickly when birds are placed on or taken offa selenium-contaminated diet (see further description of this study below under Muscle). Thus, selenium concentrations measured in the livers ofbirds sampled outside the breeding season are not good predictors ofpotential reproductive effects. In laboratory studies of 

### reproductive effects, livers of male mallards had higher concentrations ofselenium than those of females, probably because females excreted part of the selenium they had accumulated tl1rough cgg-laying. 121
•122 Nevertheless, analysis oflivers ofeither male or female field-collected birds can provide a useful indication ofthe relative level ofexposure experienced by the population. In laboratory studies with birds fed diets containing selenomethionine, selenium concentrations in liversofmallards, black-crownednight-herons(Nycticoraxnycticorax),andeasternscreech-owls (Otus asia) rangedfrom roughlyequal to the dietaryconcentrationto aboutthree times thedietarylevcl,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, andjuveniles 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 ofexposure as well as the foraging range ofthe birds at the two sites. Forexample, adults ofthe 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 injuvenilebirds

were generally similar to those in late-season adults. Selenium concentrations in the 

### 1 ivers of aquatic birds from a number ofother areas receiving subsurface agricultural drainage also were elevated and have reflected periods ofresidence in the contaminated areas.25

·137

- 139 When use ofsubsurface agricultural drainage waters for wetland management was curtailed and replaced with better-quality water, selenium concentrations in aquatic birds declined. Livers ofdivingducks (suchasseaters [Me/anitta spp.] andscaups [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 apparentreason for the higherconcentrations ofselenium in the diving ducks is that they forage on benthic organisms, which bioaccumulate selenium to a higher degree than foods ofsome of the other aquatic birds.



### 17.3.5.3 Kidneys

Seleniumconcentrations in kidneys ofbirds from selenium-normal areas were somewhathigher than those in the Jiver (liver/kidneyratios ofless 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 ofbirds are 

### 1 to 3 mg/kg.l7 Concentrations increase and decreasein response to changes in dietary exposure, but the changes occurmoreECOTOXICOLOGY OF SELENIUM 479



### slowly than those in eggs or liver. 135 

### However, for muscle as well as other tissues, when tissue levels ofselenium 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 dietcontaining I0 mg Selk:g for 6 weeks; predicted equilibrium time for concentrations in the breast muscle offemales (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 ofselenium 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 similarlevels when treatmentended. In another experimentwith 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 64days. 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. Seleniumconcentrations in breastmuscle fromjuvenileducks (Anas

### sp.) at Kesterson Reservoir and a reference site were measured because of concern about human consumption of ducks harvested in the vicinity ofKesterson.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 relationshipsbetween muscle andliver (r'= 

### 0.69) ofthe ducks were considerably more variable than those between kidneys and livers ofAmerican 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) atdietaryconcentrationsof 10, 25, or60 mg/kg had blood-selenium concentrations of4.5, 8.9, or 16 mg/L (wet-weightbasis).'"Theconcentration ofselenium 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 ofselenium in feathers are I to 4 mg/kg and are typically less than 2 mg/kg. 17 

### Seleniumconcentrations may be higherin the feathers ofbirds from areas with elevated levels of mercury because ofthe interactions between these two elements. Analysis offeathers may provideuseful information concerningexposures ofbirds 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 ofmiles 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 offive species ofseabirds were sampled atJohnston Atoll (about 1300 km southwest of Hawaii) and three species at Manana Island, Oahu, Hawaii, and analyzed for selenium and several otherelements.155 

### Selenium concentrations were generally higherin brown noddies (Anous 

### stolidus) than in otherspecies at both locations, and noddieshad significantlyhigherconcentrations 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 oflive 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 hairprovided 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 havingconcentrations in the liverorothertissues that are typical ofmammals in freshwater

or terrestrial environments and others (even ofthe same species in the same areas) having greatly elevated levels. 19

·109 

### Partofthis variation maybe 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 ofmercury. Selenium concentrations greater than 100 mg/kg have been reported for livers ofpilot whales 

### (G/obicephala macrorhync/ms), 158 

### beluga whales 

### (De/pltinaptems /eucus), 159 

### striped dolphins 

### (Stene/la coemleoa/ba),111

### nbottle-nosed dolphins 

### (Tursiops tnmcatus),11' 1

### ringed seals 

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

### vitulina), 1 w 

### gray seals 

### (Halichoerus g1)'pus), 1 M 

### 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;higherselenium concentrationsoccurred 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 spatiallyECOTOXICOLOGY OF SELENIUM 481



