Alaska is one of the last regions along the Pacific coast of the United States that still enjoys large, annual runs of spawning salmon. Upstream migrations of adult salmon provide these normally oligotrophic systems with a huge pulse of nutrients from the ocean. The asynchronous spawning runs of various salmon species as well as the retention of these nutrients in stream benthos and food webs potentially sustain this pulse of nutrients over many months. Estimates of annual nutrient delivery from spawning salmon to Alaskan streams range into the millions of tons of carbon, nitrogen, phosphorous and other nutrients. These nutrients, which enter the stream in the form of salmon eggs, sperm, waste and carcasses are termed marine-derived nutrients (MDN) and may be tightly linked to secondary production in these streams. One of the most important links between MDN and secondary production is in the production of juvenile salmon. In providing a positive feedback mechanism to ensure viability of subsequent generations, intermediate steps that connect MDN and juvenile salmon production include alterations of dissolved nutrients, biofilm production and alterations to the macroinvertebrate community. Changes due to MDN have far reaching implications in watersheds that receive salmon and may shape the dynamics of stream communities seasonally.

This research focused on aquatic insects community dynamics around the salmon runs. The objectives were to describe and quantify several community characteristics in response to MDN including abundance, biomass, functional group proportions, richness, diversity, growth and secondary production. We sampled seven streams in southeast Alaska that all receive major fall salmon runs. All the streams selected have a natural waterfall barrier to salmon, which allowed for the simultaneous sampling of stream sections with very similar habitat characteristics, but with the separation of the influence of salmon on each stream section. Samples were analyzed for mean abundance, biomass and Shannon-Weaver diversity seasonally. An additional study was conducted to estimate secondary production of midges (Chironomidae) and mayflies (primarily Heptageniidae, Baetidae, and Ephemerellidae) in stream sections or two rivers with and without annual sources of MDN. I also investigated the effect of MDN on the growth of three trichopteran genera (Dicosmoecus, Onocosmoecus, and Psychoglypha) in one anadromous stream above and below a waterfall barrier.

Results indicate upstream sections, across all streams, generally had greater abundance and biomass of mayflies, while downstream sections generally had greater abundance and biomass of midges. Results for diversity varied with season and stream system, and were not related to MDN. Results from the growth study did not indicate caddisfly larvae grew differently between stream sections. The secondary production study supported the seasonal patterns found for midges and mayflies with downstream sections having greater midge production, and upstream sections having greater mayfly production. These results implied a dichotomous role for MDN in these streams. This suggests that MDN may only enhance production of insects able to avoid the disturbance created during spawning or quickly recolonize after the runs are over. Overall this research points to a positive relationship between MDN and stream insect abundance and biomass for specific taxa with life history attributes that allow them to take advantage of the MDN enrichment.