Mobile scavengers create hotspots of freshwater productivity

Adjacent communities and ecosystems often differ in underlying productivity but are connected by flows of nutrients, energy, and matter. Pacific salmon ( Oncorhynchus spp.) transport substantial quantities of nutrients from marine ecosystems to coastal freshwater habitats when they return to spawn and die. Nutrients from their carcasses are initially concentrated in spawning streams and lakes, but are subsequently dispersed by abiotic (floods, hyporheic flow) and biotic processes (predators and scavengers). In southwest Alaska, mobile avian scavengers (gulls; Larus spp.) breed on small islands within salmon nursery lakes and consume large quantities of spawning salmon during the chick-rearing period. However the role of birds as vectors of salmon-derived nutrients remains unknown. We examined how gulls – by transporting salmon tissues to their chicks – create hotspots of biological productivity in the aquatic habitats surrounding their nesting colonies. We found that algal production was ～10× higher at islands with high gull densities compared to islands without nesting gulls, but was concentrated within 40 m of island shorelines. Carbon stable isotopes (δ¹³C) confirmed that gulls enhance primary production in local benthic communities and demonstrated that this production was transferred up the food web to grazers (snails) and carnivores (blackfish). Nitrogen stable isotopes (δ¹⁵N) confirmed that salmon dominated the diet of gulls and that nutrients from gull guano were incorporated into algae and passed up the food web. By relocating and concentrating salmon-derived nutrients into new and distant locations, gulls alter and magnify production in local aquatic communities. We offer the first evidence that the avian community can move salmon-derived nutrients great distances, enriching otherwise isolated habitats.