UMOWA began the Missouri River Macroinvertebrate Monitoring program in 2015 and has continued this effort into 2019. We established seven long-term, baseline monitoring stations for sampling benthic macroinvertebrates and water chemistry within the section of the Missouri River between Wolf Creek and Cascade. Aside from one long-term NWE (PPL) monitoring site below Holter Dam sampled in August from 1995-2019, and 5 sites sampled once in 2005 by MDEQ, very few macroinvertebrate samples have been collected below Wolf Creek Bridge (MDEQ 2007).
Record high spring flows in 2018 have played a particularly important role by flushing silts from gravels, reshaping riffles and reducing macroinvertebrate densities at many of the main stem Missouri River sites; thus, the relative composition of many benthic macroinvertebrate assemblages has been restructured. In late-September of 2019, we collected 21 quantitative macroinvertebrate samples and habitat data at 7 sites; water chemistry (WQ) samples were taken on October 1st . Total nitrogen (TN) and phosphorus (TP) levels for October 2019, as well as June & October 2018, exceeded MDEQ’s numeric nutrient standard across most sites, except downstream of LPPC and below the Dearborn River (see UMOWA 2019 WQ Report).
Other than the indirect effects of increased weed bed coverage and trapping of sediments, we can find no direct correlation between the Missouri River’s excessive nutrient concentrations and macroinvertebrate population characteristics.
Spring run-off events from Little Prickly Pear Creek (LPPC) and the Dearborn River continue to have significant effects on the densities and diversity of insect communities in the Missouri River below those tributaries, especially by adding mayfly (E), stonefly (P) and caddisfly (T) taxa (total EPT taxa); although, no new species were added in 2019. In 2016, at the Missouri River u/s of LPPC we observed the highest densities of macroinvertebrates (>20,000 per m2 ) and New Zealand mudsnails (>300 per m2 ) since 2015; these densities were significantly “knockedback” by high spring flows in June 2017 and 2018 (now benthic communities avg. ~11,000 ind. per m2 ). But surprisingly, even with the high flows, annual NZMS densities increased further at this site in 2019.
High densities and standing crops of Baetis (BWOs), Tricorythodes (Tricos), Chironomidae (midges), and Ephemerella (PMDs) in the spring samples from Wolf Creek to Craig section “hatch-out” by the summer sampling period leading to lowered summer/fall densities of these taxa, and a dominance of non-insect taxa (scuds, sowbugs, worms and snails).
EPT taxa (mostly mayflies and caddisflies) have generally been low (<40% of all inverts) during previous fall sampling at LPPC, Craig and upstream of the Dearborn (2016-2018), but the fall 2019 samples u/s of the Dearborn had a significant increase in %EPT taxa, especially with the caddisflies. Hardy Bridge has also seen an increase in the caddisfly contributions to the benthic community. Therefore, it does initially appear that the summer 2020 caddisfly hatch numbers will be increased in this reach of the river.
Of the 21 species of caddisflies that were collected across the sites, the net spinning caddis, Cheumatopsyche and the long-horned caddis, Oecetis (tan caddisflies) were usually the most prevalent. Caddisflies were more common in the summer sampling period and had been less of a percentage of total EPT than the mayflies, but in the fall of 2019, some sites reported much higher caddisfly densities and contributions to the % EPT metric.
EPT taxa richness, in general, increases with increasing distance from Holter Dam. Highest total taxa and EPT richness were reported at sites least affected by the dam, particularly the Missouri River at Hardy Bridge and Cascade which are 28 and 37 miles downstream from Holter dam, respectively. The Cascade site also contained the most unique benthic fauna reporting four mayfly species that were collected nowhere else in the study.
While not that important in the trout diet, the riffle beetle, Optioservus quadrimaculatus contributes significant numbers (avg. ~1,000 ind. per m2 ) to the benthic macroinvertebrate communities of the Missouri River sites between Wolf Creek and Craig; these are not represented in any of the metrics analyzed, because they are non-EPT insects.
Overall, macroinvertebrate communities collected in 2019 resemble those reported in 2015-2018 with similar taxa composition. There are some continued increasing trends in Total and EPT taxa richness at the Missouri River u/s LPPC, Craig, D/S of the Dearborn River, Hardy Creek and Cascade. We recorded significant reductions in Chironomidae (midges) and the % of noninsect taxa in the summer samples, especially at sites from LPPC to upstream of the Dearborn River. Increases in the percentage of non-insect taxa in the fall samples, largely from Turbellarian flatworms, worms and sowbugs, comprising the benthic samples was substantial in the Missouri River from Craig and upstream of the Dearborn River.
This invertebrate community shift from summer to fall reflects an increase in sediment build-up in many gravel areas of the stream channel, compounded by large contributions of aquatic vegetation trapping sediments. HBI tolerance scores indicated significant organic pollution at four of the seven (>57%) monitoring sites, and this metric has also been used as a surrogate for sediment impairment. We have documented significant nutrient concentration increases since the fall of 2017 in both TN and TP.
In 2015, New Zealand mudsnails (NZMS) had low densities at multiple sites in the Missouri River from Wolf Creek to Cascade. They began to substantially increase their populations upstream of Craig, especially upstream and downstream of LPPC. High densities of the NZMS in 2016 at the MO_LPPC_US site were reduced by 66% after the flushing flows of June 2017 and by 60% at Craig in 2018.
We postulate that NZMS population increases, overall non-insect macroinvertebrate density increases, significant caddisfly decreases and the expansion of rooted weed-beds in the Wolf Creek to Dearborn section is potentially correlated with lower spring flushing flows from Holter Dam since 2011. The 2019 monitoring samples have largely focused on determining macroinvertebrate community responses (i.e., caddisflies, NZMS, non-insect taxa) to the above average 2017 spring flows, and the larger flushing flows of 2018.
Average total species richness per sample continues to increase at some sites (MO_LPPC_US, MO_Craig, Hardy and MO_Cascade) despite not gaining any new species, so diversity is improving across the section. This is largely a result of the flushing flows of 2018, where significant disturbance of benthic sediments and silt removal has decreased abundant midges, worms and other non-insect taxa and opened up cleaner gravel substrates for the typical caddisfly and mayfly species usually present in riffle habitats. Based on these increasing densities, tan caddisfly hatches should be on rise from Craig to the Dearborn River down to Hardy Bridge in the summer of 2020.