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15N in riverine food webs: effects of N inputs from agricultural watersheds

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We examined the use of the natural abundance of nitrogen stable isotopes (15N) as a tracer of anthropo genic perturbations of the nitrogen (N) cycle at the watershed scale in 82 river sites draining 13 watersheds in the St. Lawrence Lowlands in Quebec. Mean 15N values of aquatic primary consumers varied greatly among sites (+2‰ to +15‰), most of this variation (88%) being attributable to site effects. Variation in 15N values among functional feeding groups of primary consumers within sites was comparatively lower (<1‰). Within watersheds, 15N values of primary consumers (and organisms of higher trophic levels) tracked longitudinal changes in the percentage of agricultural area. Overall, the percentage of total watershed area under agriculture explained up to 69% of the variation in mean primary consumer 15N values. Similar positive correlations were observed for predatory invertebrates and non-piscivorous fish. In general, our results show that 15N in riverine food webs reacts strongly to spatial patterns in the intensity of N inputs related to agricultural land use.

Nous avons évalué l'utilisation de l'abondance naturelle des isotopes stables d'azote (15N) comme traceur des perturbations anthropiques du cycle de l'azote (N) à l'échelle du bassin versant dans 82 sites de rivière dans 13 bassins hydrographiques des terres basses du Saint-Laurent au Québec, Canada. La valeur moyenne de 15N des consommateurs primaires aquatiques varie considérablement d'un site à un autre (+2 ‰ à +15 ‰), la majorité (88 %) de la variation attribuable aux effets du site. La variation de 15N entre les groupes fonctionnels de consommateurs primaires est par comparaison plus faible (<1 ‰). À l'intérieur d'un même bassin versant, les valeurs de 15N des consommateurs primaires (et des consommateurs des niveaux supérieurs) suivent les changements longitudinaux dans le pourcentage de terres agricoles. Dans l'ensemble, la pourcentage du bassin versant total utilisé pour l'agriculture explique jusqu'à 69 % de la variation de la valeur moyenne de 15N chez les consommateurs primaires. De semblables corrélations positives s'observent aussi chez les invertébrés prédateurs et les poissons non piscivores. En général, nos résultats indiquent que les valeurs de 15N dans les réseaux alimentaires des cours d'eau réagissent fortement aux patterns spatiaux d'apports de N reliés à l'utilisation des terres pour l'agriculture.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: 2005-02-01

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  • Published continuously since 1901 (under various titles), this monthly journal is the primary publishing vehicle for the multidisciplinary field of aquatic sciences. It publishes perspectives (syntheses, critiques, and re-evaluations), discussions (comments and replies), articles, and rapid communications, relating to current research on cells, organisms, populations, ecosystems, or processes that affect aquatic systems. The journal seeks to amplify, modify, question, or redirect accumulated knowledge in the field of fisheries and aquatic science. Occasional supplements are dedicated to single topics or to proceedings of international symposia.
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