David Walters, Ph.D.
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
Aquatic invertebrates are a key component of freshwater ecosystems, and an understanding of aquatic invertebrate taxonomy is central to freshwater science. The U.S. Geological Survey Aquatic Experimental Lab (AXL) at the Fort Collins Science Center has developed the North American Aquatic Macroinvertebrate Digital Reference Collection (NAAMDRC) to provide users with a graphic tool to aid in the identification and verification of aquatic macroinvertebrates.
AXL has developed a method for cataloging and digitizing reference specimens in such a way that a series of digital images provides enough information on key structures so a physical specimen is no longer necessary for verification. By using the most advanced and widely used taxonomic keys available, we have annotated important defining characteristics of each specimen so that non-expert technicians can more easily identify invertebrates found in aquatic samples.
The Aquatic Ecology and Contaminants Team investigates critical ecological processes operating in aquatic and riparian ecosystems and how these processes are affected by human activities. We address questions through a combination of field studies, laboratory experiments, and modeling, while working at multiple levels of biological organization from cells through ecosystems. Topics include land use and climate change effects on stream ecosystems and organisms, interactions between native and invasive fi shes, the flux of contaminants through aquatic and riparian food webs, and contaminant effects on aquatic-riparian linkages.
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.
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.
To create this dataset the authors exposed larval mayflies (Baetis tricaudatus) to an aqueous zinc gradient (3-340 µg Zn/l) and measured the change in zinc tissue concentrations at different stages of metamorphosis and changes in stable isotopes (δ15N and δ13C) in unexposed B. tricaudatus. Zinc concentrations in larvae and adults were positively related to aqueous concentrations.
Aquatic invertebrates are a key component of freshwater ecosystems, and an understanding of aquatic invertebrate taxonomy is central to freshwater science. The North American Aquatic Macroinvertebrate Digital Reference Collection (NAAMDRC) was created by the USGS Aquatic Experimental Lab (AXL) to provide users with high-quality digital microscopy photographs.
Holy flux: Spatial and temporal variation in massive pulses of emerging insect biomass from western U.S. rivers
The river stonefly, Pteronarcys californica (aka salmonfly), is an iconic insect in rivers of western North America due to its large size and its support of economically important species like wild trout (Nehring et al. 2011). Their emergence generates a large economic subsidy to local communities, as anglers from around the world...Walters, David; Wesner, Jeff S.; Zuellig, Robert E.; Kowalski, Dan A.; Kondratieff, Matt C.
A digital reference collection for aquatic macroinvertebrates of North America
Aquatic invertebrates are a key component of freshwater ecosystems, and understanding aquatic invertebrate taxonomy is a cornerstone of freshwater science. Physical reference collections of expertly identified voucher specimens are the ‘gold-standard’ used to confirm specimen identifications. However, most biologists lack access to such...Walters, David; Ford, Morgan A; Zuellig, Robert E.
Carbon dynamics of river corridors and the effects of human alterations
Research in stream metabolism, gas exchange, and sediment dynamics indicates that rivers are an active component of the global carbon cycle and that river form and process can influence partitioning of terrestrially derived carbon among the atmosphere, geosphere, and ocean. Here we develop a conceptual model of carbon dynamics (inputs, outputs,...Wohl, Ellen; Hall, Robert O.; Lininger, Katherine B; Sutfin, Nicholas A.; Walters, David
Zinc concentrations and isotopic signatures of an aquatic insect (mayfly, Baetis tricaudatus)
Insect metamorphosis often results in substantial chemical changes that can fractionate isotopes and alter contaminant concentrations. We exposed larval mayflies (Baetis tricaudatus) to an aqueous zinc gradient (3-340 µg Zn/l) and measured the change in zinc tissue concentrations at different stages of metamorphosis. We also measured changes in...Wesner, Jeff S; Walters, David; Schmidt, Travis S.; Kraus, Johanna M.; Stricker, Craig A.; Clements, William H.
Metamorphosis affects metal concentrations and isotopic signatures in a mayfly (Baetis tricaudatus): Implications for the aquatic-terrestrial transfer of metals
Insect metamorphosis often results in substantial chemical changes that can alter contaminant concentrations and fractionate isotopes. We exposed larval mayflies (Baetis tricaudatus) and their food (periphyton) to an aqueous zinc gradient (3-340 µg Zn/l) and measured zinc concentrations at different stages of metamorphosis: larval, subimago, and...Wesner, Jeff S.; Walters, David; Schmidt, Travis S.; Kraus, Johanna M.; Stricker, Craig A.; Clements, William H.; Wolf, Ruth E.
Pharmaceuticals and personal care products (PPCPs) are ecological disrupting compounds (EcoDC)
Pharmaceuticals and personal care products (PPCPs) are ubiquitous in freshwater ecosystems worldwide and are recognized as contaminants of concern. Currently, contaminants of concern are classified for their persistence, bioaccumulation, and toxicity (PBT criteria). PPCPs are not classified as persistent organic pollutants (POPs), although some...Richmond, Erinn; Grace, Michael; Kelly, John R.; Reisinger, Andrew; Rosi, Emma J.; Walters, David M.
Riparian spiders as sentinels of polychlorinated biphenyl contamination across heterogeneous aquatic ecosystems
Riparian spiders are being used increasingly to track spatial patterns of contaminants in and ﬂuxing from aquatic ecosystems.However, our understanding of the circumstances under which spiders are effective sentinels of aquatic pollution is limited. The present study tests the hypothesis that riparian spiders may be effectively used to track...Kraus, Johanna M.; Gibson, Polly P.; Walters, David M.; Mills, Marc A.
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...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...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)...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....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. Thus, stressors such as aquatic pollutants that greatly reduce aquatic secondary production...Kraus, Johanna M.; Pomeranz, Justin F.; Todd, Andrew S.; Walters, David M.; Schmidt, Travis S.; Wanty, Richard B.