David Walters, Ph.D.
Fort Collins Science Center
2150 Centre Avenue Bldg C
Fort Collins, CO 80526
David has been a research ecologist with the USGS since 2008. Prior to that, he was an ecologist for the U.S. EPA, National Exposure Research Laboratory for 6 years. He is a freshwater ecologist with broad training in stream ecology, human impacts on aquatic ecosystems, and ecotoxicology. His current research topics include food webs and contaminant flux, aquatic-riparian linkages, stream fish ecology, land use and climate change, and invasive species. He currently leads the Aquatic Ecology and Contaminants Team at the Fort Collins Science Center where the team investigates various human impacts to aquatic and riparian systems using field studies, manipulative experiments, and modeling.
- Ph.D. Ecology, University of Georgia (UGA), 2002
- M.S. Conservation Ecology, UGA, 1997
- B.A. Anthropology, UGA, 1991
Current Projects/Areas of Interest
- Riparian indicators of contaminant exposure at Great Lakes Areas of Concern (AOCs)
We are investigating contaminant flux from aquatic to nearby terrestrial (riparian) shoreline habitats. In particular we use riparian spiders (many of which feed almost exclusively on adult aquatic insects) to characterize contamination at these sites and to evaluate the effectiveness of their remediation.
- Leaky rivers: Nutrient retention and productivity in Rocky Mountain streams under alternative stable states
This project investigates how the volume of wood and log jams have declined in Rocky Mountain streams since European settlement, how the loss of wood affects stream geomorphology, communities, nutrient cycling, and productivity, and what management actions can be taken to restore lost ecosystem functions.
- Mechanisms for metal uptake and trophic transfer in stream and riparian food webs in mineralized landscapes
We are investigating how metals in streams (derived from natural geologic sources as well as mines) are transferred from streams to riparian zones and how this contamination alters ecological linkages between these systems. This work combines large-scale field studies with mesocosm experiments to better understand processes driving the patterns we observe in nature.
- “Metal webs” for the Grand Canyon
We are developing quantitative food webs to measure metal flux (mercury, selenium, and other trace metals) in the Colorado and Little Colorado rivers in Grand Canyon. These studies will identify key pathways of metal exposure to important fish species, such as the endangered humpback chub.
- Consequences of climate change for alpine lake-stream networks and native fishes in the southern Rocky Mountains
This project investigates the importance of alpine lakes in the ecology of native cutthroat trout and how lakes could mitigate the negative effects of rising temperatures on these threatened populations.
Science and Products
The digital macroinvertebrate reference collection was created by the USGS Aquatic Experimental Lab (AXL) to provide regional aquatic biology laboratories with a graphic tool to aid in the identification and verification of aquatic macroinvertebrates. Most aquatic biology/ecology labs possess a collection of ‘reference’ specimens so that technicians and taxonomists may compare and confirm specimens collected from a sample against expertly identified and verified type specimens of a particular taxa type. Unfortunately, specimens in these collections are often delicate and subject to damage and excessive handling will eventually render them unusable for verification purposes.
Cold-water fishes like trout, salmon, and charr are especially vulnerable to shifting conditions related to climate change; for example, warmer temperatures and more variable hydroclimate. Native cutthroat trout of the southern Rocky Mountains now only occupy a tiny fraction of their historic habitats because of stressors such as non-native fishes, habitat fragmentation, and detrimental land management practices. Using a combination of field and modeling approaches, we address how climate may influence native cutthroat trout and how conservation strategies can be tailored in a climate-smart approach to maximize conservation benefits under recent and projected climate conditions. Our research includes modeling surface temperatures for mountain lakes, and examining how altered thermal and hydrologic regimes affect critical life history events, such as spawning migrations. Ultimately, we will integrate these ecological patterns into state-of-the-art decision support models and use these models as tools to aid in the conservation of native cutthroat trout populations.
Most aquatic insects live in fresh water as larvae and move to land as flying adults to complete their life cycle. Although often ignored, the emergence of adults can transfer the effects of contamination from aquatic to terrestrial ecosystems as the adults are eaten by predators such as spiders, birds, and bats. Our scientists are investigating how contaminants move from streams, rivers, and lakes into riparian zones and how contamination alters the ecological linkages between these systems. We use large-scale field studies in various aquatic ecosystems, ranging from mountain headwater streams to the Great Lakes, and conduct complementary experiments at the new aquatic experimental laboratory at the Fort Collins Science Center. The mesocosm facility in our state-of-the-art laboratory is designed to replicate the coupled aquatic-riparian ecosystems. Results from these studies are helping managers to better evaluate water-quality criteria, to design and implement restoration plans for contaminated aquatic systems, and to assess the effectiveness of remedial actions.
