Salmon-mediated nutrient flux in selected streams of the Columbia River basin, USA

Salmon provide an important resource subsidy and linkage between marine and land-based ecosystems. This flow of energy and nutrients is not unidirectional (i.e., upstream only); in addition to passive nutrient export via stream flow, juvenile emigrants actively export nutrients from freshwater environments. In some cases, nutrient export can exceed import. We evaluated nutrient fluxes in streams across central Idaho, USA, using Chinook salmon (Oncorhynchus tshawytscha) adult escapement and juvenile production data from 1998 to 2008. We found in the majority of stream-years evaluated, adults imported more nutrients than progeny exported; however, in 3% of the years, juveniles exported more nutrients than their parents imported. On average, juvenile emigrants exported 22%+/- 3% of the nitrogen and 30%+/- 4% of the phosphorus their parents imported. This relationship was density-dependent and nonlinear; during periods of low adult abundance, juveniles were larger and exported up to 194% and 268% of parental nitrogen and phosphorus inputs, respectively. We highlight minimum escapement thresholds that appear to (i) maintain consistently positive net nutrient flux and (ii) reduce the average proportional rate of export across study streams. Our results suggest a state shift occurs when adult spawner abundance falls below a threshold to a point where the probability of juvenile nutrient exports exceeding adult imports becomes increasingly likely.