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Large quantities of material are moved annually from the ocean to freshwater systems by migrating Pacific salmon. Previous studies have focused on nitrogen and phosphorus provided by spawning salmon but largely ignored micronutrients essential to aquatic productivity. We collected salmon tissue, water, and biofilm from seven southeast Alaskan streams both before and during the salmon run to test for potential micronutrient provision by salmon and uptake by biofilm. To examine temporal patterns, one stream was also sampled with high frequency. Samples were analyzed using ICP-OES for boron (B), calcium (Ca), cobalt (Co), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), molybdenum (Mo), sodium (Na), selenium (Se), silicon (Si), and zinc (Zn). Arrival of salmon increased stream water loads for Ca, Fe, Mg, and Na and the concentration of Co in biofilm across the seven study streams. Stream loads of B and biofilm Cu and Na content decreased in the presence of salmon. By examining one stream at finer temporal resolution, biofilm increased in Ca, Fe, and Mn concentrations near the end of the salmon run, indicating possible lag effects between peak salmon densities and biofilm micronutrient uptake. The increase in stream water micronutrient loads across space for four elements and uptake of three elements in biofilm through time suggest that salmon are a potential source of essential micronutrients for freshwater ecosystems, as has been repeatedly demonstrated for macronutrients. This study expands our understanding of resource subsidies by identifying potential micronutrients important to those ecological dynamics. [ABSTRACT FROM AUTHOR]

1. Adult Pacific salmon ( Oncorhynchus spp.) transport marine nutrients to fresh waters and disturb sediments during spawning. The relative importance of nutrient fertilisation and benthic disturbance by salmon spawners can be modulated by environmental conditions (e.g. biological, chemical and physical conditions in the catchment, including human land use). 2. To determine the importance of the environmental context in modifying the uptake and incorporation of salmon-derived material into stream biota, we measured the nitrogen (δ15N) and carbon (δ13C) isotopic composition of benthic algae (i.e. epilithon) and juvenile coho salmon ( Oncorhynchus kisutch) in seven streams across a timber-harvest gradient (8-69% catchment area harvested), both before and during the salmon run. Conditional bootstrap modelling simulations were used to assess variability in the response of epilithon and juvenile coho salmon to spawning salmon. 3. In response to spawning salmon, epilithon exhibited enrichment in both δ15N (mean: 1.5‰) and δ13C (2.3‰). Juvenile coho were also enriched in both δ15N (0.7‰) and δ13C (1.4‰). Conditional bootstrap models indicate decreased variation in data as spatial replication increases, suggesting that the number of study sites can influence the results of Pacific salmon isotope studies. 4. Epilithon isotopic enrichment was predicted by environmental conditions, with δ15N enrichment predicted by stream temperature and timber harvest ( R2 = 0.87) and δ13C enrichment by discharge, sediment size, timber harvest and spawner density ( R2 = 0.96). Furthermore, we found evidence for a legacy effect of salmon spawners, with pre-spawner δ15N and δ13C of both epilithon and juvenile coho predicted by salmon run size in the previous year. 5. Our results show that the degree of incorporation of salmon-derived nitrogen and carbon differs among streams. Furthermore, the environmental context, including putative legacy effects of spawning salmon, can influence background isotopic concentrations and utilisation of salmon-derived materials in southeast Alaska salmon streams. Future studies should consider the variation in isotopic composition of stream biota when deciding on the number of study sites and samples needed to generate meaningful results. [ABSTRACT FROM AUTHOR]

Pacific salmon ( Oncorhynchus spp.) perform important ecological roles in stream ecosystems by provisioning nutrients as resource subsidies and modifying their physical habitat as ecosystem engineers. These contrasting roles result in concurrent nutrient enrichment and benthic disturbance, where local environmental characteristics potentially determine which effect predominates. Whole-stream metabolism quantifies the functional response to salmon and may identify patterns in enrichment and disturbance not apparent from structural measurements alone. We measured ecosystem respiration (ER) and gross primary production (GPP), along with chemical and physical characteristics, in seven Southeast Alaska streams and two Michigan streams, before and during the salmon run. These streams in the native and introduced ranges of salmon differed in environmental characteristics, from geomorphology at the reach scale to climate at the biome scale. Salmon consistently increased ER across streams and biomes, from an average (±SE) of 1.92 ± 0.23 g O m d before salmon to 6.30 ± 1.08 g O m d during the run. In the cobble-bottom streams of Southeast Alaska, GPP doubled from 0.29 ± 0.05 g O m d before salmon to 0.66 ± 0.16 g O m d during the run. In contrast, GPP responded inconsistently to salmon in the sand-bottom Michigan streams, increasing in one and decreasing in the other. Patterns in ER and GPP among streams and time periods were predicted by stream water nutrients (for example, ammonium, soluble reactive phosphorus) rather than by physical characteristics (for example, light, sediment size, and so on). This study demonstrates that salmon can periodically override physical controls on ER and GPP and enhance whole-stream metabolism via their dual ecological roles as both resource subsidies and ecosystem engineers. [ABSTRACT FROM AUTHOR]

