Changes in Primary Producer and Consumer Communities in Response to Upstream Nutrient Addition in the Kootenai River, Idaho

Abstract The purpose of this study was to assess potential effects of nutrient addition in the previously unassessed lower Kootenai River (Meander Reach), 44–155 km downstream from the nutrient dosing site. We hypothesized that downstream periphyton and plankton densities would increase due to nutrient spiraling and organismal drift following seasonal upstream nutrient addition. Liquid inorganic ammonium polyphosphate fertilizer (10:34:0, N:P:K) was added to the Kootenai River in Idaho immediately downstream from the Idaho-Montana border with an in-river target concentration of 3.0 µg L-1 total dissolved phosphorus for 16 weeks (01 June–01 October) from 2006 through 2012. As predicted, mean periphyton, phytoplankton, and zooplankton densities increased significantly (P < 0.05) post-nutrient addition. Periphyton density increased 5.1 times (pre: 685.1 mm-2, post: 3501.0 mm-2), phytoplankton densities increased 1.7 times (pre: 1775.9 L-1, post: 3005.4 L-1), and zooplankton densities increased by 49% (pre: 67.0 L-1, post: 99.7 L-1). While nutrient uptake, water chemistry, algal and chlorophyll accrual, periphyton, macroinvertebrate, and native fish community responses to nutrient addition have been previously published, including results from upstream reaches of the Kootenai River, this study provides the first published account of lower trophic level responses at considerable distances downstream from the dosing site in altered, low velocity habitats which are very different from the upstream gravel-bed reaches reported in other studies. Results of this study add to the growing body of empirical knowledge regarding the use of nutrient addition for restoring large culturally denutrified rivers in the Pacific Northwest and provide valuable new insights regarding the longitudinal patterns of response magnitude.