### within the site, especially as related to habitat type and history ofmanagement activities that have altered the site since 

### it served as a reservoir for agricultural drainage waters. Elevated levels ofselenium 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 ofseleniumin 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-l73 

### Several methods ofselenium 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 orderofmagnitude 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 ofbalance between meeting the dietary requirements without exceedingtoxicity thresholds. Signsofselenium deficiency havebeenreported in manydomesticated species (includingchickens,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 ofthe more common myopathies resulting from selenium deficiency (referred to as "white muscle disease") has been reported in alleasl 30 ofthe 50 U.S. stales. Selenium deficiency in some species can be ameliorated through a variety ofmeans including addition ofselenium to diets ofdomestic species, use ofintrarumcnal boluses that release selenium slowly, and application ofselenium-containing fertilizers to grazing 1ands.5

-176-l79 

### Selenium fertilization and usc ofintrarumenal boluses have been tested for improvement ofreproduction in freeranging mule deer, sometimes with significant benefit. 170

·1 7:!. 173 

### However, selenium deficiencies in otherkinds ofwildlife may not be corrected as easily as those in species such as deer.



### 17.4.2 Toxic Effects

Selenosis and seleniumtoxicosis are usedinterchangeablyto referto thepathologicalconditions resulting from ingestion of high-selenium diets. Those conditions include impaired health and reproduction, as discussed belowin relation to vertebrate animals.When wateris the onlyexposure 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 ofvertebrate animals to directe!Tccts ofwaterborne selenium. Eggs and larvae offish and birds are very sensitive to the lethal or teratogenic effects ofselenium 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 ofselenium, 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 ofwhether the effects were482 HANDBOOK OF ECOTOXICOLOGY



### measured as direct toxicity or impairment ofreproduction. 20

·1Ho.un 

### Duration ofexposure 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 ofthe midge Chironomus riparius, 

### the daphnids were more acutely sensitive than midges to the toxic effects ofinorganic 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 otherelements can affect the toxicity ofselenium. For example, sulfate can reduce the bioavailability and toxicity ofselenate 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 ofmidge 

### (Chirmwmus decants) larvae that were cultured on detrital material from these plants.79 

### Significant reductions in growth were observed in midges exposed to plant substratehavingmore than about 10mg Sc/kg, and relativebioaccumulation (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 ofthe effects ofselenium in fish and describe case studies of such effects_L!.I.J.I7•19• 23•

25•1m;,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 ofbiological (e.g., condition oflarvae/fry, reproductive success, etc.) and chemical (e.g., analysis ofovaries/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 ofrecruitment 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 ofyoung fish as well as effects on health, physiology, and survival ofolder fish. There is general agreement that early life stages (i.e., eggs, larvae, and fry) arc more sensitive to the adverse effects ofexcess selenium than arc adult fish, and that some species offish are more sensitive than others.1
•1
•17
•23•1ll6•1lJ0 

### Selenium concentrations in the ovary/eggs as well as those in whole-body fish and the diet are most useful in evaluatingrisks ofadverse effects (i.e., more useful than selenium concentrations in water or sediment, orin 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 ofadverse effects when they are greater than 10 mg/kg, 1-1·23•190 

### 7 to 13 mg/kg, 17 

### or 17 mg/kg. 1H 6 

### At concentrationsECOTOXICOLOGY OF SELENIUM 483

exceeding the threshold, there is a large increase in the 

### occmTcncc ofteratogenic deformities and

a concomitantreduction in survival ofthe larvae/fry. Although many ofthe larvae/fry die as aresult 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 ofselenium deposited there by the female parent for their nourishment and development. Feedingexcess but sublethal levels ofselenium to fry,juvenile fish,



