δ15N in riverine food webs: effects of N inputs from agricultural watersheds

We examined the use of the natural abundance of nitrogen stable isotopes (δ 15 N) 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 δ 15 N values of aquatic primary consumers varied greatly among sites (+2‰ to +15‰), most of this variation (88%) being attributable to site effects. Variation in δ 15 N values among functional feeding groups of primary consumers within sites was comparatively lower (<1‰). Within watersheds, δ 15 N 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 δ 15 N values. Similar positive correlations were observed for predatory invertebrates and non-piscivorous fish. In general, our results show that δ 15 N in riverine food webs reacts strongly to spatial patterns in the intensity of N inputs related to agricultural land use.