Linking Selenium Sources to Ecosystems: Modeling
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Selenium (Se) as a contaminant of ecosystems is bioaccumulative and causes reproductive effects in fish and wildlife. Ecosystem-scale Se modeling predicts Se bioaccumulation based on dietary biodynamics within site-specific food webs. The model can be used to forecast Se toxicity under different management or regulatory proposals or to translate a tissue guideline to a dissolved guideline.
Selenium is recognized as an important contaminant in aquatic environments because of its potency as a reproductive toxin and its ability to bioaccumulate through food webs. Selenium's role is well documented in local extinctions of fish populations and occurrences of deformities of aquatic birds in affected habitats. Details of general ecotoxicological pathways of selenium for fish and birds and effects of concern for selenium are shown in Figure 1.
Toxicity arises when dissolved selenium is transformed to organic-selenium by bacteria, algae, fungi, and plants (i.e., synthesis of selenium-containing amino acids de novo) and then passed through food webs. It is generally thought that animals are unable to biochemically distinguish selenium from sulfur, and therefore excess selenium is substituted into proteins and alters their structure and function. Other biochemical reactions also can determine and mediate toxicity. The effect of these reactions is recorded, most importantly in birds and fish, as failures in hatching or proper development (teratogenesis or larval deformities) (Figure 1). Other toxicity endpoints include growth, winter survival, maintenance of body condition, reproductive fitness, and susceptibility to disease.
Specifically for selenium in aquatic ecosystems: (1) water-column selenium has proven to be an imprecise predictor of selenium bioaccumulated in food webs because dietary selenium makes up 95% of tissue selenium in invertebrates and fish; (2) site-specific biogeochemical transformation of dissolved selenium into particulate forms (algae, microbes, seston, or sediments) determines the concentration of selenium available at the base of food webs; (3) a 38-fold variability in trophic transfer of particulate-material selenium to invertebrate species is mainly driven by physiological differences in assimilation efficiency and the rate constant of loss of selenium among invertebrates species; and (4) dietary transfer of invertebrate selenium to fish species (as measured in whole-body tissue) has a median of approximately 1, which reflects preservation of selenium as it passes up food webs but with little increase over the trophic level below. Less information is available concerning dietary selenium biodynamics of aquatic birds, but a bird species' dietary choice of prey and the bioaccumulation kinetics of that prey are still fundamental to their exposure (as measured in bird eggs). Overall, this differential response to selenium makes some predator species more vulnerable and, thus, more likely to experience demographic collapse from moderately contaminated environments than others.
Ecosystem-Scale Selenium Modeling
The main route of exposure for selenium is dietary, yet regulations lack biologically based protocols for evaluations of risk. Ecosystem-scale selenium modeling conceptualizes and quantifies the variables that determine how selenium is processed from water through diet to predators (Figure 2). This approach uses biogeochemical and physiological factors from laboratory and field studies and considers loading, speciation, transformation to particulate material, bioavailability, bioaccumulation in invertebrates, and trophic transfer to predators. Validation of the model is through data sets from 29 historic and recent field case studies of selenium-exposed sites.
The model (Figure 2) links selenium concentrations across media (water, particulate, tissue of different food-web species). It can be used to forecast toxicity under different management or regulatory proposals or as a methodology for translating a fish-tissue (or other predator tissue) selenium concentration guideline to a dissolved selenium concentration. The model illustrates some critical aspects of implementing a tissue criterion: 1) the choice of fish species determines the food web through which selenium should be modeled, 2) the choice of food web is critical because the particulate material to prey kinetics of bioaccumulation differs widely among invertebrates, 3) the characterization of the type and phase of particulate material is important to quantifying selenium exposure to prey through the base of the food web, and 4) the metric describing partitioning between particulate material and dissolved selenium concentrations allows determination of a site-specific dissolved selenium concentration that would be responsible for that fish body burden in the specific environment.