### oradult fish docs not cause dcfom1itics 

### such as those caused through maternal transfer ofselenium 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 ofthe spine), k-yphosis (convex curvature ofthe thoracic region of the spine, resulting in a "humpback" condition), missing ordefom1cd fins, missing ordcfom1ed gills and gill covers (opercle), abnormally shaped head, missing ordeformed 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, DeForestet aJ. 1Hfl evaluated results ofavailable 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 seleniumconcentrations in the range of4 to 9 mg/kg arc recognized to reflectrisk ofadverseeffects 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 I0 mg/kg for warm-water fish. l!!liAs described in the previous section (Bioaccumulation), different forms ofselenium 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 indicatethat once the selenium is biologically incorporatedin 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. Forexample, 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 I3,000 and 1000**~lg** Se/L, respectively) and selenate (98,000 and 39,000 Jlg Se/L). 1H1However, as duration ofexposure increases, the toxicity ofseleniumincreases.Forexample, when seleniumwas introducedto outdoorexperimental streams at I0 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 I0 Jlg Se/L when compared to controls. 19

·1Exposure ofadults at both lO and 30 11g Se/L for 40 weeks before spawning rcsulled in reducedembryoand larval survival and producedlarvae with ahighincidence ofedema,lordosis, and internal hemorrhaging.



### 17.4.2.3 Amphibians and Reptiles

There is only limited information about the toxic effects ofselenium 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 ofwaterborne 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, theLC50valuesdecreaseas exposureduration increases.The7-dayLC50 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 coalcombustion waste ponds showed physiological and morphological changes as well as bioaccumulation of some elcments. 112•113 For example, tadpoles ofthe bullfrog collected in a coal-ash basin and a downstream drainage swamp had areduced numberoflabialteeth and deformities oflabial papillae whencomparedwith tadpoles 

### from referenceareas. Tadpoles with defom1itics wereless able to grazeperiphyton than werenormal tadpoles, and they had lower growth rates. These tadpoles had significantly elevated concentrations ofselenium(25.7 mg/kg) bycomparison to areferencesite (3.37 mg/kg).Theyalsohad significantly greater concentrations ofarsenic, 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 oftrace clements, especially selenium and arsenic but also cadmium, chromium, and copper, by comparison to snakes from a nearbyreference site.1Jll (Results ofanalyses oflivers 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 oftheir 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 agentresponsible for mortality and reproductive failure in birds at a number ofsites in the United States (see USDI'7 and Skorupa" for a partial list). Much ofthe research evaluating the effects ofselenium in birds has been related primarily to discharges ofsubsurface 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 ofdevelopmental abnormalities and excessive embryonic and adult mortality in aquatic birds at Kesterson Reservoir, Califomia.7

·91·u::..J.t.J,.2n.J

. .205Observations in the field were corroborated through a series oflaboratory studies thatdocumented 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 otherreviews (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 ofwildbirds to determine the effects ofselenium on reproduction and other aspects of avian biology have been conducted in the field and in the laboratory since 1983. Reproductive effects ofseleniumreported in wild aquatic birds and domestic poultry include embryo mortality and teratogenesis as well as the failure ofadult birds to nest. Excess selenium in the diet of female birds during the periodjustbefore 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 Characteristiceffects observedin the field andlaboratory studiesincludereduced hatchability ofeggs (due to embryo mortality) and high incidence ofdevelopmental 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 ofthe brain]), and abdomen (gastroschisis [an open fissureECOTOXICOLOGY OF SELENIUM 485



### of the abdomen]). Most ofthese 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 gal

### lus])

..J·131.!32·1:.'A,J:.:s.!o7-210 

### Selcnomcthioninc, the mostbioavailablc and toxic ofthe 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 forusein feeding studies to parallel thedose-effectlevels found in field studies withbirds. 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 1This was attributed to high levels ofselenium in the diet ofthe 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 of0, I0, 20, 40, or 80mg Se/kg from hatching to 6 weeks ofage.212Among ducklings that received selenomethionine, 100% ofthe 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 largestcumulative 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 ofemergent vegetation (i.e., cattails, bulrush, etc.), large populations ofnesting 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 ofmagnitude(< 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 Statisticallydistinct teratogenesis response functions were delineated for ducks, stilts, and avocets using the Tulare Basin data. The Tulare curves were used to estimate expected frequencies ofteratogenesis for ducks, stilts, and avocets using other sites, and the predicted levels

were tested against the observed frequencies from the sites. The predicted andobserved frequencies ofteratogenesis were not significantly different, and therefore the data were combined to generate final response curves. Using this data set, Skorupa2 t)(; 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 EC 10 for teratogenic effects of37 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 ofavian selenosis is reduced egg hatchability due to embryo inviability.17·25·206 Egg seleniumconcentrations 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 equivalentto theEC 10 on a clutchwise (orhenwise) basis and the EC03 on an eggwise basis.33 Skorupa206 used more than486 HANDBOOK OF ECOTOXICOLOGY



