Investigating the effect of marine-derived nutrients from spawning salmon on macroinvertebrate secondary production in southeast Alaskan streams

Abstract

It is generally accepted that juvenile salmonid production is linked, via bottom-up pathways, to marine-derived nutrient (MDN) inputs from spawning salmon. Many studies have used standing stock biomass estimates of aquatic macroinvertebrates to infer relationships between MDN and secondary production in streams that receive spawners. However, no study has measured aquatic macroinvertebrate secondary production in relation to MDN. To assess the relationship between MDN and aquatic insect production, we measured secondary production of the 5 dominant mayfly genera (Baetis spp., Drunella spp., Cinygmula spp., Epeorus spp., and Rhithrogena spp.) and chironomids throughout the primary growing season in 2 streams in southeastern Alaska. Both streams had upstream reaches blocked from spawning salmonids by a waterfall barrier and downstream reaches that received large spawning runs of pink and chum salmon. Four of the mayfly genera studied (Drunella spp., Cinygmula spp., Epeorus spp., and Rhithrogena spp.) had significantly greater production in upstream than in spawning reaches. Secondary production of Baetis spp. was similar between upstream and spawning reaches. Chironomid production was significantly greater in spawning than in upstream reaches. However, biomass of each taxon was maximized in the spring and summer before the primary period of MDN input. These patterns indicate that another factor, primarily spawning disturbance, is an important driver of benthic insect secondary production in these streams and might provide the community structure within which MDN subsidies occur. If these patterns are common in streams that receive salmon runs, then secondary production-mediated links between MDN and juvenile salmonid production might be mostly the result of responses of chironomids and other benthic organisms with similar life histories.