Geology topics

Craig E. Hebert

Publications and source records attributed to Craig E. Hebert.

6 recordsLinked to original sources

USGS2020

Methylmercury exposure in wildlife: A review of the ecological and physiological processes affecting contaminant concentrations and their interpretation

Exposure to methylmercury (MeHg) can result in detrimental health effects in wildlife. With advances in ecological indicators and analytical techniques for measurement of MeHg in a variety of tissues, numerous processes have been identified that can influence MeHg concentrations in wildlife. This review presents a synthesis of theoretical principals and applied information for measuring MeHg exposure and interpreting MeHg concentrations in wildlife. Mercury concentrations in wildlife are the net result of ecological processes influencing dietary exposure combined with physiological processes that regulate assimilation, transformation, and elimination. Therefore, consideration of both physiological and ecological processes should be integrated when formulating biomonitoring strategies. Ecological indicators, particularly stable isotopes of carbon, nitrogen, and sulfur, compound-specific stable isotopes, and fatty acids, can be effective tools to evaluate dietary MeHg exposure. Animal species differ in their physiological capacity for MeHg elimination, and animal tissues can be inert or physiologically active, act as sites of storage, transformation, or excretion of MeHg, and vary in the timing of MeHg exposure they represent. Biological influences such as age, sex, maternal transfer, and growth or fasting are also relevant for interpretation of tissue MeHg concentrations. Wildlife tissues that represent current or near-term bioaccumulation and in which MeHg is the predominant mercury species (such as blood and eggs) are most effective for biomonitoring ecosystems and understanding landscape drivers of MeHg exposure. Further research is suggested to critically evaluate the use of keratinized external tissues to measure MeHg bioaccumulation, particularly for less-well studied wildlife such as reptiles and terrestrial mammals. Suggested methods are provided to effectively use wildlife for quantifying patterns and drivers of MeHg bioaccumulation over time and space, as well as for assessing the potential risk and toxicological effects of MeHg on wildlife.

Science of the Total Environment↗

USGS2019

Ecosystem change and population declines in gulls: Shifting baseline considerations for assessing ecological integrity of protected areas

In Lake Superior's Pukaskwa National Park (PNP) in northern Ontario, Canada, herring gull ( Larus argentatus ) population size is used as an indicator of ecological integrity. Since the 1970s, gull populations have declined by 70% suggesting deteriorating park conditions. However, most other rated park indicators show stable or positive trends. Here, we focus on reconciling these seemingly disparate trends through a better understanding of factors regulating PNP gull populations. Lake-wide declines in surface-schooling prey fish may be limiting aquatic food resources for PNP gulls. To investigate this, we examined gull population trends in different parts of the park in the context of food availability. Gull diets were assessed using regurgitated pellets, egg stable isotopes (nitrogen, carbon) and fatty acids. Population declines were more severe in southern PNP compared to northern PNP and inter-region differences in bird diets likely contributed to these population trends. Gulls in the south relied to a greater extent on dwindling aquatic food resources, i.e., prey fish, while birds in northern PNP supplemented their diets with anthropogenic foods, i.e., garbage. Recognizing that regional declines in aquatic food availability have occurred across eastern Lake Superior is important for the interpretation of PNP gull population trends. Wide-scale ecological changes affecting PNP suggest that factors limiting PNP's herring gull population are not park-specific but, instead, reflect broader ecosystem-wide changes. Defining an appropriate threshold based on current knowledge of ecological conditions on Lake Superior is critical for using herring gull populations as an indicator of park ecological integrity.

Journal of Great Lakes Research↗

USGS2016

Amino acid specific stable nitrogen isotope values in avian tissues: Insights from captive American kestrels and wild herring gulls

Through laboratory and field studies, the utility of amino acid compound-specific nitrogen isotope analysis (AA-CSIA) in avian studies is investigated. Captive American kestrels ( Falco sparverius ) were fed an isotopically characterized diet and patterns in δ 15 N values of amino acids (AAs) were compared to those in their tissues (muscle and red blood cells) and food. Based upon nitrogen isotope discrimination between diet and kestrel tissues, AAs could mostly be categorized as source AAs (retaining baseline δ 15 N values) and trophic AAs (showing 15 N enrichment). Trophic discrimination factors based upon the source (phenylalanine, Phe) and trophic (glutamic acid, Glu) AAs were 4.1 (muscle) and 5.4 (red blood cells), lower than those reported for metazoan invertebrates. In a field study involving omnivorous herring gulls ( Larus argentatus smithsonianus ), egg AA isotopic patterns largely retained those observed in the laying female’s tissues (muscle, red blood cells, and liver). Realistic estimates of gull trophic position were obtained using bird Glu and Phe δ 15 N values combined with β values (difference in Glu and Phe δ 15 N in primary producers) for aquatic and terrestrial food webs. Egg fatty acids were used to weight β values for proportions of aquatic and terrestrial food in gull diets. This novel approach can be applied to generalist species that feed across ecosystem boundaries.

