Nitrogen stable isotope ratios of aquatic biota as indicators of perturbations on adjacent watersheds

G. Cabana, C. Anderson, and M. Bertrand. Département de Chimie-Biologie, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canda G8A 5H7

We measured the stable nitrogen isotope ratios (δ¹⁵N ) of biota collected from 16 small streams draining contrasting forested and agricultural watersheds and 82 sites situated along larger rivers draining 13 watersheds in Quebec, Canada. Using GIS we obtained information about land use and the amounts and sources of N (manure and fertilizers) applied to the watersheds. We observed a strong correlation between the estimated quantity of N applied to a watershed and the δ¹⁵N values of either herbivorous or carnivorous invertebrates. Mean δ¹⁵N of aquatic primary consumers varied greatly among sites (-1 to +15‰), most of this variation (86 %) being attributable to watershed and within-river site effects. Overall, percentage of total watershed area under agriculture explained 69% of the variation in mean primary consumer δ¹⁵N. Within watershed, δ¹⁵N of biota tracked longitudinal changes (or lack thereof) in percent agricultural area. Forested watersheds also had δ¹⁵N lower than partially or totally cleared ones. Our results suggest that δ¹⁵N of aquatic invertebrates is an excellent predictor of the N charge received by a river from the watershed but is not a precise indicator of the relative importance of recycled (manure) and new (synthetic fertilizers) sources of N.