Pacific salmon (Oncorhynchus spp.) disturb sediments and fertilize streams with marine-derived nutrients during their annual spawning runs, leading researchers to classify these fish as ecosystem engineers and providers of resource subsidies. While these processes strongly influence the structure and function of salmon streams, the magnitude of salmon influence varies widely across studies. Here, we use meta-analysis to evaluate potential sources of variability among studies in stream ecosystem responses to salmon. Results obtained from 37 publications that collectively included 79 streams revealed positive, but highly inconsistent, overall effects of salmon on dissolved nutrients, sediment biofilm, macroinvertebrates, resident fish, and isotopic enrichment. Variation in these response variables was commonly influenced by salmon biomass, stream discharge, sediment size, and whether studies used artificial carcass treatments or observed a natural spawning run. Dissolved nutrients were positively related to salmon biomass per unit discharge, and the slope of the relationship for natural runs was five to ten times higher than for carcass additions. Mean effects on ammonium and phosphorus were also greater for natural runs than carcass additions, an effect attributable to excretion by live salmon. In contrast, we observed larger positive effects on benthic macroinvertebrates for carcass additions than for natural runs, likely because disturbance by live salmon was absent. Furthermore, benthic macroinvertebrates and biofilm associated with small sediments (<32 mm) displayed a negative response to salmon while those associated with large sediments (>32 mm) showed a positive response. This comprehensive analysis is the first to quantitatively identify environmental and methodological variables that influence the observed effects of salmon. Identifying sources of variation in salmon-stream interactions is a critical step toward understanding why engineering and subsidy effects vary so dramatically over space and time, and toward developing management strategies that will preserve the ecological integrity of salmon streams.

We tested the hypotheses that marine-derived resource subsidies (salmon carcasses) increase the growth rates of stream-resident salmonids in southeastern Alaska and that more carcasses translate into more growth. Five carcass treatments of pink salmon Oncorhynchus gorbuscha (0, 1, 2, 3, and 4 carcasses/m 2 or 0, 1.9, 3.7, 5.6, and 7.4 kg wet mass/m 2 ) were replicated six times in once-through artificial channels, then each channel was stocked with three live age-0 coho salmon O. kisutch. The experiment spanned more than 9 weeks: 16 August to 24 October 1998. The body mass and fork length of the young coho salmon significantly increased from carcass additions, but the incremental increases sharply diminished at carcass-loading levels above 1 carcass/m 2 . Further, in a small stream in which we added salmon carcasses to a cumulative density of 0.54 carcasses/m 2 , both cutthroat trout O. clarki and Dolly Varden Salvelinus malma grew significantly faster during the 2 months in which carcasses were added (September-October) compared with fish in control reaches. Fish maintained their assimilated body mass through winter into the following spring. This study illustrates that marine nutrients and energy from salmon spawners increase growth rates of resident and anadromous salmonids in streams. This elevated growth should translate into increased survival and reproduction, ultimately elevating freshwater and marine salmon production. Ecological relationships between salmon runs and aquatic community nutrition and productivity may be important considerations for salmon stock protection and restoration and for freshwater and marine ecosystem management.

1. Pacific salmon are thought to stimulate the productivity of the fresh waters in which they spawn by fertilising them with marine-derived nutrients (MDN). We compared the influence of salmon spawners on surface streamwater chemistry and benthic biota among three southeastern Alaska streams. Within each stream, reaches up-and downstream of barriers to salmon migration were sampled during or soon after spawners entered the streams. 2. Within streams, concentrations of dissolved ammonium and soluble reactive phosphorus (SRP), abundance of epilithon (chlorophyll a and ash-free dry mass) and biomass of chironomids were significantly higher in reaches with salmon spawners. In contrast, biomass of the mayflies Epeorus spp. and Rhithrogena spp. was significantly higher in reaches lacking spawners. 3. Among streams, significant differences were found in concentrations of dissolved ammonium, dissolved organic carbon, nitrate and SRP, abundance of epilithon, and the biomass of chironomids and Rhithrogena. These differences did not appear to reflect differences among streams in spawner density, nor the changes in water chemistry resulting from salmon spawners. 4. Our results suggest that the 'enrichment' effect of salmon spawners (e.g. increased streamwater nutrient concentrations) was balanced by other concurrent effects of spawners on streams (e.g. sediment disturbance). Furthermore, the collective effect of spawners on lotic ecosystems is likely to be constrained by conditions unique to individual streams, such as temperature, background water chemistry and light attenuation.

