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

δN 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 δN throughout the growing season, and compared the average δN to estuarine nitrogen loads and sources for several estuaries receiving different watershed N-loads. δN of S. alterniflora differed among estuaries, and these differences were maintained even as δN declined during the end of the growing season. δN values increased with increasing nitrogen loads to the subestuaries and with increasing percent wastewater-derived nitrogen load. The response of δN 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.