Ecological effects of spawning salmon on several southcentral Alaskan streams

Abstract

The ecological effects of salmon (Oncorhynchus spp.) carcasses on southcentral Alaskan streams were studied by: (1) observing salmon carcass decomposition and use; (2) comparing the macroinvertebrate community structure of streams receiving different inputs of salmon carcasses; and (3) quantifying the amount of marine-derived nitrogen (MDN) entering stream food webs using stable-isotope analysis. Abiotic mechanisms, such as large woody debris and the slow waters of stream margins and eddies were important in initial retention of salmon carcasses. Once entrained, carcasses decayed rapidly due to intense microbial processing. Stream insects and fishes were observed consuming carcasses, eggs, and smolts. Macroinvertebrate communities in streams receiving runs of salmon or in lake outlet streams were more diverse taxonomically. One functional feeding group, filterers (including net-spinning caddisflies (Hydropsychidae) uncommon in Alaska), increased in relative abundance. Although many other taxa also responded positively to enrichment, some taxa responded negatively. A significant difference existed in $\partial\sp{15}$N values between MDN and terrestrial sources but natural dissolved inorganic nitrogen contributions to stream food webs ($\approx$90-95% of total N) from groundwater generally overwhelmed the marine signal ($\approx$5-10% of total N). $\partial\sp{15}$N values generally suggested that some MDN ($\approx$15% of total N) entered into food webs after its incorporation into algal biomass but values for certain macroinvertebrate taxa (Arctopsyche and Plumiperla), salmon fry (Oncorhynchus spp.) grayling (Thymallus arcticus), rainbow trout (O. mykiss) and American dippers (Cinclus mexicanus) suggest these biota directly consume substantial amounts (40%-90%) of salmon protein. $\partial\sp{15}$N values in individual macroinvertebrate taxa usually cycled seasonally. All three elements of this investigation support the hypothesis that salmon carcasses can be important in structuring aquatic food webs.