Fig. 4. (a) Wastewater as a percentage of total N load as calculated with the N-loading model of Valiela et al. (1997)(2000) vs. ¹⁵N of Spartina alterniflora for two geographic groups of ponds and estuaries: Cape Cod (CC) and Rhode Island (RI). (b) Mean annual water-column dissolved inorganic nitrogen (DIN) concentrations vs. ¹⁵N of macrophytes grouped in four geographic regions. ¹⁵N values are means of all sampling dates, and error bars represent standard error.