By , Published: June 20, 2023

A researcher brought her love of birds to a watershed in Summit County, Colorado, affected by acid mine drainage. Recently published in the journal Diversity, the resulting research connects some dots between water quality, life in streams, and life on land in the granitic landscape of the West.

Former CU graduate student Kelly Watson is lead author of the study with Professor and INSTAAR Fellow Diane McKnight. Based on her master’s degree research, the study aimed to see if breeding birds were avoiding streams with high metal concentrations.

The study found that, despite an order of magnitude increase in metal concentrations in some streams, birds did not seem to be avoiding them.

A stream in the Snake River watershed flows over rocks with a red coating, the result of high metal concentrations in the stream.

A stream in the Snake River watershed flows over rocks with a red coating, the result of high metal concentrations in the stream.

Diane McKnight has led investigations of water quality and stream ecology in Colorado’s Snake River watershed for decades. The watershed has a section with pristine streams, and a section with streams impacted by acid mine drainage.

Acidic drainage can happen wherever geology puts sulfide-based minerals together with air and water. But this natural weathering process is greatly intensified by mining activities that expose more rock. As a result, sulfuric acid and dissolved metals, like iron, drain into streams, where the more acidic water can further dissolve heavy metals and rare earth elements. Warmer, earlier summers are causing longer, lower stream flows, making it easier for metals to leach into streams.

“Acid mine drainage is a source of water pollution in the Rocky Mountains and around the world,” said Watson. “We know it has a lot of impacts on organisms living in the stream.”

McKnight has conducted research on many aspects of water quality in affected streams, as well as how it affects life within streams. The acidity of the water controls which species can live in it. For instance, the nymphs and larvae of mayflies populate the pristine streams of the Snake River watershed, but only the acid-tolerant larvae of stoneflies live in streams affected by acid mine drainage. Streams with particularly high metal content may develop a coating on their beds that inhibits insect and other life.

Relatively little is known, however, about how elevated metals and rare earth elements may affect terrestrial species living near polluted streams.

Then Watson joined her research program. Watson “had studied birds as an undergraduate at the College of William & Mary,” said McKnight. “She wanted to work with birds for her master’s thesis.” It was an opportunity to investigate links between stream chemistry and terrestrial life in the Snake River watershed.

“Something to consider is, what should we be looking at when we remediate mine drainage sites?” said Watson. “We’ve known for a long time that benthic macroinvertebrates are indicators of stream health. What about species that live on land around the streams?”

McKnight and Watson designed a research project to look at water quality, insect and other life in the streams, and birds breeding in the watershed. Were birds avoiding sites next to streams affected by acid mine drainage?

Because the insects and other invertebrates living in acidic streams accumulate metals in their bodies, there is concern that birds or fish that use them as a food source might also start to accumulate metals. There was some evidence of concern from a previous study in the area, which found elevated metal concentrations in tree swallow nestlings. But little was known about potential effects on migratory birds that breed in the area.

“As an undergraduate, I was part of a bird monitoring program in Virginia,” said Watson, who is now the Principal Floodplain Planner for Boulder County. “I learned how to do point counts and run the occupancy models we used in this study.”

The takeaway is that that birds that live by the stream are probably fine even though metals in the water have gone up by an order of magnitude.
- Diane McKnight

Watson worked together with Stella Koliavas, a student in the Undergraduate Research Opportunities Program (UROP), to conduct bird counts at twenty study sites along the main streams of the Snake River watershed during breeding season. They observed robins, dark-eyed juncos, Wilson’s warblers, Lincoln’s sparrow, mountain chickadees, ruby-crowned kinglets, white-crowned sparrows, and yellow-rumped warblers at many of the sites.

She also took samples of the water and macroinvertebrates living in the stream and analyzed them for concentrations of trace metals and rare earth elements. She then used a computer model to relate the presence of birds to various types of habitat and conditions, and to the potential for trace metal and rare earth element exposure.

The most acidified stream was about the acidity of soda or grapefruit juice, and showed more dissolved metals and very little resident life. Stonefly larvae dominated the acidified streams, which were about as acid as a cup of black coffee; while mayflies were the most abundant macroinvertebrate in the pristine stream, with a neutral pH. Overall, the water quality was similar to that found in many prior studies of the same watershed, though quantities of metals and rare earth elements in the streams are generally increasing.

Watson found that birds were found throughout the watershed area. The only pattern in their residency seemed to be habitat—a few birds are “habitat specialists” who nest in shrubby environments rather than forest. Habitat, but not water quality or invertebrate prey quality, is an important factor for these species during the breeding season.

The study also showed that the relationship between the amount of metals in a stream and the amount of metals accumulated in the bodies of insects living in the stream is not straightforward. This may affect how many metals birds ingest if they prey on the insects. While the species present in the streams are controlled by the acidity of the water, insects’ metal accumulations seem more related to what they eat and whether those food sources originate in the stream or out of it.

Watson and McKnight recently adapted Watson’s master’s thesis into the journal article. “I have a full-time day job, so on nights and weekends, Diane and I would collaborate and work on revisions,” said Watson.

“The takeaway is that that birds that live by the stream are probably fine,” said McKnight, “even though metals in the water have gone up by an order of magnitude.”

“The reason that’s interesting is because the stream conditions are getting worse,” she said. While the study was not designed to show any possible negative effects of birds breeding near acidic streams, the birds may be an example of resilience in a changing climate.

Another aspect of the study was finding that the concentrations of some rare earth elements in invertebrates were high and aligned with concentrations of lead. There is little information about rare earth elements in aquatic organisms, and this finding might point the way to a need for further research on how metals and rare earth elements are transferred through ecosystems.

“This study indicates further inquiry on how metals may or may not be moving into terrestrial ecosystems is needed,” said Watson.

McKnight will give a free public talk on the study at the Mountain Research Station, near Nederland, Colorado, on Wednesday June 28. All are invited.