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


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 δ15N and δ13C 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.

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We thank Gordon Reeves for helping initiate this study, and Ken Hodges, Merlyn Schelske and Samantha Greenwood for field and planning assistance, and Don Phillips for assistance with implementing the mixing model analysis. The manuscript benefited from presubmission reviews by Pete Bisson and Don Phillips, and from formal reviews by Mat Wooller and one anonymous reviewer. Bob Bilby helped estimate the carcass loadings for East Fork, Grizzly, Kennedy, and Griffin Creeks. Kristine Martin helped with fieldwork and laboratory analysis, and Anjana Rajendram of the Waikato Stable Isotope Unit, Department of Biological Sciences, University of Waikato, conducted the isotope analyses. Funding for this study was provided by the USDA Forest Service, Alaska Region, Juneau, Alaska, the Pacific Northwest Research Station, Portland, Oregon, and the University of Waikato, Hamilton, New Zealand. The sampling protocol and experimental enrichment complied with the current laws of Alaska. Use of trade or firm names in this publication is for reader information only and does not imply endorsement by the U.S. Department of the interior of any product or service.

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Correspondence to Brendan J. Hicks.

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Communicated by Jim Ehleringer

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Hicks, B.J., Wipfli, M.S., Lang, D.W. et al. Marine-derived nitrogen and carbon in freshwater-riparian food webs of the Copper River Delta, southcentral Alaska. Oecologia 144, 558–569 (2005). https://doi.org/10.1007/s00442-005-0035-2

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  • Stable isotopes
  • Marine nutrients
  • Dietary analysis
  • Coho salmon
  • Aquatic invertebrates