We measured ?15N signatures of macrophytes and particulate organic matter (POM) in six estuaries and three freshwater ponds of Massachusetts to assess whether the signatures could be used as indicators of the magnitude of land-derived nitrogen loads, concentration of dissolved inorganic nitrogen in the water column, and percentage of N loads contributed by wastewater disposal. The study focused specifically on sites on Cape Cod and Nantucket Island, in the northeastern United States. There was no evidence of seasonal changes in ?15N values of macrophytes or POM. The ?15N values of macrophytes and POM increased as water column dissolved inorganic nitrogen concentrations increased. We found that ?15N of macrophytes, but not of POM, increased as N load increased. The ?15N values of macrophytes and groundwater NO3 tracked the percent of wastewater contribution linearly. This research confirms that ?15N values of macrophytes and NO3 can be excellent indicators of anthropogenic N in aquatic systems.