Benthic Insect Assemblage and Species-Level Responses to Eleven Years of Nutrient Addition in the Kootenai River, Idaho

Abstract

Because of their abundance and intermediate trophic level positions, benthic invertebrates can affect trophic dynamics in aquatic ecosystems by routing and regulating energy through aquatic food webs. Significant reductions in nutrient loading and primary production downstream from dams can profoundly affect benthic insect assemblages. The Kootenai River, a large 7th order river in Idaho, Montana, and British Columbia, experienced such changes following construction of Libby Dam in 1972. A nutrient addition program was initiated in 2005 to mitigate the resulting cultural oligotrophication. The goal of this study was to evaluate long-term responses to nutrient addition among benthic insects at the assemblage and species levels, using replicated data collected from 2003 to 2016, and classified into pre-, early post-, and late post-nutrient addition periods. Response metrics included total abundance and biomass measures across all species, as well as aggregated species of major insect orders, five functional feeding groups, and species richness. We also included abundance responses from a substantial number of individual Chironomidae, and non-Chironomidae insect species. Effects of nutrient addition were assessed using before-after-control-impact and before-and-after analyses. Spearman's Rank correlation coefficients were used to express the degree of association between biotic and abiotic factors. Nutrient addition consistently contributed to increased individual and total insect abundance each year for over a decade without inducing substantive changes in community composition or species dominance. Overall, benthic insects responded positively to nutrient addition and did so in a similar fashion in the early and late nutrient addition periods.