Incorporation of marine-derived nutrients (MDN) into freshwater food webs of southeastern Alaska was studied by measuring the natural abundance of nitrogen and carbon stable isotopes in biota from artificial and natural streams. Biofilm, aquatic macroinvertebrates (detritivores, shredders, and predators), and fish (coho salmon, Oncorhynchus kisutch, and cutthroat trout, Oncorhynchus clarki) were sampled from streams in which Pacific salmon (Oncorhynchus spp.) carcasses had been artificially placed or were present naturally. In the presence of carcasses, all trophic levels incorporated marine-derived nitrogen (range, 22–73% of total N) and carbon (range, 7–52% of total C). In general, chironomid midges assimilated more marine-derived nitrogen and carbon than did other consumers. The assimilation of MDN by aquatic organisms and subsequent isotopic enrichment (5–6‰ for 15N, 3–4‰ for 13C) were similar in experimentally and naturally carcass-enriched streams. For specific taxa, however, percent assimilation for marine nitrogen and carbon were often dissimilar, possibly because of fractionation or transfer inefficiencies. These results suggest that pathways of MDN incorporation into stream food webs include both consumption of salmon material by macroinvertebrates and fish and uptake of mineralized MDN by biofilm. Incorporation of MDN into multiple trophic levels demonstrates the ecological significance of annual returns of anadromous fishes for sustaining the productivity of freshwater food webs.

Abstract:We conducted this study to determine the relationship between salmon spawner abundance and stream biofilm and benthic macroinvertebrate abundance in Southeast Alaska. Experiments took place in outdoor artificial and natural streams. Six pink salmon (Oncorhynchus gorbuscha) carcass treatments (0.00, 1.45, 2.90, 4.35, 5.80, and 7.25 kg wet mass) placed in artificial channels were subsampled repeatedly for biofilm ash-free dry mass (AFDM), chlorophyll a, and macroinvertebrates. In a small (nonanadromous) forest stream, we sampled benthos throughout a 66-m reach 17 days after distributing 60 carcasses along the lower half of that reach. All response variables significantly increased in response to carcass additions in both artificial and natural streams. Chlorophyll a continued to increase across all loading rates, while AFDM and total macroinvertebrate densities showed no further response to loading beyond the first treatment (1.45 kg) in artificial streams. In the natural stream, AFDM and chironomid densities continued increasing across loading levels. These results indicated that increased spawner densities increased lower trophic level abundance until a trophic capacity was reached. Salmon escapement goals should consider food web effects, especially on trophic levels that support juvenile salmonids, that ultimately affect freshwater salmon production.Résumé : Nous avons entrepris cette étude pour déterminer la relation entre, d'une part, l'abondance des saumons reproducteurs et, d'autre part, l'abondance des films biologiques fluviaux et des macroinvertébrés dans le sud-est de l'Alaska. Les expériences ont été effectuées à l'extérieur dans des canaux artificiels et un cours d'eau naturel. On a procédé à cinq traitements consistant en l'introduction de carcasses de saumon rose (Oncorhynchus gorbuscha) (1,45, 2,90, 4,35, 5,80, et 7,25 kg poids humide) avec témoin (0,00 kg) dans des canaux artificiels; on y a prélevé à plusiers reprises des sous-échantillons pour mesurer la masse à sec sans cendre des films biologiques ainsi que la chlorophylle a et les macroinvertébrés. Dans un petit cours d'eau forestier non fréquenté par des saumons anadromes, nous avons échantillonné le benthos d'un tronçon de 66 m 17 jours après avoir distribué 60 carcasses dans la moitié inférieure de ce tronçon. Toutes les variables mesurées se sont significativement accrues en réponse aux introductions de carcasses tant dans les canaux artificiels que dans le cours d'eau naturel. La quantité de chlorophylle a n'a pas cessé de s'accroître d'un traitement à l'autre, tandis que la masse à sec sans cendre des films biologiques et les densités de macroinvertébrés ont plafonné dès le premier traitement (1,45 kg) dans les canaux artificiels. Dans le cours d'eau naturel, la masse à sec sans cendre des films biologiques et les densités des chironomides n'ont cessé de s'accroître d'un traitement à l'autre. Ces résultats ont indiqué que l'accroissement des densités de reproducteurs a eu pour effet d'accroître l'abondance des organismes des ...