Nonnative Pacific salmon alter hot spots of sediment nitrification in Great Lakes tributaries

Biogeochemical transformations may represent an important pathway influencing the fate of nutrient subsidies in stream ecosystems. Pacific salmon (Oncorhynchus spp.) provide an ammonium (NH4+) subsidy to streams during their annual spawning runs, which may be transformed to nitrate (NO3-) via sediment nitrification. Increases in either forms of dissolved inorganic nitrogen may have ecosystem effects both at the reach and watershed scales, including the fertilization of algal biofilms and elevated export of nutrients to downstream ecosystems. In the nonnative range of salmon, where spawning runs are a relatively new phenomenon, few studies have explored the effect of introduced salmon on ecosystem processes. To assess the effect of nonnative salmon on dissolved inorganic nitrogen dynamics in Great Lakes tributaries, we quantified sediment nitrification in five streams before, during, and after the spawning run in 2009. Overall, sediment nitrification rates were higher in the channel thalweg (meanSE=1.90.1mgN/gAFDM/d) compared to channel margins (meanSE=0.90.1mgN/gAFDM/d). In the two streams with the largest salmon runs, nitrification was highest in the channel thalweg prior to salmon, but margin sediments had higher nitrification during the run. Among all streams, variation in nitrification rates was habitat specific, predicted by exchangeable NH4+ in sediments from the thalweg and predicted by salmon biomass for sediments in the channel margin. Nonnative salmon provide a pulsed source of inorganic nitrogen to Great Lakes tributaries, yet dissimilatory biogeochemical transformations such as nitrification may alter the form of the NH4+ subsidy and potentially influence downstream lakes via export of both NH4+ and NO3-.