Increases in benthic community production and metabolism in response to marine‐derived nutrients from spawning Atlantic salmon (Salmo salar)

Atlantic salmon (Salmo salar) and other anadromous fishes represent a major vector for transporting marine‐derived nutrients (MDNs) to Atlantic rivers. Marine‐derived nutrient subsidies may be key for maintaining ecological processes and ecosystem function in river basins. Stream channels and mesocosms designed to approximate natural river systems were used to measure the response of stream productivity in two treatments, one with marine‐derived nutrients from spawning Atlantic salmon and one without marine‐nutrient subsidies (control). Biofilm biomass (measured as chlorophyll a) and benthic metabolism (measured as the net change in dissolved oxygen) were evaluated from artificial substrata encompassing pre‐spawning, spawning and post‐spawning periods. We calculated marine‐nutrient contributions from spawning salmon using a simple empirical process model. We found that biofilm accrual was significantly greater with MDN inputs, with mesocosms having higher biomass (34.8 ± 1.2 mg chl a m⁻²) and growth rates (0.071 ± 0.002 mg m⁻² day⁻¹) than the stream channels (24.4 ± 4.8 mg chl a m⁻² and 0.052 ± 0.003 mg m⁻² day⁻¹, respectively). Both control mesocosms and stream channels had significantly lower biomass (10.4 ± 0.7 mg chl a m⁻² and 6.3 ± 2.5 mg chl a m⁻², respectively) and growth rates (0.042 ± 0.008 mg m⁻² day⁻¹ and 0.029 ± 0.009 mg m⁻² day⁻¹, respectively) than treatment channels. Despite having a lower biofilm biomass, stream channels yielded a significantly greater final gross primary production of 2343.5 mg C m⁻² day⁻¹. The presence of MDNs in the stream channels shifted biofilm metabolism from heterotrophy (P/R = 0.283) to autotrophy (P/R = 2.422) within 112 days. Spawning Atlantic salmon contributed a total of 20.98 g m⁻² of nitrogen and 1.04 g m⁻² of phosphorous to each stream channel, through excretion and gametes alone. There was a strong predictable positive relationship between the increase in productivity and the amount of marine‐derived nitrogen and phosphorous delivered. This study highlights the importance of marine‐derived nutrients from Atlantic salmon for driving and maintaining freshwater productivity. Marine‐nutrient subsidies relieve the ‘bottom‐up’ constraints on stream productivity by facilitating the production of enough energy to support the food web, lessoning the reliance on outside energy sources.