### 400 sample eggs from black-necked stiltnests monitored atTulare Basin, Kesterson Reservoir, and Salton Sea, California to identify the background rate ofinviable stilt eggs (8.9%) and developed the following equation to determine the numberofclutches containing at leastone inviableembryo:



### Y = 

### exp(-2.327 

### + 0.0503X)/(l 

### + exp(-2.327 

### + 0.0503X)); r' = 

### 0.18 (17.1)

whereY 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 of6 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 ofthe available information concerning reproductive effects ofselenium in birds, Heinz13 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 etal.32

·3

·1 

### used some of the data available from experimental studies with mallards and concluded that the threshold for reduced hatchability (ECwlis 16 mg Sc/kg in the egg. However, Skorupa" disagreed wilh this conclusion. Thedataavailablefrom six studiesin whichmallards werefed sclenomethionine121. 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 briefsummmy ofthe relevant studies, including the dietary treatment level ofselenium, 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 L 0 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% LCLonlolled 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, drywt.) 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 ofselenium 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 ofreporting on fertility effects may be due in part to a general practice ofsimply including infertile eggs as inviable eggs in studies ofselenium effects in birds (i.e., "infertility" effects may not be separated from "embryotoxic" effects in the overall measurement ofhatchability). 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 amechanism acting on adult, rather than embryonic, physiology. Results obtained in kestrels suggest infertility may be a potentially importantfactorcontributingto theoverallreproductiveimpairmentin somespecies. However, inmallards 

### andblack-crowned night-herons fed 10 mg Se/kg as selenomethionine,egg fertility was notreduced 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 offertile 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 ofthe 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 othermeasurementsthatcan be used fordiagnosis ofselenium toxicosis in birds. 1 ·Hl,:::IJ.:::t.J 

### In general, ducks that received diets containing more than 

### 20 mg Se/kg developed a number oflesions ofthe 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 orbroken}). When corroboratedby elevated seleniumconcentrations in tissues (especially the liver), these integumentary and hepatic lesions, as weil as weight loss, can serve for diagnosis ofselenium 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 seleniumconcentrations greaterthan I00 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 I6-weekexposure to evaluate susceptibility ofmallards to over-wintermortality 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 1week oftreatment, 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 controldiet; after4 weeks, theirbody weight was similarto thatofthecontrolbirds. Concentrations ofselenium in blood were related to dietary treatment levels, but mortality was not clearly related to a threshold selenium concentration in the blood. A number ofstudies (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 ofselenium 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 ofthe effects ofexcess 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 mostcommoncause ofchronic selenosis is consumption offorage containingmore 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 ofthe various kinds of lesions associated with acute-subchronic and chronic selenosis in mammalian species, and they stress the importance ofusing multiple lines ofevidence (e.g., selenium concentrations in tissues,

# i I

I I I ECOTOXICOLOGY OF SELENIUM 489



### presence and types oflesions) in diagnosingselenosis. Subchronicandchronic selenosis (sometimes called "alkali disease") commonlyinvolves changes in the integument,especiallythe hooves, horns, and hair. Loss ofhair is a common sign of selenosis in a number of mammalian species and is similar to the bilateral loss offeathers observed in selenium-poisoned birds (described above). 

### Lesions of the liver occur in some species ofmammals (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 ofselenium. 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 ofadult animals ortheirreproduction. 15h,IM' 

### Despite very high concentrations ofselenium in livers or whole-body small mammals, or in livers, hair, feces, or blood ofraccoons, 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). Thosemeasures as well as histopathology ofselectedtissues (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 orantagonistic in terms ofeffects on uptake and metabolic effects, and the degree ofinteraction is affectedby numerous factors. Thus, the topic ofinteractions 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 ofthe 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 offield or laboratory studies and in the evaluation ofresults, and they should be taken into consideration. Interactions between mercury and selenium have probably been studied more intensively in

more kinds ofanimals than most ofthe others listed above. Mercury and selenium concentrations in the livers ofvarious free-living carnivorous mammalsoften arehighly correlated in a molarratio of I:1. 19•109• 223

·22

·1

### However, there is no consistentpattern for such a correlation in the livers ofbirds. For example, hepatic mercury and selenium were correlated, with an overall mercury:sclenium molarratio of l:6, in diving ducks (surfseaters [!vlelanittaperspicillata] 

### and greaterscaups [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 werepositivelycorrelated in some bird livers, butnolin others,orthey werenegatively correlated(seereviewbyOhlendorf-'16).

