15N enrichment in agricultural catchments: field patterns and applications to tracking Atlantic salmon (Salmo salar)

Nitrogen isotopes were used to study the source of nitrate and its uptake into the food web of a northeastern river in Vermont, USA. In six tributaries of the White River (Vermont) nitrate concentrations were elevated in streams flowing through areas with agricultural land use as compared to streams flowing through pristine forested areas. We observed a strong positive correlation between δ15N values of stream water nitrate and percent of agricultural land-use within a given catchment. Agricultural sites had relatively high δ15N values of nitrate (+7.3‰) compared to forested sites (+2.0‰). These relatively high δ15N values coupled with relatively high concentrations of nitrate in agricultural streams suggest the introduction of 15N-enriched nitrate draining from agricultural areas. Additionally, elevated δ15N values of algae, aquatic insects, and Atlantic salmon (Salmo salar) in agricultural sites compared to lower δ15N values of their counterparts in forested sites suggest the uptake of nitrate from agricultural sources by stream organisms. We observe a similar positive correlation between the δ15N of Atlantic salmon and the percent of agricultural land in a catchment. The nitrogen isotope values of both Atlantic salmon and water nitrate reflect land-use. Thus, nitrogen isotope ratios in salmon, especially when coupled with strontium isotope ratios (which reflect catchment geology), create unique isotopic identities for Atlantic salmon stocking streams and thereby provide a means of determining the natal origin of juvenile salmon.