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Bighorn River Macroinvertebrate Monitoring Summary

Jan 15, 2021

Lead Scientist

David Stagliano

study objectives

The main objectives of the macroinvertebrate portion of the BHRA Research Initiative are:

1. To develop a long-term data set on benthic macroinvertebrate populations and community assemblage structure along the study reach of the Bighorn River (Map 1) by collecting repeatable, quantitative, baseline data using standardized methods (Photo 1).

  • What historical macroinvertebrate data has been collected and how can they be used in comparison to current communities?

  • How can we relate these biological indicators to spatial and temporal trends in aquatic habitat health as influenced

2.  Evaluate longitudinal macroinvertebrate patterns to help BHRA understand how the river’s water quality, hydrology, flow management and temperature regimes has changed through time, and lead to an understanding of the overall biological health of the Bighorn River.

  1. Benthic macroinvertebrate densities reported at Split Island on September 2020 averaged 4,434 individuals per m2 (± 210 SE); this is significantly (9 times) lower than reported in Sept. 1987 (41,761 ind. per m2) (Table 1). Benthic macroinvertebrate densities at Three Rivers in September 2020 averaged 21,970 individuals per m2 (± 1,509 SE); this is significantly (>3 times) lower than densities reported in Sept. 1987 (75,670 ind. per m2) (Table 1). Despite the absence of more recent comparison data, this type of decrease can often be attributed to large flushing flows (2017-2019) that scour benthic sediments and reduce the presence of silt-tolerant organisms (midges and worms), and allow the colonization of caddisflies and mayflies that prefer cleaner gravels.

  2. Multiple species reported in the 2020 Bighorn River samples were not observed during the 1986-1987 sampling (Figure 3). No individuals of the aquatic moth, Petrophila, were reported during any sample period in 1986-1987, but in 2020, we documented this species at densities of 438 individuals per m2 (9.8% of the community) at Split Island and 400 per m2  (1.8%) at Three Rivers. Similarly, the Isopod, Caecidotea was not reported during 1986-1987, but we documented this taxon at moderately abundant numbers (684 ind. per m2) at Split Island (15% of the community) and 7,213 ind. per m2 at Three Rivers (34%).  Likewise, the non-native, New Zealand mudsnail (NZMS) was not present at either upper site in 1987, but in 2020, Three Rivers has benthic densities of 880 ind. per m2. The presence of NZMS at these densities in the Bighorn River is not a surprise; they occur up and down the Madison and Missouri Rivers at heavily fished access points. 

  3. The biggest differences observed in the composition of the benthic community at Split Island in 2020 vs. 1987 are the decreases in dominance of the midges, and the increase in abundance of the caddisflies, Amiocentrus aspilus and Hydropsyche spp. (which were both present in low numbers in 1987), but now contribute ~18% of the community. Additionally, the aquatic moth Petrophila which was not reported in 1987, but now contributes ~10% to the benthic community.  

  • From the fall 2020 macroinvertebrate data at these two sampling sites, we have already documented large decreases in benthic densities since the late 1980s, and shifts in the benthic community, including decreases of midges and mayflies, increases in caddisflies and the addition of New Zealand mudsnails, aquatic moths and the isopod, Caecidotea.

  • We will continue to analyze the macroinvertebrate samples from the remaining six sampling sites. The data will be compiled to compare current conditions across all eight sites and to compare current conditions with older community data.

  • We will attempt to answer additional questions: What is the current health status of the macroinvertebrate community at these sites, since not having been sampled in 14 years or longer? Did the declining macroinvertebrate trends seen in the historical data at Manuel Lisa FAS continue or were they altered by changing environmental conditions?

  • These and other interesting questions will be addressed in the final report which will become available year-end.

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