### Theserelationships may change as birds remain at the sampling location (due to differential accumulation and loss rates for mercury and selenium), they may vary because ofdiffering relative concentrations ofthe two clements, and other factors (such as the chemical forms present) also may complicate the patterns ofbioaccumulation. In the selenium-mercury studies discussed above, concentrations were reported for total selenium and total mercury. However, recentstudiesby Bennyet aJ.227 

### and Spaldinget alY11 

### have shown high correlations ofselenium with inorganic mercury on a molar basis in fish-eating birds. Those authors suggested that selenium may contribute to the sequestration ofinorganic 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 HANDBOOKOF ECOTOXICOLOGY



### Interactive effects of selenium with arsenic, boron, or mercury have been evaluated in reproductive studies with mallards.L2·1.1 25

,208 

### Each ofthe studies involved varying levels ofdietary exposures ofbreeding 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 athighertreatment 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 ofselenium 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 ofyoung producedper female and the frequency ofteratogenic effects were significantly affectedby the combination ofmercury and selenium in the diet, and mercury also enhanced the storage ofselenium 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 offish orbirds at dietary concentrations abouttwo orthree 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. Seleniumconcentrations 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 ofan 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 ofbioaccumulation ofselenium. Studies of the effects ofselenium 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), vitaminsA, 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 ofparticular interest, and there is nouniversal ruleofthumb as to the netresultofthese interactions.Forexample,in mallards, selenium provided a protective effect that reduced the toxicity of' mercury in adult ducks, but theECOTOXJCOLOGY OF SELENIUM 491

effects ofmercury plus selenium in the diet caused greater impairment ofreproduction than either chemical alone. Overall, there is enough information to thoroughly evaluate the significance ofselenium as an 

### environmental contaminant 

### ifit is included among the chemicals ofpotential concern when ecological risk assessments are planned and conducted.:20

·1The main difference between selenium and mostothercontaminantsis thatitis essential to haveagoodunderstandingofselenium'soccurrence, 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 ofselenium in the eggs ordiet. Gary H. Heinz reviewed the manuscript and provided useful comments.



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# I I ECOTOXICOLOGY OF SELENIUM 493

39. Presser, T. S. and Ohlendorf, H. M., Biogeochemical cycling ofselenium in the San Joaquin Valley, California, USA, En\'imn. Mmwgc., 11, 805, 1987. 40. Saiki, M. K. and Lowe, T. P., Selenium in aquatic organisms from subsurface agricultural drainage water, San Joaquin Valley, California,Arch. Environ. Contam. 10xicol., 16, 657, 1987. 41. Hothem, R. L. and Ohlendorf, H. M., Contaminants in foods ofaquatic birds at Kesterson Reservoir, California, 1985,Arch. Environ. Contam. Toxicol., 18, 773, 1989. 42. Schuler, C. A., Anthony, R. G., and Ohlendorf, H. M., Selenium in wetlands and waterfowl foods at Kesterson Reservoir, California, 1984,Arch. Em•iron. Contam. Toxicol., 19, 845, 1990. 43. Zhang,Y. Q. and Moore, J. N., Controls on selenium distribution in wetland sediment, Benton Lake, Montana, WaterAirSoil Po!!w., 97, 323, 1997. 44. IGshchak, I. T., Supplementation of selenium in the diets of domestic animals, in Environmmtal Chemistry of Selenium, Frankenberger,W. T., Jr. and Engberg, R. A., Eds., Marcel Dekker, New York, 1998, 143 (Chap. 9). 45. Lo, M. T. and Sandi, E., Selenium: Occurrence in foods and its toxicological significance. A review, J. Environ. Patlwl. Toxicol., 4, 193, 1980. 46. National Academy of Sciences -National Research Council, Selenium in Nutrition, Subcommittee

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# I

I



# I
l



### 1 

## I ECOTOXICOLOGY OF SELENIUM 495

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