Abstract
Resource flows and disturbance from species migrations can alter the productivity, structure and function of an ecosystem. Annual mass migrations of Pacific salmon (Oncorhynchus spp.) to coastal watersheds import vast quantities of potentially limiting nutrients that have been shown to increase primary and secondary productivity in streams and lakes. Substrate disturbance during spawning can also export nutrients and reduce primary and secondary production. Here we study the impacts of these dual roles of salmon on stream invertebrates. We collected benthic macroinvertebrates in 15 streams prior to and following peak salmon spawning on British Columbia's central coast. Along with other habitat measurements including stream water chemistry, temperature, and watershed size, we investigated the effects of salmon on invertebrate δ15N, δ13C and biomass density (mg/m2) among 15 streams and within 5 streams, upstream and downstream of barriers to spawning salmon. We found that stream invertebrates assimilate salmon‐derived nutrients in proportion to availability but invertebrate biomass density declines in both seasons with increasing salmon density. Benthic disturbance appears to be the cause of this decline in the fall, but the decline in the spring may be due to the slow recovery of invertebrates from substrate disturbance the previous fall or salmon nutrients may be indirectly driving declines in spring invertebrate biomass by subsidizing other trophic levels and eliciting a trophic cascade.
Highlights
Understanding the sources, magnitude and flow of nutrients between ecosystems has been a major theme in ecology (Elton 1924, Odum 1971, Polis and Hurd 1996, Vanni et al 2004)
We focused on 15 streams in the Great Bear Rainforest on British Columbia, Canada’s central coast to test predictions on how natural variation in salmon density and other abiotic and biotic factors contribute to the family-level composition of freshwater invertebrates
This study is one of the few studies to examine the effects of salmon-derived material and disturbance on invertebrate family diversity and is the only study to link invertebrate beta diversity to mean salmon density
Summary
Understanding the sources, magnitude and flow of nutrients between ecosystems has been a major theme in ecology (Elton 1924, Odum 1971, Polis and Hurd 1996, Vanni et al 2004). Subsidies through species movements can increase production in nutrient-limited recipient ecosystems (Vanni 2002, Vanni et al 2004, Payne and Moore 2006) and have been shown to have the greatest effects in such hydrologically-linked systems (Polis and Hurd 1996, Bain and Stevenson 1999, Marczak et al 2007, Leroux and Loreau 2008) While these habitats are inherently variable and disturbed frequently through natural events such as floods and droughts, species migrations can be an additional mechanism that alters the trophic interactions and physical landscape in these communities (Lake 2000, Vanni et al 2004, Moore et al 2004, Verspoor et al 2010, Winemiller et al 2010). With the influx of terrestrial and marine subsidies, in addition to in-situ freshwater production, stream consumers have the ability to capitalize on a variety of resources year-round
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