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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]

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Using nutrient-diffusing substrata (NDS) in seven streams in southeast Alaska, USA, we tested whether (i) nutrient limitation of autotrophic and heterotrophic biofilms was alleviated by salmon resource subsidies, and (ii) whether the degree of alleviation could be predicted by environmental variables. Before salmon spawners arrived, autotrophic biofilms were nitrogen (N)-limited, or co-limited by N and phosphorus (P), whereas heterotrophic biofilms were either P-limited, or co-limited by N and P. Combined N and P amendments resulted in a 2.6-fold increase in biofilm chlorophyll a, and a 3.2-fold increase in community respiration. After salmon arrived, autotroph nutrient limitation was alleviated in six of the seven streams. Heterotrophs still exhibited nutrient limitation in six streams, but most streams shifted from co-limitation to P-limitation. Nutrient-diffusing substrata amended with salmon tissue indicated that salmon could also be an important source of organic carbon for biofilms. Autotrophs responded less to N and P amendments as streamwater ammonium concentration increased with the arrival of salmon. For heterotrophs, ammonium concentration and N:P ratio best predicted changes in response following the arrival of salmon. We provide the first direct evidence that biofilm nutrient limitation can be alleviated by salmon spawners in nutrient-poor streams. À l'aide de substrats diffuseurs de nutriments (NDS) dans sept cours d'eau du sud-est de l'Alaska, .-U., nous avons testé (i) si la limitation par les nutriments des biofilms autotrophes et hétérotrophes est réduite par les apports de ressources provenant des saumons et (ii) si l'importance de la réduction peut être prédite à partir des variables environnementales. Avant que les saumons n'arrivent pour frayer, les biofilms autotrophes sont limités par l'azote (N) ou limités conjointement par N et le phosphore (P), alors que les films hétérotrophes sont limités par P ou conjointement par N et P. Des amendements combinés en N et P entraînent une augmentation de la chlorophylle a des biofilms par un facteur de 2,6 et un accroissement de la respiration de la communauté par un facteur de 3,2. Après l'arrivée des saumons, la limitation des autotrophes par les nutriments a été réduite dans six des sept cours d'eau. Chez les hétérotrophes, six des cours d'eau montraient encore une limitation par les nutriments, bien que la plupart des cours d'eau aient changé d'une limitation conjointe pour une limitation par P. Les NDS amendés avec du tissu de saumons montrent que les saumons peuvent aussi représenter une source importante de carbone organique pour les biofilms. Les autotrophes réagissent moins aux amendements en N et en P, alors que la concentration d'ammonium augmente dans l'eau des cours d'eau avec l'arrivée des saumons. Chez les hétérotrophes, la concentration d'ammonium et le rapport N :P sont les meilleures variables prédictives des changements consécutifs à l'arrivée des saumons. Nous présentons les premières données qui démontrent directement que la limitation des biofilms par les nutriments peut être atténuée par les saumons reproducteurs dans les cours d'eau pauvres en nutriments. [ABSTRACT FROM AUTHOR]

The stated goal of Levi et al. (2011, Can. J. Fish. Aquat. Sci. 68: 1316-1329) was to determine the influence of geomorphic complexity on the dynamics of salmon-derived nutrients. We studied seven streams in Southeast Alaska with varying degrees of historical (mid-1900s) timber harvest and, as a result, differences in stream geomorphology. In a comment on our study, Jackson and Martin (2012, Can. J. Fish. Aquat. Sci. 69: this issue) suggest that the geomorphic complexity we ascribe to timber harvest may be due to natural variation in watershed characteristics and offer alternative hypotheses. We sought to reduce the natural variation among our study streams by using a stratified sampling design (i.e., selecting reaches classified as floodplain 4 or 5 by the USDA Forest Service), but acknowledge that, as with any ecological field study, alternative hypotheses may exist to explain observed patterns in ecological responses. We maintain that our study design was sufficiently robust (i.e., 300 m reaches studied in seven streams for 3 years, totaling 21 stream-years) to draw inferences about the influence of salmon on streamwater nutrients and, secondarily, the role of geomorphic variation in mediating nutrient dynamics. Our data also support our finding that the legacy of timber harvest altered nutrient dynamics in salmon-bearing streams via alterations to stream geomorphology that were quantifiable. [ABSTRACT FROM AUTHOR]

Although species commonly modify habitats and thereby influence ecosystem structure and function, the factors governing the ecological importance of these modifications are not well understood. Pacific salmon have repeatedly been shown to positively influence the abundance of benthic biota by annually transferring large quantities of nutrients from marine systems to the nutrient-poor freshwaters in which they spawn. Conversely, other studies have demonstrated that salmon can negatively influence the abundance of freshwater biota, an effect attributed to bioturbation during upstream migration and nest construction. The factors determining which of these contrasting ecological effects predominates are unknown, including how human activities, such as land use, influence ecological responses to salmon. We sampled a key basal food resource, sediment biofilm, in seven southeast Alaskan streams impacted to varying degrees by timber harvest. Biofilm abundance (measured as chlorophyll a and ash-free dry mass) was positively related to timber-harvest intensity prior to salmon arrival. However, during the salmon run, an inverse relationship emerged of more abundant biofilm in less-harvested watersheds. Among-stream variability in biofilm response to salmon was largely explained by sediment particle size, which was larger in less-harvested watersheds. Collectively, these results suggest that, by altering stream sediment size, timber harvest transformed the dominant effect of salmon from nutrient enrichment to physical disturbance, thus modifying nutrient linkages between marine and freshwater ecosystems.