Beyond nitrogen and phosphorus subsidies: Pacific salmon (Oncorhynchus spp.) as potential vectors of micronutrients

Large quantities of material are moved annually from the ocean to freshwater systems by migrating Pacific salmon. Previous studies have focused on nitrogen and phosphorus provided by spawning salmon but largely ignored micronutrients essential to aquatic productivity. We collected salmon tissue, water, and biofilm from seven southeast Alaskan streams both before and during the salmon run to test for potential micronutrient provision by salmon and uptake by biofilm. To examine temporal patterns, one stream was also sampled with high frequency. Samples were analyzed using ICP-OES for boron (B), calcium (Ca), cobalt (Co), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), molybdenum (Mo), sodium (Na), selenium (Se), silicon (Si), and zinc (Zn). Arrival of salmon increased stream water loads for Ca, Fe, Mg, and Na and the concentration of Co in biofilm across the seven study streams. Stream loads of B and biofilm Cu and Na content decreased in the presence of salmon. By examining one stream at finer temporal resolution, biofilm increased in Ca, Fe, and Mn concentrations near the end of the salmon run, indicating possible lag effects between peak salmon densities and biofilm micronutrient uptake. The increase in stream water micronutrient loads across space for four elements and uptake of three elements in biofilm through time suggest that salmon are a potential source of essential micronutrients for freshwater ecosystems, as has been repeatedly demonstrated for macronutrients. This study expands our understanding of resource subsidies by identifying potential micronutrients important to those ecological dynamics.