The linked approach illustrates that environmentally safe dissolved selenium concentrations will differ among ecosystems depending on the ecological pathways and biogeochemical conditions in that system (Figure 3). Uncertainties and model sensitivities can be directly illustrated by varying exposure scenarios based on site-specific knowledge. The model can also be used to facilitate site-specific regulation and to present generic comparisons to illustrate limitations imposed by ecosystem setting and inhabitants. Used optimally, the model provides a tool for framing a site-specific ecological problem or occurrence of selenium exposure, quantify exposure within that ecosystem, and narrow uncertainties about how to protect it by understanding the specifics of the underlying system ecology, biogeochemistry, and hydrology.
Ecosystem-scale selenium modeling methodology:
Chapman, P.M., Adams, W.J., Brooks, M.L., Delos, C.G., Luoma, S.N., Maher, W.A., Ohlendorf, H.M., Presser, T.S., and Shaw, D.P., eds., 2010, Ecological Assessment of Selenium in the Aquatic Environment: Society of Environmental Toxicology and Chemistry (SETAC) Press, Pensacola Florida, 339 p.
Presser, T.S., and Luoma, S.N., 2010, A Methodology for ecosystem-scale modeling of selenium: Integrated Environmental Assessment and Management, v. 6, no. 4, p. 685-710.
Luoma, S.N., and Presser, T.S., 2009, Emerging opportunities in management of selenium contamination: Environmental Science and Technology, v. 43, no. 22, p. 8483-8487.
Luoma, S.N., and Rainbow, P.S., 2005, Why is metal bioaccumulation so variable? Biodynamics as a unifying concept: Environmental Science and Technology, 39:1925–1931.
In-depth publications for site-specific application of ecosystem-scale selenium modeling:
Luoma, S.N., and Presser, T.S., 2018, Status of selenium in south San Francisco Bay—A basis for modeling potential guidelines to meet National tissue criteria for fish and a proposed wildlife criterion for birds: U.S. Geological Survey Open-File Report 2018–1105, 75 p.
Presser, T.S., and Naftz, D.L., 2018, USGS Measurements of Dissolved and Suspended Particulate Material Selenium in Lake Koocanusa in the Vicinity of Libby Dam (MT), 2015-2017 (update): U.S. Geological Survey data release.
Presser, T.S., and Naftz, D.L., 2017, USGS Measurements of Dissolved and Suspended Particulate Material Selenium in Lake Koocanusa in the Vicinity of Libby Dam (MT), 2015-2016: U.S. Geological Survey data release.
Jenni, K.E., Naftz, D.L., Naftz, Presser, T.S., 2017, Conceptual Modeling Framework to Support Development of Site-Specific Selenium Criteria for Lake Koocanusa, Montana, U.S.A., and British Columbia, Canada: U.S. Geological Survey Open-File Report 2017-1130, 14 p.
Presser, T.S., 2013, Selenium in Ecosystems within the Mountaintop Coal Mining and Valley-Fill Region of Southern West Virginia-Assessment and Ecosystem-Scale Modeling, U.S. Geological Survey Professional Paper 1803, 86 p.
Presser, T.S., and Luoma, S.N., 2013, Ecosystem-scale selenium model for the San Francisco Bay-Delta Regional Ecosystem Restoration Implementation Plan (DRERIP): San Francisco Estuary and Watershed Science, v. 11, no. 1, p. 1-39.
Presser, T.S., and Luoma, S.N., 2010, Ecosystem-scale selenium modeling in support of fish and wildlife selenium criteria development for the San Francisco Bay-Delta Estuary, California: U.S. Geological Survey Administrative Report, 101 p. and Appendices A-D. [Published 12/14/2010; released by USEPA (Region 9, San Francisco, California) 8/29/2011]
Presser, T.S., and Luoma, S.N., 2009, Modeling of selenium for the San Diego Creek watershed and Newport Bay, California: U.S. Geological Survey Open-File Report 2009-1114, 48 p.