Spartina alterniflora δ¹⁵N as an indicator of estuarine nitrogen load and sources in Cape Cod estuaries

δ¹⁵N values of coastal biota have been used as indicators of land-derived N-loads and sources to estuarine systems and should respond predictably to differences in nitrogen and be sensitive to changes in nitrogen, preferably at the low end of eutrophication. We evaluated Spartina alterniflora as an indicator species of N-loads and sources of δ¹⁵N throughout the growing season, and compared the average δ¹⁵N to estuarine nitrogen loads and sources for several estuaries receiving different watershed N-loads. δ¹⁵N of S. alterniflora differed among estuaries, and these differences were maintained even as δ¹⁵N declined during the end of the growing season. δ¹⁵N values increased with increasing nitrogen loads to the subestuaries and with increasing percent wastewater-derived nitrogen load. The response of δ¹⁵N of S. alterniflora to increased N loads was greater at low N-loads, and decreased as N-loads increased, suggesting that S. alterniflora is a good indicator of incipient nitrogen load.