National Park Service


Date of this Version



Tronstad, L. 2012. Aquatic invertebrate monitoring at Agate Fossil Beds National Monument: 2011 annual report. Natural Resource Technical Report NPS/NGPN/NRTR—2012/653. National Park Service, Fort Collins, Colorado.


Aquatic invertebrates are excellent animals to use for monitoring ecosystem quality; however, how to sample aquatic invertebrates for such monitoring efforts is a central question. All samplers have advantages and disadvantages, and finding the sampler that minimizes bias and fulfills the objectives is crucial. The ecosystem quality of the Niobrara River at Agate Fossil Beds National Monument has been measured for 15 years using aquatic invertebrates colonizing Hester-Dendy samplers. These artificial substrate samplers are useful in rivers that are difficult to sample, but previous studies demonstrated that they bias results toward certain insect orders. Additionally, large debris dams formed upstream of these samplers in the Niobrara River potentially altering samples. Therefore, we compared aquatic invertebrates collected using Hester-Dendy samplers and a Hess sampler in the Niobrara River. Hester-Dendy and Hess samplers collected a similar invertebrate assemblage; however, Hess samples collected fewer mayflies, and fewer true flies, but more dragonflies and damselflies compared to Hester-Dendy samplers. Bioassessment metrics calculated using the two samplers were not statistically different. Three bioassessment metrics changed over time. Hilsenhoff’s Biotic Index (HBI) increased over the last 15 years, indicating that invertebrates living in the Niobrara River are more tolerant of pollution. Mayfly, stonefly, and caddisfly (EPT) taxa richness and the proportion of EPT taxa have declined over time, showing a decline in the number of sensitive invertebrates. I recommend collecting aquatic invertebrates using a Hess sampler in the Niobrara River at Agate Fossil Beds National Monument, because the Hess sampler will reduce the number of visits to each site reducing overall costs. Furthermore, Hess samples collect the natural density and diversity of invertebrates, and results are compared to other ecosystems.