Salmon Nutrient Mitigation Effects on Bottom-up Processes in Streams: Lessons from Large-Scale Experiments in Central Idaho

Declines of anadromous salmon throughout the Pacific Northwest have reduced delivery of marine-derived nutrients (MDN) to streams and rivers, which may have profoundly affected nutrient-poor stream ecosystems and food webs.  Additions of salmon carcasses, salmon carcass analog (pasteurized, pelletized fish meal), and inorganic fertilizers are proposed to mitigate for the loss of MDN in sub-drainages of the Columbia River Basin.  These treatments are predicated on the widely-held paradigm that added nutrients will transmit up the food web from primary producers to animal consumers in nutrient-limited systems.  Yet, the microbial dynamics and nutrient uptake patterns that underpin this bottom-up rationale are rarely studied in the context of MDN mitigation.  Our objective is to use the tools of nutrient cycling and ecosystem ecology to quantify the capacity of streams to retain and process MDN introduced via mitigation treatments in two studies in central Idaho.  In the North Fork Boise River, we are in year 4 of an experimental comparison of salmon carcass, analog, and inorganic fertilizer additions to 500-m stream reaches (n=3 for each treatment).  In the headwater region of the Salmon River, we are in the second year of a large-scale analog experiment (3000-m reaches; n=4 treatment, n=6 control).  In both study areas, nutrient diffusing substrates indicated that stream biofilms were nitrogen-limited before treatments, and remained nitrogen limited in analog, but not carcass-treated reaches, following treatments.  Biofilm biomass on natural stream substrates increased 2-5X within 4-wk post-treatment, suggesting that biofilms have the short-term capacity to respond to MDN mitigation.  However, in the Boise River experiment, concentrations of dissolved nutrients remained elevated for as much as 6-wk post-treatment, after biofilm responses had subsided.  Moreover, dissolved nutrients accumulated along the study reaches; for example, ammonium-N concentrations increased 2-3X along treated reaches 3-wk post-treatment, despite that algae and microbes highly favor ammonium as an N source. Nutrient uptake estimates suggested that treatments did not change the nutrient uptake capacity of stream biota, as uptake velocity and lengths were similar in treated vs. control streams, when detectable.  Therefore, biofilms can utilize added MDN until they are saturated, but because treatments do not increase the nutrient uptake capacity of the biofilm community, large amounts of nutrients introduced via mitigation may be exported.  Understanding the nutrient concentrations where biotic uptake becomes saturated, and the downstream distance that exported nutrients travel can help design more biologically- and cost-effective strategies for MDN mitigation throughout the Pacific Northwest.