The human population and associated watershed development has risen steadily since the 1850s in Rhode Island. With these increases, human-derived wastewater has also risen dramatically, resulting in increasing nitrogen loads to estuarine systems. In this study, we examined relationships of modeled watershed nitrogen loads of 6 coastal subwatersheds of varying land development with the stable nitrogen isotope ratio (δ¹⁵N) of salt marsh fish and larvae. There was a significant positive relationship (r = 0.97, P < 0.05) between the watershed modeled percent wastewater and δ¹⁵N in Fundulus heteroclitus L. (Common Mummichog), and significantly higher (P < 0.05) δ¹⁵N in fish larvae collected from developed mainland marsh sites compared to less-developed island marsh sites. Our results support earlier published findings that fish in coastal marshes are assimilating nitrogen derived from watershed wastewater sources. Furthermore, there was an inverse relationship (P = 0.05) between the modeled percentage of human wastewater and mummichog size. The increasing loads of watershed nitrogen entering into coastal salt marshes are a concern because it is unclear how well salt marsh ecosystems can continue to assimilate high nitrogen inputs especially when also subjected to a warming climate.