UMOWA's Baseline Macroinvertebrate Study
by Dave Stagliano, Montana Biological Survey
2019 marks the fifth year that UMOWA has funded the collection of macroinvertebrates at 7-9 sites in the Upper Missouri River. Scientifically valid macroinvertebrate data was collected during a variety of river conditions from a low mean discharge year (2016) to one of the highest (2018). Figure 1 - redline denotes average flow.
Macroinvertebrate populations across the study demonstrated patterns associated with streamflow: higher abundances and more silt-tolerant species (scuds, midges, aquatic worms) during below average flow years, and “better” insect communities with more caddisflies and mayflies in high flow years. Dominant taxa nearest Holter Dam (upstream of Little Prickly Pear Creek-LPPC) have experienced a significant shift to more Trico mayflies and tan caddis with fewer scuds and blackflies (Table 1). The invasive New Zealand mudsnail densities decreased at this site from ~373 per meter squared in 2016 to <200 per m2 in 2018; decreases in NZMS abundances have been documented at 5 of the 7 sites in 2018.
Table 1. Abundance of dominant taxa at the Missouri River upstream of LPPC between 2016 and 2018.
Regarding insect community “health”, the long-term monitoring site below Holter Dam (Northwestern Energy data) has seen bioassessment scores increase with each spring flushing flow and scores this high (65.7% in 2018) have not been seen since 1998 (Figure 2).
To be able to compare communities at sites over decades and document these trends underscores the importance of maintaining continuous, long-term data sets. UMOWA is committed to collect this important data at multiple sites in the Missouri River so that we may understand how insect communities respond to restoration management activities or modified dam operations.