|Abstract or Summary
- Human alterations of landscapes take many forms, one of which is anthropogenic pollution. Mercury (Hg) is a complex contaminant because its uptake into the food web is not driven entirely by loading to the system; methylation is necessary to make Hg bioavailable and toxic to fish and wildlife. Because methylation takes place primarily in aquatic systems, research has historically focused on local Hg exposure in fish and fish-eating taxa (including the human health implications of eating exposed fish). As our understanding of Hg movement out of the aquatic and piscivorous food web advances, we must shift focus to other potential recipients and spatial scales. This dissertation seeks to understand how Hg moves through food webs with avian endmembers, both with traditionally-studied piscivores at novel spatial scales and novel endmembers (riparian songbirds in western North America) at more traditional spatial scales. Moving from broad to narrow, I consider Hg exposure across continental (Chapter 2), regional (Chapter 3), and local scales (Chapter 4). At the largest scale, in Chapter 2, I used fish Hg concentrations compiled across western North America at a 1 degree-by-1 degree grid cell resolution. Fish Hg concentrations were size corrected to reflect the primary prey base for a suite of five avian piscivores commonly used in Hg ecotoxicological studies: Bald Eagle, Osprey, Loons (Common and Yellow-billed), Grebes (Clark’s and Western) and Belted Kingfisher. At a continental scale, I identified taxa and regions of increased potential Hg risk to avian endmembers. Avian piscivores foraging on larger-sized fish generally were at higher relative risk to Hg. Habitats with relatively high risk included wetland complexes (e.g., prairie potholes in Saskatchewan), river deltas (e.g., San Francisco Bay, Puget Sound, Columbia River), and arid lands (Great Basin and central Arizona). Chapter 3 focuses on one river basin in particular – the Willamette Valley in western Oregon. Mercury in fish and Osprey have been previously studied in this region; this study characterized differences in aquatic invertebrate and riparian songbird Hg exposure, starting at the headwater area which contains a point source of Hg from a historic Hg mine, moving downstream to a reservoir known to methylate Hg, and including all subsequent downstream reaches. While Hg exposure was highest in songbirds near the Black Butte Hg Mine, Hg concentrations were also elevated at the Cottage Grove Reservoir and several wetland complexes within the valley, reinforcing the importance of habitat on Hg methylation rates. Within the main stem Willamette River, birds in backwater areas had higher Hg concentrations than birds in main channel areas. Chapter 4 examines why individual songbirds and different species of songbirds can vary dramatically in their Hg exposure. I sampled 11 sites within the main stem Willamette River and found that both aquatic invertebrates and riparian songbird Hg concentrations were higher in backwater habitats than main channel habitats, even over relatively small distances. After sampling both aquatic and terrestrial invertebrates, I used a two end member mixing model for δ13C to determine the proportion of aquatic prey in the diet of composited riparian spiders and individual riparian songbirds. Birds sampled early in the season exhibited higher reliance on aquatic prey than those sampled later in the season and also had correspondingly higher blood Hg concentrations. Taken together, the findings of this dissertation show that, for many avian species, Hg exposure is mitigated by what, when, where, and how they eat. What birds eat is driven both by taxonomic and behavioral differences (piscivores versus insectivores vs omnivores) and source of diet (aquatic versus terrestrial prey). For riparian songbirds, those individuals foraging early in the season rely more heavily on the pulsed aquatic carbon subsidy of emergent aquatic insects. Birds foraging near habitats that efficiently methylate Hg, such as backwaters or wetlands, exhibit higher Hg levels.
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