Mobile scavengers create hotspots of freshwater productivity

Abstract

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.

Salmonid influences on rivers: A geomorphic fish tail

“…For SDN to have an influence, it must, therefore, be retained somehow. Four primary potential mechanisms exist by which SDN may be retained and assimilated within the riparian zone: (i) deposition of carcasses on streambank and floodplain surfaces and vegetation during high water, (ii) transference of dissolved nutrients through the hyporheic zone to groundwater and roots, (iii) via terrestrial and avian predators and scavengers, and (iv) freezing and ice formation in northern latitudes (Ben-David et al, 1998;Helfield and Naiman, 2001;Gende et al, 2002;Payne and Moore, 2006). Some of the imported nutrients are exported back downstream out of the riverine ecosystem as carcasses, dissolved nutrients, and juvenile outmigrants (Gende et al, 2002;Naiman et al, 2002;Scheuerell et al, 2005;Moore et al, 2007).…”

Impacts of seabird-derived nutrients on water quality and diatom assemblages from Cape Vera, Devon Island, Canadian High Arctic

“…Yet, while seabirds have been used to monitor environmental change in both marine (reviewed in Diamond & Devlin, 2003) and terrestrial (reviewed in Ellis, 2005) ecosystems, few studies have assessed the impact of seabird-derived nutrients on freshwater quality or biota (Izaguirre et al, 1998;Harding et al, 2004). In temperate and subarctic regions, nonmarine birds, including ducks, geese, and gulls, deposit significant amounts of nutrients into freshwater lakes (Manny et al, 1994;Kitchell et al, 1999;Payne & Moore, 2006). In Arctic regions, only a few studies have documented the transport of nutrients and pollutants into freshwater ponds and lakes via seabird guano (Evenset et al, 2004(Evenset et al, , 2007Blais et al, 2005;Brimble et al, submitted).…”

Deep-sea scavenging amphipod assemblages from the submarine canyons of the Western Iberian Peninsula

“…In an environment where food-supply is often a limiting factor, they play a pivotal role in the degradation and redistribution of organic matter (Christiansen and Diel-Christiansen, 1993). Scavengers breakdown, disperse, and reintegrate organic carbon from food falls into the deep-sea food web via predator-prey interactions (Payne and Moore, 2006) and by faecal enrichment of sediments (Stockton and DeLaca, 1982;Jones et al, 1998).…”