Ecological Applications of Stable Isotopes

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Environmental contaminants of natural and anthropogenic origin represent a major stressor to ecosystems, including human and wildlife populations. FORT scientists are studying these stressors in order to understand the contaminant cycling under natural environmental conditions. Stable isotope techniques are extremely useful in resolving trophic pathways by which contaminants become bioavailable, bioaccumulate, and biomagnify.

Stable isotope geochemistry represents a powerful tool for better understanding biogeochemical cycles, species’ distributions, contaminant cycling, food webs, and ecosystem structure. Using stable isotopes, TSH scientists are able to capitalize on two essential attributes. First, isotopic ratios in an organism’s tissue refl ect the isotopic ratios of what it eats or grows in, and second, local isotopic ratios vary spatially across the face of the earth. Combining this knowledge, our scientists can make inferences into the dietary habits and migration pathways of birds, mammals and fi sh, as well as contaminant cycling and accumulation in species and their habitats. There is a growing need for such information and stable isotopes offer a new approach to integrate earth and life sciences in the context of conservation and environmental change.

The Stable Isotope Laboratory maintained by the Crustal Geophysics and Geochemistry (CGG) and Fort Collins Science Center (FORT) is a state-of-the-art facility with modern instrumentation capable of analyzing the isotopic composition of nearly any conceivable sample matrix. Laboratory staff and principal investigators have significant expertise in isotope systematics of the geosphere, biosphere, and hydrosphere. The Laboratory has demonstrated innovative approaches to challenging questions and as such, has significant latitude to address emerging issues in ecosystem science such as this.

Scientists study contaminants in the water in areas like this rice field.
Scientists study contaminants in the water in areas like this. USGS photo.

Contaminant Biology: Stable Isotope Applications - Principal Investigator - Craig Stricker

Environmental contaminants of natural and anthropogenic origin represent a major stressor to ecosystems, including human and wildlife populations. 

A better mechanistic understanding of contaminant cycling under natural environmental conditions is necessary to mitigate and protect our natural resources. Stable isotope techniques are extremely useful in resolving trophic pathways by which contaminants become bioavailable, bioaccumulate, and biomagnify.  The objectives of this project are to: 

  • Utilize stable isotopes to better resolve the geochemical cycling of contaminants that influences bioavailability.
  • Use stable isotope data to constrain contaminant sources.Employ stable isotope techniques to infer trophic transfer pathways of select contaminants.
Biogeochemical cycling diagram showing climatic processes, hydrologic cycle and biogeochemical cycles.

Biogeochemistry: Stable Isotope Applications - Principal Investigator - Craig Stricker

Biogeochemical cycling is a cornerstone of ecosystem function and structure. Much has been learned about element cycles in a variety of systems using standard geochemical techniques. 

Isotope geochemistry can offer unique and additional insights into sources, processes, and mechanisms. Further, many light stable isotope techniques are amenable to studying element cycling, but logically can be extended to consumer organisms and therefore integrate food webs, offering a more complete assessment of ecosystems. Light isotopes commonly used in this context include hydrogen, carbon, nitrogen, oxygen, and sulfur. Further, such techniques can complement studies of related biogeochemical cycles that have proven challenging to study directly with isotopic techniques.

The Stable Isotope Laboratory maintained by the Crustal Geophysics and Geochemistry (CGG) and Fort Collins (FORT)Science Centers is a state-of-the-art facility with modern instrumentation capable of analyzing the isotopic composition of nearly any conceivable sample matrix. Laboratory staff and PI's have significant expertise in isotope systematics of the geosphere, biosphere, and hydrosphere. The Laboratory has demonstrated innovative approaches to challenging questions and as such, has significant latitude to address emerging issues in ecosystem science. In addition, the Laboratory is highly competent in standard mass spectrometric methods and maintains a library of standard operating procedures. The full resources of the Laboratory are available to this Task and relevant research projects.

The objectives of this project are to: employ isotope techniques in tandem with standard geochemistry to better resolve critical biogeochemical processes; to continue to develop sulfur isotope techniques to refine this element cycle, particularly relationships with non-essential metals such as Hg and to couple biogeochemical cycles to food webs through intensive and interdisciplinary place-based studies.

Migratory birds in a wetland.

Biogeography: Stable Isotope Applications - Principal Investigator - Craig Stricker

The distribution of species across the landscape is of great interest to conservation biology. Habitat quality and quantity are thought to be important drivers of occurrence and distribution, and numerous studies have demonstrated fitness-related consequences. However, for species with cryptic or migratory life histories, we often lack sufficient detail about habitat usage and in many cases, general location and activity information are unavailable. Extrinsic markers, such as tags and bands, have proven to be rather ineffective due to low returns. Intrinsic markers, such as stable isotopes, on the other hand have great potential because inference is not constrained by recapture. Rather, important insights regarding movement and habitat use can be derived from both metabolically active and inactive tissues of many wildlife species. This approach is cost-effective and has led to important findings regarding the location of wintering and breeding grounds, stop over sites, and the concept of migratory connectivity.

Polar bear mother and two cubs on the Beaufort Sea ice.
Polar bear mother and two cubs on the Beaufort Sea ice. USGS photo.

Foraging Ecology Using Stable Isotopes - Principal Investigator - Craig Stricker

Understanding species habitat requirements is incomplete without insight into nutrition, including various aspects of foraging ecology. Traditional diet studies can be challenging because of logistics, issues related to resource availability, and observations are often short-term in nature based on gut contents or scat. Additionally, perturbations such as species introductions, habitat degradation, pollution, and climate change can drastically alter the availability and quality of dietary resources. Stable isotope techniques offer a useful tool for gaining longer-term insight into the dietary habits for a variety of wildlife species. 

In collaboration with other USGS scientists, Federal and State agencies, and university researchers, Trust Species and Habitats scientists are using stable isotopes to: 

  • understand the foraging habits of polar bears, providing context from which future changes because of habitat loss can be inferred;
  • investigate nutritional stress as the cause of population decline in Stellar sea lions;
  • assess the effects of a gillnet ban on bottlenose dolphin;
  • reconstruct aquatic food web pathways before and after exotic species invasions; 
  • determine the arrival time on arctic breeding grounds of a long-distance migratory shorebird based on a shift in diet; and
  • clarify the role of marine-derived nutrients to juvenile salmon. Findings from this work are providing valuable insight into the foraging ecology of species of management or conservation concern.

The Stable Isotope Laboratory maintained by the Crustal Geophysics and Geochemistry (CGG) and Fort Collins Science Center (FORT) is a state-of-the-art facility with modern instrumentation capable of analyzing the isotopic composition of nearly any conceivable sample matrix. Laboratory staff and principal investigators have significant expertise in isotope systematics of the geosphere, biosphere, and hydrosphere. The Laboratory has demonstrated innovative approaches to challenging questions and as such, has significant latitude to address emerging issues in ecosystem science such as this.

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