Glacial Meltwater Controls the Distribution of Benthic Invertebrate Communities in Alpine Lakes

Research Summary

Written by Danielle Haskett, University of Georgia

The Question: Is the meltwater emanating from glacial retreat changing the insect communities that live on the sediment bottom of high elevation lakes? How will this affect paleoclimatic studies that use these insects to reconstruct temperatures over millennial time scales?

Chironomids (or midges) are true flies that possess identifiable features that allow identification down to the species level. Chironomus spp. (left) and Heterotrissocladius spp. (right) are prevalent in all samples collected from Rocky Mountain National Park.

Photos courtesy of Danielle Haskett

Chironomids, or midges, are true flies that are known to be sensitive to changes in temperature and are used as a biological proxy in paleoclimate studies. They are used to model changes in temperature over varying time scales and are found all over the world. While the adults can fly to their preferred air temperature range, the majority of the insect’s life is spent as larvae at the bottom of the lake. Which temperature (air or water) is responsible for the distribution of chironomids when the two temperatures no longer covary? To date, the studies that have assessed the response of midges to glacial melt in alpine settings have focused solely on alpine streams. These studies indicate that chironomid communities are responsive to glacier meltwater input. However, no work has assessed how glacial meltwater has affected the midge communities in high elevation lakes. Danielle Haskett, a Ph.D. student at the University of Georgia, was the lead investigator.

The Project: Collect environmental variables during core sediment collection to categorize the limnology of each lake. Analyze sediment cores from ten alpine lakes for chironomid community assemblages.

Map of Study Sites. All lakes were located on the eastern side of the Continental Divide.

Map created by Danielle Haskett

Scientists collected sediment cores from the deepest points in ten lakes. These lakes comprised five sets of paired lakes where all environment variables, such as elevation, vegetation, and bedrock were controlled for as much as possible. The only difference between the pairs was the presence or absence of glacial meltwater in the catchment. The pairs were: 1) Cony and Pipit Lakes, 2) Hutcheson and Falcon Lakes, 3) Eagle and Box Lakes, 4) Black and Thunder Lakes, and 5) Odessa and Spruce Lakes. During the sediment core collection, a total of 43 environmental variables were also examined and collected. These included lake depth, secchi depth, pH, specific conductivity, and nutrients. Scientists then sub sectioned the ~20 cm sediment cores into 0.25cm sections. They identified individual chironomid species using standard laboratory procedures. The activity of naturally occurring lead isotopes was used to develop age chronologies for historical sediment.

The Results: Evidence suggests that the glacial retreat in most lakes is shaping the modern distribution of chironomid communities. However, changes over time are more variable due to within-lake variability.

Scientists on an inflatable boat collect environmental data and a lake core from Pipit Lake.

NPS/J Westfall

Statistical analysis indicates that the chironomid taxa associated with the colder lakes typical of glacial input are different than those found in warmer lakes. These taxa include Diamesinae, Heterotrissocladius, Diplocladius, and Rheocricotopus. Heterotrissocladius was a very common taxon and is typically found in the profundal zone of cold, non-productive lakes. The remaining three are not common in lake sediment and are often associated with cold, running water. Diamesinae is of particular interest in that the remains of this taxa are extremely rare in lake sediment and poorly studied. Recent studies of chironomids in alpine streams find that the presence of Diamesinae increases with closer proximity to the terminus of melting glaciers (Lencioni, 2018). For this study, multiple species were grouped into the subfamily Diamesinae in analysis as they were not found in the multiple taxonomic keys and libraries available for chironomid identification. Future research will investigate these species as possible new species. The presence of Diamesinae may also be used as a qualitative indicator of glacial meltwater and may assist historical reconstructions that use chironomids as a biological proxy for temperature.

Figure 1. Rank clocks of the most common and indicator species present: Top pair: Cony (glacial) and Pipit (snow-fed) lakes; middle: Eagle (glacial) and Box (snow-fed) lakes; bottom: Black (glacial) and Thunder (snow-fed) Lakes. The clocks assess changes in abundance of individual taxa over time.

Figure courtesy of Danielle Haskett

Important players in lakes receiving no meltwater were more likely influenced by within-lake variability and location with regards to tree line. Pipit Lake lies within a rocky cirque along the continental divide with no vegetation in the catchment. Chironomus, Heterotrissocladius, and Procladius were the dominant species present. Chironomus spp. are eurythermic and can be present in a wide range of temperatures and is the most common taxa found in all lakes sampled. They are mostly confined to the profundal zone of lakes and may survive periods with low oxygen availability. The increasing abundance of Procladius in Pipit Lake over the 20th century and into the 21st century suggests an increasingly productive environment. Box Lake, located 3274 meters above sea level (m asl), lies at tree-line. The chironomid assemblage was unlike any other lake sampled. The taxa with the highest abundance belonged to Corynocera oliveri-type, and Corynocera ambigua-type as the second most prominent taxa present (not pictured). The presence of both species suggests that the location at tree line is the factor most likely to be responsible for the chironomid assemblage found in this lake. Of further note, iron nodules were found in sediment that corresponded with the late 1970s thru the early 1980s. These nodules can only form in sediment where oxygen has been depleted and would suggest a period of high algal productivity (Davison, 1993). Chironomid communities in Box Lake exhibited an extreme drop in abundance at this time and did not recover until 2010 (Figure 1). Thunder Lake is located below tree line and is the most diverse lake sampled. Chironomus and Heterotrissocladius dominated the chironomid assemblage. These taxa were inversely related to one another, and as the abundance of one taxon increased, the other declined.

Overall, lakes receiving glacial meltwater were 2.62°C colder than their nonglacial counterparts. Nitrate (NO₃) was 66% higher in glacial lakes. Statistical analyses indicate that only one environmental variable was statistically significant. The variable responsible for the distribution of modern chironomid assemblages is surface water temperature (p=0.04). However, the relationships between SWT and NO₃ were explored using Pearson’s Correlation coefficient and were found to be highly and negatively correlated (-0.82). The mechanism that delivers cooler surface water temperatures and increased levels of nitrate are both related to glacial meltwater and indicate that chironomid communities in the alpine lakes are being controlled by glacial meltwater.

Management Implications: The benthic invertebrate communities in the high elevation lakes of Rocky Mountain National Park will change dramatically once cirque glaciers are no longer present on the landscape.

Danielle Haskett collects a sediment core at Falcon Lake

Photo taken by Gretchen Sneegas, courtesy of Danielle Haskett

Chironomid communities are drastically different between glacial and nonglacial lakes. Taxa, such as Diamesinae, will disappear from assemblages altogether and Chironomus will replace Heterotrissocladius as temperatures increase. The abundances (or the number of bugs present in any given sample) were much lower in glacial lakes. Once glaciers are no longer on the landscape, temperatures in these lakes will increase, and their benthic bug communities will shift and alter dramatically over a very short period. Specialized taxa will disappear from the assemblages altogether, and biodiversity will increase. This situation holds large ramifications for the cold-adapted species that are endemic to the front range, such as the Greenback Cutthroat Trout.

References:
Davison, W., (1993). Iron and manganese in lakes. Earth-Science Reviews, 34(2), pp.119-163.
Lencioni, V., (2018). Glacial influence and stream macroinvertebrate biodiversity under climate change: Lessons from the Southern Alps. Science of the Total Environment, 622, pp.563-575.

This project was funded through the Continental Divide Research Learning Center's annual funding opportunity targeting park management relevant research, technical assistance and education projects.