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We tested the hypothesis that the carcasses of anadromous Pacific salmon ( Oncorhynchus spp.) constitute a significant source of nutrients in the nutrient-poor freshwaters where these fish migrate, spawn, senesce, and die. In a 110 m-long stream reach in Southeast Alaska, we retained nearly 3000 salmon carcasses and compared streamwater nitrogen (N), phosphorus (P), and the biomass of benthic biofilm in this reach with an upstream reference reach. The study spanned 5 months, bracketed the entire salmon run, and encompassed significant seasonal variation in abiotic stream conditions. Concentrations of dissolved and particulate N and P followed distinctly unimodal patterns through time, which tracked the abundance of live salmon, and we observed strong predictive relationships between live-salmon abundance and streamwater-nutrient concentrations. In contrast, we did not observe clear relationships between salmon carcasses and streamwater nutrients. Biofilm biomass within our study reaches seemed to more closely track the abundance of live salmon than the abundance of carcasses. The experimental retention of carcasses had a minor or undetectable influence on nutrient concentrations and biofilm within the study reach as compared to the reference reach. We conclude that physical factors such as temperature, discharge, nutrient limitation, and irradiance vary seasonally in ways that maximize the influence of nutrients provisioned by live salmon and minimize the influence of carcass-derived nutrients on the aspects of stream ecosystems that we examined. Overall, our results promote a new perspective on the ecological role of salmon in freshwaters, and contribute to a more mechanistic understanding of how migratory fishes can influence aquatic ecosystems. [ABSTRACT FROM AUTHOR]

1. Pacific salmon ( Oncorhynchus spp.) deliver marine-derived nutrients to the streams in which they spawn and die, and these resource subsidies can increase the abundance of stream biota. In strong contrast, physical disturbance from salmon spawning activity can reduce the abundance of benthic organisms. Previous experimental designs have not established the relative effects of these two contrasting processes on stream organisms during a salmon run. 2. We combined manipulative and observational field studies to assess the degree of nutrient enrichment, physical disturbance, and the net effect of salmon on the abundance of benthic periphyton. Related salmon-mediated processes were also evaluated for benthic macroinvertebrates. Mesh exclosures (2 × 2 m plots) prevented salmon from disturbing areas of the stream channel, which were compared with areas to which salmon had access. Sampling was conducted both before and during the late-summer spawning run of pink ( O. gorbushca) and chum ( O. keta) salmon. 3. Streamwater nitrogen and phosphorus concentrations increased sharply with the onset of the salmon run, and highly significant positive relationships were observed between the numbers of salmon present in the stream and these dissolved nutrients. Before the salmon run, periphyton biomass (as chlorophyll a) and total macroinvertebrate abundance were very similar between control and exclosure plots. During the salmon run, exclosures departed substantially from controls, suggesting significant disturbance imparted on benthic biota. 4. Comparing exclosures before and during the salmon run enabled us to estimate the effects of salmon in the absence of direct salmon disturbance. This ‘nutrient enrichment potential’ was significant for periphyton biomass, as was a related index for macroinvertebrate abundance (although enhanced invertebrate drift into exclosures during the salmon run could also have been important). Interestingly, however, the net effect of salmon, evaluated by comparing control plots before and during the salmon run, was relatively modest for both periphyton and macroinvertebrates, suggesting that nutrient enrichment effects were largely offset by disturbance. 5. Our results illustrate the importance of isolating the specific mechanisms via which organisms affect ecosystems, and indicate that the relative magnitude of salmon nutrient enrichment and benthic disturbance determines the net effect that these ecologically important fish have on stream ecosystems. [ABSTRACT FROM AUTHOR]