Marine-derived nitrogen and carbon in freshwater-riparian food webs of the Copper River Delta, southcentral Alaska

Abstract

After rearing to adulthood at sea, coho salmon (Oncorhynchus kisutch) return to freshwater to spawn once and then die on or near their spawning grounds. We tested the hypothesis that spawning coho salmon return marine N and C to beaver (Castor canadensis) ponds of the Copper River Delta (CRD), Cordova, southcentral Alaska, thereby enhancing productivity of the aquatic food webs that support juvenile coho salmon. We sampled three types of pond treatment: (1) natural enrichment by spawning salmon, (2) artificial enrichment via addition of salmon carcasses and eggs, and (3) ponds with no salmon enrichment. All ponds supported juvenile coho salmon. Seasonal samples of stable isotopes revealed that juvenile coho salmon, threespine sticklebacks (Gasterosteus aculeatus), caddisfly larvae, leeches, and chironomid midge larvae were enriched with marine N and C. The aquatic vascular plants bur reed (Sparganium hyperboreum), pondweed (Potamogeton gramineus), and mare's tail (Hippuris vulgaris) were enriched with marine N only. Riparian vegetation (Sitka alder Alnus viridis ssp. sinuata and willow Salix spp.) did not show enrichment. Artificial additions of adult carcasses and eggs of coho salmon increased the delta15N and delta13C values of juvenile coho salmon. In this dynamic and hydrologically complex coastal environment, spawning coho salmon contributed marine N and C comprising 10-50% of the dietary needs of juvenile coho salmon through direct consumption of eggs and carcass material. Invertebrates that have assimilated marine N and C yield a further indirect contribution. This perennial subsidy maintains the productivity of the ecosystem of the coho salmon on the CRD.

Opposing forces: Evaluating multiple ecological roles of Pacific salmon in coastal stream ecosystems

“…Some studies have found that invertebrates from streams with higher salmon density have a more enriched nitrogen and carbon isotope signature (Bilby et al 1996, Hicks et al 2005 and are found in greater numbers (Wipfli et al 1999, Verspoor et al 2011, while other studies have found fewer invertebrates in the presence of spawning salmon (Moore et al 2004, Moore andSchindler 2008). These studies seem to contradict each other.…”

Effects of subsidies from spawning chum and pink salmon on juvenile coho salmon body size and migration timing

“…Our results suggest that body size in age 1 coho salmon had stronger relationships with adult chum salmon than body size in young-ofthe-year, which may reflect greater benefits when salmon nutrients are available directly to the juvenile fish, through preferential diet switching to eggs and tissue (Hicks et al 2005, Scheuerell et al 2007). These diets can have dramatically improved energy rations compared to diets not containing eggs (Armstrong et al 2010).…”

“…Notably, this study design does not take the full effect of spawning salmon into account (Tiegs et al 2011). For example, many of these studies do not include eggs, which are preferred by juvenile salmonids (Hicks et al 2005, Scheuerell et al 2007. Exceptions that did include egg provision were studies by and Lang et al (2006).…”

Stable isotopic (δ15N, δ13C) analysis of wood in trees growing in past and present colonies of burrow‐nesting seabirds in New Zealand. I. δ15N in two species of conifer (Podocarpaceae) from a mainland colony of Westland petrels (Procellaria westlandica), Punakaiki, South Island

“…Reduction in the sea-land nutrient flux led to major changes in the islands' vegetation (Croll et al 2005). Decreases in salmon (Oncorhynchus species) runs may also affect the well known marine subsidy of stream and riparian ecosystems along the Pacific coast of North America (Gende et al 2002;Hicks et al 2005).…”