Water Quality, Chlorophyll, and Periphyton Responses to Nutrient Addition in the Kootenai River, Idaho

Abstract

During the past century, the Kootenai River, Idaho (USA), has experienced cultural oligotrophication following extensive levee construction, channelization, wetland drainage, and impoundment. A multiyear, whole-river nutrient-addition experiment was undertaken to mitigate these effects. The river was dosed with liquid agricultural-grade ammonium polyphosphate fertilizer (10-34-0) from June through September 2006-2010 to achieve an in-river total dissolved P (TDP) concentration of 3.0 μg/L. A fine-scale monitoring program included 8 sites over a 20-km reach (2 upstream control sites, one injection site, and 5 downstream treatment sites). Nutrient addition did not significantly increase N and P concentrations in the water column, but it significandy increased chlorophyll accrual rates and densities of edible green algae and diatoms. Nutrient addition significantly reduced NO3-+NO2- concentrations, atomic TN:TP ratios, and densities of inedible cyanophytes. Mean NO3- +NO2- values decreased along a downstream gradient below the nutrient-addition site, whereas chlorophyll accrual rate typically peaked immediately downstream from the nutrient addition site then decreased progressively down-stream. Our study showed that nutrient addition is a useful river restoration technique for the Kootenai River.