Habitat influences Pacific salmon {Oncorhynchus spp.) tissue decomposition in riparian and stream ecosystems

摘要

Decomposition incorporates organic material delivered by Pacific salmon (Oncorhynchus spp.) into aquatic and terrestrial ecosystems of streams where salmon spawn. We hypothesized that salmon tissue decomposition would be faster, and macroinvertebrate abundance and bio-mass higher, in terrestrial compared to aquatic habitats, and this would be reflected in the nutritional quality of the tissue. Salmon tissue in coarse-mesh bags was placed in four habitats [terrestrial: riparian (RIP), gravel bars (GRA); aquatic: stream sediment surface (STR), buried in sediments (BUR)] in four southeast Alaska watersheds. After 2 (RIP, GRA) or 4 (STR, BUR) weeks of decomposition, tissue dry mass, macronutrient content, and macroinvertebrate colonizer abundance and biomass were determined. Overall, tissue decomposition was rapid (mean k = 0.088 day~(-1)), while nutritional quality remained high based on elemental ratios (mean C:N = 4.9; C:P = 140; N:P = 30), and differed among habitats (Linear-mixed effects model p ＜ 0.05). Macroinvertebrate assemblages colonizing carcasses were unique to each habitat, although Diptera generally dominated. In terrestrial habitats, the dominant macroinvertebrates were Sphaeroceridae (96 % of invertebrate abundance in RIP habitat) and Calliphoridae larvae (98 % in GRA habitat). In aquatic habitats, the dominant macroinvertebrates were Chironomidae (48 % in STR habitat) and Chloroperlidae (72 % in BUR habitat). Macroinvertebrate colonizer abundance and biomass were higher in RIP (mean 286 individuals and 22 mg g~(-1)) than in other habitats (mean 4 individuals and 3 mg g~(-1)) (Friedman p ＜ 0.05). Rapid decomposition rates and high invertebrate biomass, combined with the high nutritional quality of tissue, suggest rapid incorporation of critical salmon nutrients and energy into both aquatic and terrestrial ecosystems.