Does δ15N in river food webs reflect the intensity and origin of N loads from the watershed?

Stable nitrogen isotope ratios (delta N-15) were measured in invertebrates and fish collected from 82 river sites located in the Saint-Lawrence Lowlands in Quebec, Canada, to examine the relationship between aquatic biota delta N-15 and anthropogenic nitrogen (N) loads. Mean delta N-15 values of all three trophic levels examined (primary consumers, predatory invertebrates and invertebrate-feeding fish) were highly correlated with total anthropogenic N loads on the watershed (kg N km(-2) year(-1); r(2)> 0.61, p < 0.0001) and with N loads originating from livestock manure (r(2)> 0.62, p < 0.0001), synthetic fertilizers (r(2)> 0.45, p < 0.000 1), and human population (r(2)> 0.29, p < 0.0001), respectively. Significant relationships were also observed between primary consumer delta N-15 and N loads originating from each of the three livestock species examined (bovines, pigs and poultry; p < 0.000 1). Furthermore, all three animal species contributed significantly and independently in elevating primary consumer delta N-15 (multiple r(2)=0.67, p < 0.0001). Curvilinear regressions were observed at all levels of analysis, delta N-15 values increasing slowly over a wide range of low levels of N loads, but increasing much faster as N loads grew larger. The three anthropogenic N sources examined were highly correlated with one another, preventing us from statistically isolating their respective effects on delta N-15. When these loads were expressed as a proportion of total N load, delta N-15 of aquatic biota was still highly correlated with N from livestock and fertilizers, but not with N from human population. Overall, these results suggest that delta N-15 values of aquatic consumers could be used as indicators of the intensity of anthropogenic N loading on watersheds, but not as tracers of the relative importance of individual N sources. (c) 2006 Elsevier B.V. All rights reserved.