Temporal patterns of isotopic enrichment in a Southeast Alaska stream foodweb: The role of salmon spawners and environmental context

The ecological effects of Pacific salmon (Oncorhynchus spp.) on stream ecosystems reflect the contribution of nutrients and energy from spawner excretion and carcass decomposition, as well as benthic disturbance from upstream migration and redd construction. The net outcome of spawner-mediated resource enrichment and disturbance is thought to be influenced by the environmental context, and reflected in the extent of salmon nitrogen (N) and carbon (C) incorporation into foodwebs. As part of a five-month salmon carcass augmentation experiment in Maybeso Creek, Southeast Alaska, we sampled foodweb compartments at approximately two-week intervals to establish enrichment by N and C stable isotopes of salmon origin. Consistent with other studies, presence of salmon increased isotopic enrichment of all compartments, but the magnitude and enrichment rate were closely linked to the abundance of live spawners rather than carcasses. Macroinvertebrate scrapers showed the most isotopic enrichment, followed by biofilm, and then juvenile coho salmon, suggesting indirect pathways of incorporation of salmon-derived nutrients and energy into stream foodwebs. Environmental factors, including temperature and discharge, did not appear to strongly influence isotopic enrichment by salmon spawners. Overall, our results confirm the importance of resources delivered by live salmon while the role of the environmental context appears more complex.