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.

Differences in urbanization and degree of marine influence are reflected in delta13C and delta15N of producers and consumers in seagrass habitats of Puerto Rico.

Couplings between land use and marine food webs in tropical systems are poorly understood. We compared land-sea coupling in seven sites around Puerto Rico, differing in the degree of precipitation and urbanization, by measuring delta(13)C and delta(15)N in producers and consumers. delta(15)N values were influenced by human activity: the food web from sites near urbanized centers was on average 1 per thousand heavier in delta(15)N compared to undeveloped sites. This is most likely due to wastewater inputs from septic systems relatively near the shoreline. Changes in delta(13)C were best explained by differences in the degree of marine influence. Where terrestrial inputs from a major river dominated, delta(13)C values were lighter, whereas sites further from land and in locations exposed to oceanic currents had heavier delta(13)C values, characteristic of a marine source of dissolved organic carbon. We found no significant effect of precipitation on connectivity in spite of a twofold difference in annual average rainfall between the north and south coast. The results suggest there is some connectivity between land and sea in Puerto Rico, despite high rates of evaporation relative to precipitation.