Macrophytes as indicators of land‐derived wastewater: Application of a δ¹⁵N method in aquatic systems

We measured δ 15 N 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 δ 15 N values of macrophytes or POM. The δ 15 N values of macrophytes and POM increased as water column dissolved inorganic nitrogen concentrations increased. We found that δ 15 N of macrophytes, but not of POM, increased as N load increased. The δ 15 N values of macrophytes and groundwater NO 3 tracked the percent of wastewater contribution linearly. This research confirms that δ 15 N values of macrophytes and NO 3 can be excellent indicators of anthropogenic N in aquatic systems.