Environmental Science & Technology↗

USGS2014

Ecological tracers reveal resource convergence among prey fish species in a large lake ecosystem

1. We measured stable isotopes of carbon (δ 13 C) and nitrogen (δ 15 N) and fatty acid profiles in Lake Ontario alewife ( Alosa pseudoharengus ), rainbow smelt ( Osmerus mordax ), slimy sculpin ( Cottus cognatus ) and round goby ( Neogobius melanostomus ) collected from 1982 to 2008 to investigate how temporal variability in these ecological tracers can relate to ecosystem-level changes associated with the establishment of highly invasive dreissenid mussels. 2. Prey fish δ 15 N values remained relatively constant, with only slimy sculpin exhibiting a temporal increase in δ 15 N. In contrast, δ 13 C values for alewife, rainbow smelt and, especially, slimy sculpin became less negative over time and were consistent with the benthification of the Lake Ontario food web associated with dreissenids. 3. Principal components analysis revealed higher contributions of 14:0 and 16:1n-7 fatty acids and increasingly negative δ 13 C values in older samples in agreement with the greater historical importance of pelagic production for alewife, rainbow smelt and slimy sculpin. 4. Temporal declines in fatty acid unsaturation indices and Σn-3/Σn-6 ratios, and also increased 24:0/14:0 ratios for alewife, rainbow smelt and slimy sculpin, indicated the increasing importance of nearshore production pathways for more recently collected fish and resulted in values more similar to those for round goby. 5. These results indicate a temporal convergence of the food niche, whereas food partitioning has historically supported the coexistence of prey fish species in Lake Ontario. This convergence is consistent with changes in food-web processes associated with the invasion of dreissenid mussels.

Lake Ontario↗

USGS2012

Long-term impacts of invasive species on a native top predator in a large lake system

1. Declining abundances of forage fish and the introduction and establishment of non-indigenous species have the potential to substantially alter resource and habitat exploitation by top predators in large lakes. 2. We measured stable isotopes of carbon (δ 13 C) and nitrogen (δ 15 N) in field-collected and archived samples of Lake Ontario lake trout ( Salvelinus namaycush ) and five species of prey fish and compared current trophic relationships of this top predator with historical samples. 3. Relationships between δ 15 N and lake trout age were temporally consistent throughout Lake Ontario and confirmed the role of lake trout as a top predator in this food web. However, δ 13 C values for age classes of lake trout collected in 2008 ranged from 1.0 to 3.9‰ higher than those reported for the population sampled in 1992. 4. Isotope mixing models predicted that these changes in resource assimilation were owing to the replacement of rainbow smelt ( Osmerus mordax ) by round goby ( Neogobius melanostomus ) in lake trout diet and increased reliance on carbon resources derived from nearshore production. This contrasts with the historical situation in Lake Ontario where δ 13 C values of the lake trout population were dominated by a reliance on offshore carbon production. 5. These results indicate a reduced capacity of the Lake Ontario offshore food web to support the energetic requirements of lake trout and that this top predator has become increasingly reliant on prey resources that are derived from nearshore carbon pathways.

Freshwater Biology↗

USGS2011

Polymethylene-interrupted fatty acids: Biomarkers for native and exotic mussels in the Laurentian Great Lakes

Freshwater organisms synthesize a wide variety of fatty acids (FAs); however, the ability to synthesize and/or subsequently modify a particular FA is not universal, making it possible to use certain FAs as biomarkers. Herein we document the occurrence of unusual FAs (polymethylene-interrupted fatty acids; PMI-FAs) in select freshwater organisms in the Laurentian Great Lakes. We did not detect PMI-FAs in: (a) natural seston from Lake Erie and Hamilton Harbor (Lake Ontario), (b) various species of laboratory-cultured algae including a green alga ( Scenedesmus obliquus ), two cyanobacteria ( Aphanizomenon flos-aquae and Synechococystis sp.), two diatoms ( Asterionella formosa , Diatoma elongatum ) and a chrysophyte ( Dinobryon cylindricum ) or, (c) zooplankton ( Daphnia spp., calanoid or cyclopoid copepods) from Lake Ontario, suggesting that PMI-FAs are not substantively incorporated into consumers at the phytoplankton–zooplankton interface. However, these unusual FAs comprised 4-6% of total fatty acids (on a dry tissue weight basis) of native fat mucket ( Lampsilis siliquoidea ) and plain pocketbook ( L. cardium ) mussels and in invasive zebra ( Dreissena polymorpha ) and quagga ( D. bugensis ) mussels. We were able to clearly partition Great Lakes' mussels into three separate groups (zebra, quagga, and native mussels) based solely on their PMI-FA profiles. We also provide evidence for the trophic transfer of PMI-FAs from mussels to various fishes in Lakes Ontario and Michigan, further underlining the potential usefulness of PMI-FAs for tracking the dietary contribution of mollusks in food web and contaminant-fate studies.

Michigan, Ontario, Prince Edward Island, Wisconsin↗