A modeling approach to compare ΣPCB concentrations between congener-specific analyses
Changes in analytical methods over time pose problems for assessing long-term trends in environmental contamination by polychlorinated biphenyls (PCBs). Congener-specific analyses vary widely in the number and identity of the 209 distinct PCB chemical configurations (congeners) that are quantified, leading to inconsistencies among summed PCB concentrations (ΣPCB) reported by different studies...Gibson, Polly P.; Mills, Marc A.; Kraus, Johanna M.; Walters, David M.
Assessing atmospheric concentration of polychlorinated biphenyls (PCBs) by evergreen Rhododendron maximum next to a contaminated stream
Conifers are often used as an “air passive sampler”, but few studies have focused on the implication of broadleaf evergreens to monitor atmospheric semivolatile organic compounds such as polychlorinated biphenyls (PCBs). In this study, we used Rhododendron maximum (rhododendron) growing next to a contaminated stream to assess atmospheric PCB concentrations. The study area was located in a rural...Dang, Viet D.; Walters, David; Lee, Cindy M.
Rapid movement and instability of an invasive hybrid swarm
Unstable hybrid swarms that arise following the introduction of non-native species can overwhelm native congeners, yet the stability of invasive hybrid swarms has not been well documented over time. Here we examine genetic variation and clinal stability across a recently formed hybrid swarm involving native blacktail shiner (Cyprinella venusta) and non-native red shiner (C. lutrensis) in the...Glotzbecker, Gregory J.; Walters, David; Blum, Michael J.
Trophic magnification of organic chemicals: A global synthesis
Production of organic chemicals (OCs) is increasing exponentially, and some OCs biomagnify through food webs to potentially toxic levels. Biomagnification under field conditions is best described by trophic magnification factors (TMFs; per trophic level change in log-concentration of a chemical) which have been measured for more than two decades. Syntheses of TMF behavior relative to chemical...Walters, David; Jardine, T.D.; Cade, Brian S.; Kidd, K.A.; Muir, D.C.G.; Leipzig-Scott, Peter C.
Aquatic pollution increases use of terrestrial prey subsidies by stream fish
Stream food webs are connected with their riparian zones through cross-ecosystem movements of energy and nutrients. The use and impact of terrestrial subsidies on aquatic consumers is determined in part by in situ biomass of aquatic prey.Kraus, Johanna M.; Pomeranz, Justin F.; Todd, Andrew S.; Walters, David M.; Schmidt, Travis S.; Wanty, Richard B.
Methylmercury bioaccumulation in stream food webs declines with increasing primary production
Opposing hypotheses posit that increasing primary productivity should result in either greater or lesser contaminant accumulation in stream food webs. We conducted an experiment to evaluate primary productivity effects on MeHg accumulation in stream consumers. We varied light for 16 artificial streams creating a productivity gradient (oxygen production =0.048–0.71 mg O2 L–1 d–1) among streams...Walters, David; D.F. RaikowC.R. HammerschmidtM.G. MehlingA. KovachJ.T. Or
Turbidity alters pre-mating social interactions between native and invasive stream fishes
Environmental degradation can result in the loss of aquatic biodiversity if impairment promotes hybridisation between non-native and native species.Glotzbecker, Gregory J.; Ward, Jessica L.; Walters, David M.; Blum, Michael J.
The natural sediment regime in rivers: broadening the foundation for ecosystem management
Water and sediment inputs are fundamental drivers of river ecosystems, but river management tends to emphasize flow regime at the expense of sediment regime. In an effort to frame a more inclusive paradigm for river management, we discuss sediment inputs, transport, and storage within river systems; interactions among water, sediment, and valley context; and the need to broaden the natural flow...Wohl, Ellen E.; Bledsoe, Brian P.; Jacobson, Robert B.; Poff, N. LeRoy; Rathburn, Sara L.; Walters, David M.; Wilcox, Andrew C.
Metamorphosis alters contaminants and chemical tracers in insects: implications for food webs
Insects are integral to most freshwater and terrestrial food webs, but due to their accumulation of environmental pollutants they are also contaminant vectors that threaten reproduction, development, and survival of consumers. Metamorphosis from larvae to adult can cause large chemical changes in insects, altering contaminant concentrations and fractionation of chemical tracers used to establish...Kraus, Johanna M.; Walters, David M.; Wesner, Jeff S.; Stricker, Craig A.; Schmidt, Travis S.; Zuellig, Robert E.