Short-term ecosystem responses to experimental salmon carcass and analog additions in headwater streams

Background/Question/Methods

Returns of anadromous salmon have declined dramatically in the past century throughout the Pacific Northwest, reducing delivery of marine-derived nutrients (MDN) to streams and rivers.  The loss of salmon could have had profound effects on stream nutrient processing and ecosystem production, particularly in nutrient-poor systems.  Additions of salmon carcasses and pelletized salmon (analog) have been proposed as a way to mitigate for the loss of MDN, and ultimately boost stream-riparian productivity, in sub-drainages of the Columbia River Basin.  Our objective is to determine short-term responses of nutrient cycling (periphyton nutrient limitation, whole-stream nutrient uptake) and production (biofilm standing stock and production, fish growth and standing stock) to MDN from salmon.  We have completed year one of a five-year experiment in the North Fork Boise River, Idaho, with three streams receiving additions of salmon carcasses, three streams receiving analog, and three control streams with no additions.

Results/Conclusions

Two weeks following additions, nutrient diffusing substrates indicated nitrogen limitation of stream biofilms in both the control and analog streams; however, biofilms were not nutrient limited in carcass streams.  Additionally, short-term nitrate additions revealed that nitrogen demand relative to availability (v_f­) was similar in the analog stream and control streams, but was undetectable in carcass-treated streams.  Two weeks after treatment application, biofilm chlorophyll was three and six times greater in the analog and carcass streams compared to the control streams, respectively.  However, total biofilm biomass, including algae, microbes and fungi, increased only two-fold in both the carcass and analog streams. Together, these suggest a proportionally higher response of biofilm heterotrophs to analog and of biofilm autotrophs to salmon carcass additions.  Fish growth doubled in both the carcass and the analog treatment within six weeks.  Diet data suggests that this short-term increase in fish growth might be due to direct consumption of carcass material.

Salmon carcasses and analogs produced different nutrient limitation and recycling efficiencies in these streams, although both increased periphyton biomass and fish growth. Two mechanisms might cause these effects on stream ecosystems and consumers; either by increased availability of MDN with carcass and analog additions, or by the introduction of a novel food resource to stream consumers.  Knowing the mechanisms and the extent to which stream-riparian systems respond to MDN additions, in both salmon carcass and analog forms, will help evaluate the efficacy of mitigation efforts designed to replenish productivity lost by the elimination of wild salmon runs.