AQUATIC INVERTEBRATE COMMUNITY STRUCTURE IN THE NIOBRARA RIVER, AGATE FOSSIL BEDS NATIONAL MONUMENT, NEBRASKA, 1996—2009

Aquatic invertebrates were sampled annually in the Niobrara River, Nebraska, during the period 1996—2009 using Hester-Dendy multiplate samplers. Collections indicated the invertebrate community in the river has shifted from one dominated by Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa to one dominated by Chironomidae and Amphipoda. Generally, EPT richness and percentage abundance of EPT of the total community, as well as percentage abundance of Heptageniidae and Leptophlebiidae, has declined across the years. During that same period, percentage abundance of Amphipoda and Chironomidae, taxa evenness, Shannon's Index, and the Hilsenhoff Biotic Index have increased. Stream discharge decreased significantly during the 15-year period when invertebrates were collected (Mann-Kendall trend test, P = 0.04). Water-surface elevation of the river measured with staff gages also gradually increased over time, although the trends were not significant (P ≥ 0.15). Although not analyzed statistically, water temperature (°C), conductivity (μS/cm), and pH gradually increased over time while dissolved oxygen concentration (mg/liter) decreased over time. Although other factors cannot be ruled out, a proposed reason for the observed changes in invertebrate community structure may be related to changes in the physical habitat condition in the Niobrara River associated with encroachment of the nonnative and invasive emergent aquatic plant, yellow flag (Iris pseudacorus L.). This weed has expanded into the stream channel, where it blocks streamflow.