Does delta15N in river food webs reflect the intensity and origin of N loads from the watershed?
Stable nitrogen isotope ratios (delta15N) were measured in invertebrates and fish collected from 82 river sites located in the Saint-Lawrence Lowlands in Québec, Canada, to examine the relationship between aquatic biota delta15N and anthropogenic nitrogen (N) loads. Mean delta15N 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; r2>0.61, p<0.0001) and with N loads originating from livestock manure (r2>0.62, p<0.0001), synthetic fertilizers (r2>0.45, p<0.0001), and human population (r2>0.29, p<0.0001), respectively. Significant relationships were also observed between primary consumer delta15N and N loads originating from each of the three livestock species examined (bovines, pigs and poultry; p<0.0001). Furthermore, all three animal species contributed significantly and independently in elevating primary consumer delta15N (mult
