Aquatic insects play a minor role in dispersing salmon-derived nutrients into riparian forests in southwestern Alaska Tessa B. Francis, Daniel E. Schindler, and Jonathan W. Moore Abstract: Recent research has highlighted the importance of nutrients derived from Pacific salmon (Oncorhynchus spp.) carcasses for coastal freshwater and riparian ecosystems. To investigate the role of emerging aquatic insects in dispersing salmon nutrients from spawning streams to riparian habitats, we quantified the emergence and return rates of mayflies (Ephemeroptera), stoneflies (Plecoptera), and caddisflies (Trichoptera) on Pick Creek in southwestern Alaska and, using stable isotopes, estimated the associated flux of nutrients derived from sockeye salmon (Oncorhynchus nerka) to streamside forests. Between June and September of 2004, 7.6 mg·m –2 of salmon-derived nitrogen emerged from Pick Creek in the form of aquatic invertebrates, 6.7 mg·m –2 of which was transferred to the terrestrial ecosystem. Dispersal patterns on four area streams showed that the majority of stream-borne nutrients are deposited within 25 m of the stream. Aquatic insects represent a minor vector for salmon nutrients to terrestrial systems, dispersing less than 0.03% of total nitrogen imported to Pick Creek by spawning salmon. Nevertheless, emerging insects make available salmon-derived resources otherwise inaccessible to some terrestrial consumers. Résumé : Des recherches récentes soulignent l’importance des nutriments dérivés des carcasses de saumons du Paci- fique (Oncorhynchus spp.) pour les écosystèmes côtiers d’eau douce et les écosystèmes des rivages. Afin de déterminer le rôle de l’émergence des insectes aquatiques dans la dispersion des nutriments de saumons depuis les cours d’eau de fraye vers les habitats du rivage, nous avons mesuré les taux d’émergence et de retour des éphémères (Ephemeroptera), des perles (Plecoptera) et des phryganes (Trichoptera) à Pick Creek dans le sud-ouest de l’Alaska; nous avons ensuite estimé le flux correspondant de nutriments dérivés des saumons rouges (Oncorhynchus nerka) vers les forêts riveraines à l’aide d’isotopes stables. Entre juin et septembre 2004, 7,6 mg·m –2 de nutriments provenant des saumons ont émergé de Pick Creek sous forme d’invertébrés aquatiques, dont 6,7 mg·m –2 ont été transmis à l’écosystème terrestre. Les pa- trons de dispersion dans trois sections du cours d’eau montrent que la majorité des nutriments provenant du cours d’eau sont déposés à moins de 25 m de la rive. Les insectes aquatiques représentent un vecteur peu important pour le transfert des nutriments provenant des saumons vers les systèmes terrestres et sont responsables de la dispersion de moins de 0,03 % de l’azote total importé dans Pick Creek par les saumons en fraye. Néanmoins, les insectes en émer- gence rendent disponibles des nutriments dérivés des saumons qui autrement seraient inaccessibles à certains consom- mateurs terrestres. [Traduit par la Rédaction] Francis et al. 2552 Introduction Pacific salmon (Oncorhynchus spp.) represent an impor- tant link between marine and freshwater ecosystems because of their anadromous and semelparous life history. Pacific salmon accumulate over 95% of their body mass in the ma- rine environment (Groot and Margolis 1991), and their nutri- ent-rich carcasses are deposited in freshwater ecosystems upon their return migration, spawning, and eventual death. Recent evidence indicates that this seasonal pulse of energy and nutrients is an important subsidy to freshwater systems. Much attention has been drawn to the contribution by spawning salmon of marine-derived nutrients (MDN), in- cluding nitrogen (N) and phosphorus, which are incorpo- rated into aquatic biota (Naiman et al. 2002; Schindler et al. 2003). Indeed, lotic organisms across nearly all freshwater trophic levels accumulate MDN relative to biota from streams or reaches inaccessible to salmon, including peri- phyton (Kline et al. 1990), invertebrates (Kline et al. 1990; Bilby et al. 1996), resident fishes (Wipfli et al. 2003), and juvenile salmonids (Bilby et al. 1998). While there is debate over the impact of MDN on the productivity of aquatic eco- systems, there exists some evidence that salmon carcasses in streams can increase the density of juvenile salmonids (Bilby et al. 1998) and some aquatic invertebrates (Wipfli et al. 1998; Lessard and Merritt 2006) at local scales. Salmon-derived nutrients deposited in freshwater systems are also incorporated into terrestrial food webs and riparian Can. J. Fish. Aquat. Sci. 63: 2543–2552 (2006) doi:10.1139/F06-144 © 2006 NRC Canada 2543 Received 6 January 2006. Accepted 29 August 2006. Published on the NRC Research Press Web site at http://cjfas.nrc.ca on 2 November 2006. J19091 T.B. Francis 1 and J.W. Moore. Department of Biology, University of Washington, P.O. Box 355020, Seattle, WA 98195-5020, USA. D.E. Schindler. School of Aquatic and Fisheries Sciences, University of Washington, P.O. Box 355020, Seattle, WA 98195-5020, USA. 1 Corresponding author (e-mail: tessa@u.washington.edu).