Research
Current research
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My research focuses on the trophic transfer of heavy metals (Hg, Pb, As, Cu, Zn, Cd, Se) in aquatic food webs. I am interested in how ecological factors (food quality, diet breadth and omnivory) affect the transfer of elements between organisms. Specifically, I am studying the effects of prey quality on growth and mercury accumulation in aquatic organisms.
Questions
- How does the elemental composition of organisms vary over space and time?
- Do fish with broad diets accumulate more mercury (Hg) from benthic or pelagic prey?
- How does growth affect the trophic transfer of contaminants such as mercury in aquatic food webs?
Previous Research
Do taxonomic groups of organisms share elemental fingerprints?
A goal of ecological stoichiometry (Sterner and Elser, 2002) is to extend its principles beyond the macronutrients (C, N, P) to elements throughout the periodic table. Some of the first attempts to explore multi-element composition and expand the Redfield ratio for phytoplankton (C106N16P1) began in the 1970s (Martin, 1973; Morel, 1985; reviewed in Stumm, 1996). Since then, the technology to measure ultra trace amounts of elements has become widely available. An expanded view of element composition would enhance our understanding of trace metal cycling in ecosystems and food webs, consumer-resource mismatches and homeostatic regulation of trace metals, and provide the opportunity to explore relationships between trace metals and macronutrients from an ecological perspective. However, inherent variation in organismal trace element concentrations could limit these advances, if such variation substantially exceeds that observed in macronutrients. The primary goal of this study is to compare spatial and temporal variation among macronutrients (C, N, P), essential micronutrients (Zn, Cu, As, Se) and nonessential, potentially toxic metals (Pb, Hg, Cd) in freshwater benthic and pelagic invertebrates.
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Figure 1. Physiological effects and biological control over different
element types. Modified from (Stumm and Morgan, 1996). |
Due to differences in the extent to which organisms regulate elements with different functions, concentrations of essential micronutrients and nonessential elements in freshwater invertebrates are likely more variable than macronutrient concentrations. Compared to macronutrients, organisms have a limited ability to regulate essential micronutrients, and almost no ability to regulate nonessential elements. As shown in Figure 1, organisms are likely to regulate essential elements along some supply range near the optimum concentration, maintaining some internal concentration. Above this range, regulation breaks down, and the otherwise essential element can exert toxic effects. Outside of the regulated supply range, organisms can have highly variable somatic element concentrations. Nonessential elements (Hg, Cd, Pb) are generally unregulated and simply cause negative effects with increasing concentration.
We hypothesized that spatial and temporal variation of element concentration within invertebrate taxa reflects element function. We found that elements indeed are increasingly variable from macronutrients to essential metals and to nonessential, potentially toxic metals. Despite this variation, patterns of relative element concentrations among taxa emerged.
Benthic and Pelagic Hg Sources for Freshwater Fish
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Figure 2. Zooplankton tend to have higher Hg burdens
than benthic invertebrates. |
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Figure
3. Model predictions that benthivory may reduce Hg levels in fish. |
Previous studies of Hg accumulation in freshwater fish have emphasized the transfer of Hg from pelagic sources in freshwater food webs. This is despite the proclivity of fish and other higher trophic level organisms to consume prey from littoral and benthic sources (Schindler and Scheuerell 2002, VanderZanden and Vadeboncoeur 2002). This study is the first direct comparison of Hg levels in common benthic and pelagic invertebrate prey for fish from multiple lakes in New England. We further investigate the effect of benthic versus pelagic diet composition and prey Hg levels on Hg accumulation in fish using a bioenergetics-Hg trophic transfer model (Hanson, 1997). Zooplankton consistently have higher Hg burdens than the littoral-benthic invertebrates sampled in all lakes in this study, and throughout a season (Figure 2). (In fact, zooplankton have consistently higher burdens of other metals such as Zinc, Arsenic and Selenium.) The model results make evident that consumption of littoral-benthic prey reduces fish total Hg burdens (Figure 3). This leads to the expectation that littoral-benthic prey sources attenuate the transfer of Hg to higher trophic levels, and that in general, benthivorous lentic fish may have lower Hg burdens than planktivorous fish in nature.
Growth Dilution
The ecophysiological factors that govern contaminant accumulation in individuals are not well understood. I revisited some basic predictions put forth by different models that describe contaminant accumulation in individual organisms, from either a toxicology/biochemistry perspective, or a bioenergetics/ecology perspective. Sensitivity analyses of these models consistently show that both contaminant accumulation and trophic transfer (for contaminants that are obtained through prey consumption) are sensitive to individual growth rate. They further predict that fast growth can actually reduce contaminant concentrations in individuals (growth dilution). Of the multiple factors that can cause growth dilution, prey quality appears to be one of the most effective, yet the least empirically studied.
Does algal nutrient composition (C:N:P) modify Hg accumulation in Daphnia
through growth dilution?
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Figure
4. Hypothetical effect of algal nutrient content on Hg burdens in
zooplankton through growth dilution. |
Previous Research Projects
- Heavy metal loading from abandoned mines to headwater streams in the Rockies, UT.
- Remediation of heavy metals by aquatic plants and soil in manmade wetlands, UT.
- Distribution and abundance of salmonids and amphibians in alpine systems, Mount Rainier NP.
- The effects of rainforest fragmentation on the relative abundance of nocturnal mammals, Queensland, Australia.
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