Using δ 15N in Fish Larvae as an Indicator of Watershed Sources of Anthropogenic Nitrogen: Response at Multiple Spatial Scales

There is growing interest in applying δ¹⁵ N in biota as an indicator of anthropogenic nutrient inputs to coastal environments because changes in δ¹⁵ N correlate to inputs of land-based nutrients. In complex coastal receiving waters, however, land-use effects on biota δ¹⁵ N may be masked by local hydrologic processes, especially exchange with coastal waters of different geochemical character. We examined δ¹⁵ N differences among larval fish, a novel biotic indicator, in coastal receiving waters at both among and within watershed scales. Our goal was to characterize how hydrologic processes within coastal river mouths and embayments mediate the effect of land-based N sources on larval fish δ¹⁵ N. We sampled three Lake Superior river-embayment systems from watersheds that span a large population density gradient. Over all stations, mean fish δ¹⁵ N ranged from 2.7 ‰ to 10.8 ‰. Within each system, we found a different pattern in δ¹⁵ N across the river–lake transition zone. Correlations between fish δ¹⁵ N and water quality, particularly NH₄ ⁺ and total nitrogen, were highly significant and corresponded to known differences in sewage waste water inputs. A multivariate model that included both watershed-based population density and NH₄ ⁺ was found to provide the best fit to the δ¹⁵N data among a series of multi- and univariate candidate models. These results demonstrate that: (1) fish larvae δ¹⁵ N responded at within watershed scales, and (2) within coastal receiving waters, fish larvae δ¹⁵ N was related to waste water inputs at the watershed scale; however, expression at specific locations within a coastal system was strongly influenced by local hydrologic processes.