Effects of nutrient replacement on benthic macroinvertebrates in an ultraoligotrophic reach of the Kootenai River, 2003–2010
G. Wayne Minshall (1), Bahman Shafii (2), William J. Price (2), Charlie Holderman (3), Paul J. Anders (4), Gary Lester (5), and Pat Barrett (5)
(1) Stream Ecology Center, Idaho State University, Pocatello, Idaho 83209 USA
(2) Statistical Programs, University of Idaho, Moscow, Idaho 83844 USA
(3) Kootenai Tribe of Idaho, Bonners Ferry, Idaho 83805 USA
(4) Cramer Fish Sciences, Moscow, Idaho 83843 USA
(5) EcoAnalysts, Inc. Moscow, Idaho 83843 USA
ABSTRACT
Large impoundments remove substantial amounts of sediment and nutrients from rivers and often limit production by downstream primary producers and secondary consumers. Nutrient levels and macro- invertebrate and fish abundance in the lower Kootenai River (7th order, mean annual discharge = 454 m3/s) in Idaho and Montana declined dramatically after Libby Dam was built in 1972. A subsequent study implicated ultraoligotrophic conditions (total dissolved P [TDP] ≤ 2 μg/L TDP) as a principal causative agent and prompted an on-going experimental nutrient-addition program for the Kootenai River downstream from Libby Dam, with dosing at the Idaho–Montana border. Pre-treatment monitoring began in 2003 and liquid ammonium polyphos- phate fertilizer (10-34-0) was added each year during the growing season from 2006 through 2010 with a target TDP concentration of 3 μg/L and TN∶TP near 20∶1. We studied benthic macroinvertebrate responses to the experimental addition and hypothesized moderate increases in invertebrate richness, abundance, and bio- mass with little change in assemblage structure. We used a before–after control–impact BACI design with macroinvertebrate samples collected pre- and post-treatment from July to early November 2003–2010 from fertilized and unfertilized reaches. After treatment, mean modified (Oligochaeta and Chironomidae subtaxa excluded) total abundance increased 72%, mean total abundance increased 69%, and mean biomass increased 48%. Abundance of Ephemeroptera, the principal insect order in the study area increased 66%. Filter-feeder abundance also increased, indicating increased suspended organic matter in addition to the attached forms consumed by other benthic macroinvertebrates. The first 5 y of experimental treatment resulted in increased food resources for resident native fishes with no major alteration of macroinvertebrate community structure or trophic pathways.