Nitrogen‐stable isotope signatures in estuarine food webs: A record of increasing urbanization in coastal watersheds

Nutrient enrichment as a result of anthropogenic activity concentrated along the land‐sea margin is increasing eutrophication of near‐shore waters across the globe. Management of eutrophication in the coastal zone has been hampered by the lack of a direct method to trace nitrogen sources from land into coastal food webs. Stable isotope data from a series of estuaries receiving nitrogen loads from 2 to 467 kg N ha−1 yr−1 from the Waquoit Bay watershed, Cape Cod, Massachusetts, indicate that producer and consumer 15N‐to‐14N ratios record increases in wastewater nitrogen inputs. Nitrate from groundwater‐borne wastewater introduces a 15N‐enriched tracer to estuaries. This study explicitly links anthropogenically derived nitrogen from watersheds to nitrogen in estuarine plants and animals, and suggests that wastewater nitrogen may be detectable in estuarine biota at relatively low loading rates, before eutrophication leads to major changes in species composition and abundance within estuarine food webs.