Going against the current: Evidence of labile organic carbon limitation in southeastern Alaska salmon streams

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Chelsea Crenshaw, Self employed, Gallina Inc., SANTA FE, NM, Richard T. Edwards, Pacific Northwest Research Station, Juneau, AK, Dominic Chaloner, Department of Biological Sciences, University of Notre Dame, Notre Dame, IN, Jason Fellman, Environmental Science Arts and Sciences - Natural Sciences, University of Alaska Southwest, Juneau, AK and Erik Norberg, Stormwater program, Alaska Department of Transortation and Public Facilities, Juneau, AK

Background/Question/Methods
Migrating Pacific salmon (Oncorhynchus spp.) deliver nitrogen (DIN), phosphorus (P), and carbon (C) to streams during their spawning runs. Salmon runs have been shown to increase dissolved nutrient concentrations in N and P. However, little evidence exists of significant or consistent salmon spawner-mediated increases in dissolved organic carbon (DOC) despite most of an adult salmon biomass being carbon. Furthermore, there has been no consideration of whether streams are limited by the availability of labile C that could strongly influence microbial N and P biogeochemical pathways. Laboratory and field experiments were performed to determine whether labile DOC additions alter N and P concentrations in streams during salmon runs in southeastern Alaska streams. For laboratory experiments, sediments from streams with and without spawning salmon were incubated with DOC (acetate), dissolved organic nitrogen (DON), and DOC+DON additions. For field experiments, whole-stream DOC (glucose/fructose) additions were performed on a stream during and after the salmon run.
Results/Conclusions
During laboratory incubations both dissolved ammonium and nitrate (DIN) concentrations decreased in water overlying stream sediment where salmon were present when labile DOC was added. Despite DOC addition, there was no net change in DOC or DON concentrations relative to controls with either sediment type. Field experiments showed that DOC uptake lengths were shorter, uptake rates higher, and uptake velocities faster in the presence of spawning salmon. Furthermore, DIN decreased (>200 μg/l) during the additions, and DIN uptake rates were higher in all experiments when DOC was added. Therefore, nutrients that salmon deliver to stream ecosystems may not be fully utilized without sufficient labile DOC as a substrate for those microorganisms involved in the mineralization and transformation of different forms of N and P.