Reply to comment by Jackson and Martin on 'does timber harvest influence the dynamics of marine-derived nutrients in Southeast Alaska streams?'

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. Le but enonce de Levi et al. (2011, J. Can. Sci. Halieutiques Aquat. 68 : 1316-1329) consistait a determiner l'influence de la complexite geomorphologique sur la dynamique des nutriments provenant des saumons. Nous avons etudie sept cours d'eau du sud-est de l'Alaska presentant differentes intensites de coupe de bois historique (au milieu du 20e siecle) et, du coup, des differences sur le plan de leur geomorphologie fluviale. Dans un commentaire concernant notre etude, Jackson et Martin (2012, J. Can. Sci. Halieutiques Aquat. 69 : present numero) suggerent que la complexite geomorphologique que nous attribuons a la coupe de bois pourrait etre le fait de la variabilite naturelle des caracteristiques des cours d'eau et ces auteurs presentent d'autres explications possibles. Bien que nous ayons tente de limiter la variabilite naturelle parmi les cours d'eau etudies en utilisant un plan d'echantillonnage stratifie (c.-a-d. en choisissant des passages designes plaine inondable 4 ou 5 par le Service des forets du USDA), nous admettons que, comme il en va de toute etude ecologique de terrain, differentes hypotheses peuvent expliquer les patrons de reactions ecologiques observes. Nous maintenons cependant que notre methodologie etait assez robuste (c.-a-d. des passages de 300 m etudies dans sept cours d'eau pendant 3 ans, pour un total de 21 annees-cours d'eau) pour permettre de tirer des conclusions sur l'influence du saumon sur les nutriments dans l'eau et, secondairement, sur le role de la variabilite geomorphologique dans la mediation de la dynamique des nutriments. Nos donnees appuient egalement notre conclusion a l'effet que la coupe de bois a modifie la dynamique des nutriments dans les cours d'eau a saumons en modifiant de maniere quantifiable la geomorphologie de ces derniers. [Traduit par la Redaction]
In Levi et al. (2011), our research objective was to 'explain spatial and temporal variation in the magnitude of nutrient enrichment' from salmon-derived nutrients during annual spawning runs. Numerous studies [...]