Salmon-derived nitrogen in terrestrial invertebrates from coniferous forests of the Pacific Northwest

Abstract

Background: Bi-directional flow of nutrients between marine and terrestrial ecosystems can provide essential resources that structure communities in transitional habitats. On the Pacific coast of North America, anadromous salmon (Oncorhynchus spp.) constitute a dominant nutrient subsidy to aquatic habitats and riparian vegetation, although the contribution to terrestrial habitats is not well established. We use a dual isotope approach of delta15N and delta13C to test for the contribution of salmon nutrients to multiple trophic levels of litter-based terrestrial invertebrates below and above waterfalls that act as a barrier to salmon migration on two watersheds in coastal British Columbia.

Results: Invertebrates varied predictably in delta15N with enrichment of 3-8 per thousand below the falls compared with above the falls in all trophic groups on both watersheds. We observed increasing delta15N levels in our invertebrate groups with increasing consumption of dietary protein. Invertebrates varied in delta13C but did not always vary predictably with trophic level or habitat. From 19.4 to 71.5% of invertebrate total nitrogen was originally derived from salmon depending on taxa, watershed, and degree of fractionation from the source.

Conclusions: Enrichment of delta15N in the invertebrate community below the falls in conjunction with the absence of delta13C enrichment suggests that enrichment in delta15N occurs primarily through salmon-derived nitrogen subsidies to litter, soil and vegetation N pools rather than from direct consumption of salmon tissue or salmon tissue consumers. Salmon nutrient subsidies to terrestrial habitats may result in shifts in invertebrate community structure, with subsequent implications for higher vertebrate consumers, particularly the passerines.

Figure 1

δ¹⁵N values in four trophic groupings of litter-based invertebrates collected above and below waterfall barriers to salmon migration on the Clatse and Neekas Rivers, British Columbia. Invertebrates are ranked (left to right) based on increasing consumption of animal protein (see methods). t-test results: ^** denotes p < 0.01; ^*** denotes p < 0.001.

Figure 2

δ¹³C values in four trophic groupings of litter-based invertebrates collected above and below waterfall barriers to salmon migration on the Clatse and Neekas Rivers, British Columbia. Invertebrates are ranked (left to right) based on increasing consumption of animal protein (see methods). t-test results: ^* denotes 0.01 < p < 0.05

Figure 3

δ¹⁵N values in ground beetles (Carabidae) and spiders (Araneae) with distance of collection upstream from the estuary (m) on the Clatse and Neekas Rivers, British Columbia.

Figure 4

δ¹⁵N and δ¹³C values in ground beetles (Carabidae) and spiders (Araneae) below and above waterfalls on the Clatse and Neekas Rivers, British Columbia.