Varying stable nitrogen isotope ratios of different coastal marsh plants and their relationships with wastewater nitrogen and land use in New England, USA

The stable nitrogen isotope ratios of some biota have been used as indicators of sources of anthropogenic nitrogen. In this study the relationships of the stable nitrogen isotope ratios of marsh plants, Iva frutescens (L.), Phragmites australis (Cav.) Trin ex Steud, Spartina patens (Ait.) Muhl, Spartina alterniflora Loisel, Ulva lactuca (L.), and Enteromorpha intestinalis (L.) with wastewater nitrogen and land development in New England are described. Five of the six plant species (all but U. lactuca) showed significant relationships of increasing delta N-15 values with increasing wastewater nitrogen. There was a significant (P < 0.0001) downward shift in the delta N-15 of S. patens (6.0 +/- 0.48 parts per thousand) which is mycorrhizal compared with S. alterniflora (8.5 +/- 0.41 parts per thousand). The downward shift in delta N-15 may be caused by the assimilation of fixed nitrogen in the roots of S. patens. P. australis within sites had wide ranges of delta N-15 values, evidently influenced by the type of shoreline development or buffer at the upland border. In residential areas, the presence of a vegetated buffer (n = 24 locations) significantly (P < 0.001) reduced the delta N-15 (mean = 7.4 +/- 0.43 parts per thousand) of the P. australis compared to stands where there was no buffer (mean = 10.9 +/- 1.0 parts per thousand; n = 15). Among the plant species, I. frutescens located near the upland border showed the most significant (R-2 = 0.64; P = 0.006) inverse relationship with the percent agricultural land in the watershed. The delta N-15 of P. australis and I. frustescens is apparently an indicator of local inputs near the upland border, while the delta N-15 of Spartina relates with the integrated, watershed-sea nitrogen inputs.