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δ 15 N 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 δ 15 N throughout the growing season, and compared the average δ 15 N to estuarine nitrogen loads and sources for several estuaries receiving different watershed N-loads. δ 15 N of S . alterniflora differed among estuaries, and these differences were maintained even as δ 15 N declined during the end of the growing season. δ 15 N values increased with increasing nitrogen loads to the subestuaries and with increasing percent wastewater-derived nitrogen load. The response of δ 15 N 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. [ABSTRACT FROM AUTHOR]

Abstract: 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 δ 13C and δ 15N in producers and consumers. δ 15N values were influenced by human activity: the food web from sites near urbanized centers was on average 1‰ heavier in δ 15N compared to undeveloped sites. This is most likely due to wastewater inputs from septic systems relatively near the shoreline. Changes in δ 13C were best explained by differences in the degree of marine influence. Where terrestrial inputs from a major river dominated, δ 13C values were lighter, whereas sites further from land and in locations exposed to oceanic currents had heavier δ 13C 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. [Copyright &y& Elsevier]