Tracing Land-Based Nutrients Through Coastal Food Web Networks Using Stable Isotopes

There is growing interest in applying δ¹⁵N in biota to indicate anthropogenic nutrient inputs to coastal environments because changes in δ¹⁵N correlate to inputs of land-based nutrients. In complex coastal receiving waters, however, land-use effects on biota δ¹⁵N may be masked by local hydrologic processes, especially exchange with coastal waters of different geochemical character. We examined δ¹⁵N differences among larval fish 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 δ¹⁵N. We sampled three Lake Superior river-embayment systems from watersheds that span a large population density gradient. Across stations, mean fish δ¹⁵N ranged from 2.7‰ to 10.8‰. Within each watershed, we found a different pattern in δ¹⁵N values across the river-lake transition zone, indicating that larval fish δ¹⁵N response scaled down to a site-basis. At the site scale, correlations between fish δ¹⁵N 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 metrics of population density and NH₄⁺ explained most of the variation in larval fish δ¹⁵N among sampling locations (r² = 0.89). The results indicate that within coastal receiving waters, biota δ¹⁵N is broadly influenced by land-based N sources; however, expression at the site level is strongly influenced by local hydrologic processes. Near point sources of effluent, contribution of land-based nutrients to the food web was substantial, plausibly supporting up to 50% of fish production. This abstract does not necessarily reflect U.S. EPA policy.