δ15N variation in Ulva lactuca as a proxy for anthropogenic nitrogen inputs in coastal areas of Gulf of Gaeta (Mediterranean Sea)

We tested the capacity of Ulva lactuca to mark N sources across large marine areas by measuring variation in its d15N at several sites in the Gulf of Gaeta. Comparisons were made with the macroalga Cystoseira amentacea. Variation of d15N values was assessed also in the coastal waters off the Circeo Natural Park, where U. lactuca and C. amentacea were harvested, as these waters are barely influenced by human activities and were used as reference site. A small fragment from each frond was preserved before deployment in order to characterize the initial isotopic values. After 48 h of submersion, U. lactuca was more responsive than C. amentacea to environmental variation and d15N enrichment in the Gulf of Gaeta was observed. The spatial distribution of d15N enrichment indicated that different macro-areas in the Gulf were affected by N inputs from different origins. Comparison of the d15N values of fragments taken from the same transplanted frond avoided bias arising from natural isotopic v