Effects of spawning Pacific salmon on the isotopic composition of biota differ among southeast Alaska streams

摘要

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 (15N) and carbon (13C) isotopic composition of benthic algae (i.e. epilithon) and juvenile coho salmon (Oncorhynchus kisutch) in seven streams across a timber-harvest gradient (869% 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 15N (mean: 1.5 parts per thousand) and 13C (2.3 parts per thousand). Juvenile coho were also enriched in both 15N (0.7 parts per thousand) and 13C (1.4 parts per thousand). 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 15N enrichment predicted by stream temperature and timber harvest (R2=0.87) and 13C 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 15N and 13C 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.