Mercury Concentrations in Snowy Plovers and Sediments of their Breeding Environments in the Southern Great Plains
Date of Award
Master of Science - Environmental Sciences
Western snowy plovers (Charadrius nivosus) are philopatric shorebirds sporadically distributed during the breeding season at saline lake and alkali flat breeding environments of the Southern Great Plains (SGP) of Oklahoma, New Mexico, and Texas. The major conservation threat to interior snowy plovers breeding on the SGP derives from anthropogenic modification of breeding environments. The SGP is one of the most intensively cultivated regions in the world, and excessive groundwater mining and resulting deterioration of artesian spring flows reduces site quantity and quality critical for regional successful nesting. Most saline lakes now primarily receive freshwater inputs via irregular precipitation events and overland flow. Snowy plovers are dependent on consistent spring flows and shallow surface water to maintain foraging habitats and satisfy thermoregulatory needs. Therefore, limited surface water availability effectively reduces snowy plover habitat availability, occupancy, and reproductive success in these degraded habitats. Snowy plovers may also be vulnerable to elevated contaminant exposure, such as mercury (Hg), in these environments. Presently, there is a paucity of published research regarding Hg exposure for avifauna inhabiting saline lake and alkali flat environments of the SGP.
The health consequences of Hg exposure are dependent on the dose-response relationship for methylmercury (MeHg) and the organism. Acute exposure may cause death, while lower chronic doses compromise avian learning ability, motor skills, body condition, and response to environmental stimuli. Shorebirds are at high risk of contaminant exposure due to their consumption of aquatic invertebrates found in sediments, surface water, and agricultural fields where pollutants tend to accumulate. Therefore, the objectives of this research were to 1) to quantify baseline blood total mercury (THg) concentrations to evaluate risks of Hg concentrations to regional snowy plover survival and reproductive success and 2) provide baseline sediment THg levels for potentially at-risk snowy plover breeding sites on saline lake and alkali flat habitats in the SGP to assess potential regional THg contaminant threats.
We collected whole blood from 130 adult snowy plovers in 2014, of which 97 were analyzed for THg concentrations. All analyzed blood samples were above instrument detection limits (DL), ranging from 0.0222-0.7060 ppm THg. The arithmetic mean blood THg level was 0.2582 ppm across all body conditions, sexes, collection months, and study sites. All blood samples contained quantifiable THg concentrations (i.e., >DL), thereby implicating some degree of chronic local exposure for snowy plovers breeding in the SGP. However, all plover blood THg concentrations were below blood adverse effect thresholds of 1.0 ppm for avian piscivores and within the normal range of 0.02-2.5 ppm for shorebird blood THg exposure. Snowy plover blood THg concentrations were similar between sexes and body condition was not related to blood THg concentration, but did vary among sites and months. Overall, site was the most predictive modifier in the top ranked candidate concentration model. While Hg exposure may not constitute a singular threat to regional snowy plover declines, Hg interactions with other bioavailable contaminants, such as selenium, could prove relevant to long-term snowy plover population dynamics.
A total of 34 sediment samples were collected from seven of eight study sites in 2014 and analyzed for percent moisture content and THg concentrations. Sediment moisture content ranged from 16.2-71.8% depending on substrate particle size at each sampling site. As with blood, all sediment samples were >DL, ranging from 0.0004-0.0268 ppm THg. The arithmetic mean sediment THg concentration was 0.0079 ppm among all moisture contents, spring types, potential snowy plover nesting areas, and sites. Low sediment THg levels indicate some extent of consistent regional Hg bioavailable within the environment. Yet, all sediment THg concentrations were below suggested sediment adverse effect thresholds of 0.2 ppm for freshwater invertebrates and <0.05 ppm THg detected during past regional saline lake research. Sediment moisture content proved the strongest modifier on sediment THg concentrations, followed by spring type, and snowy plover nesting activity. Negligible THg variation occurred among sites. An array of factors dictates environmental (sediment) THg availability. Consequently, baseline sediment THg levels only offer partial insight into regional Hg risk assessments. Long-term monitoring should further incorporate sediment particle size and a suite of other environmental factors that influence baseline THg levels or methylation potential. Vital research needs should address isotopic contaminant origins to identify potential regional sources and attempt to establish linkages between sediment THg levels and biomagnification potential, or trophic transfer, throughout local food chains.
Very few THg adverse effect threshold data are available to quantify toxicity risk for shorebirds, but snowy plover blood THg concentrations were consistent with results reported from other studies. The greatest blood Hg concentrations occurred in Oklahoma (Salt Plains NWR), followed by New Mexico (Bitter Lake NWR), and Texas. Saline lake sediment THg concentrations also aligned with findings from past regional contaminant investigations. Although baseline sediment THg levels offer a temporal snapshot of Hg presence (or absence) in the environment, representative sediment concentrations cannot fully depict Hg mobility, distribution, or toxicity risk without regard for other sediment physio-geochemical properties. Snowy plover blood THg concentrations are indirectly linked to geographic variations in baseline environmental Hg availability, predominantly derived from differences in hydrological or geological influences on sediment moisture content. Consequently, differential baseline sediment concentrations and methylation rates among moisture contents and spring types could drive variations in plover THg levels. While a direct relationship may not exist between snowy plover blood THg and sediment THg concentrations, intermediate trophic levels may provide linkages for MeHg bioaccumulation and biomagnification through the system. Future research should establish adverse effect threshold levels for species-specific shorebird blood, evaluate non-breeding season snowy plover Hg exposure, and monitor plausible snowy plover MeHg exposure pathways through THg quantification in intermediary vegetation and potential invertebrate prey assemblages indicative of regional nesting environments.
Duffie, Laura, "Mercury Concentrations in Snowy Plovers and Sediments of their Breeding Environments in the Southern Great Plains" (2017). Electronic Theses and Dissertations. 93.
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