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 delta N-15 at several sites in the Gulf of Gaeta. Comparisons were made with the macroalga Cystoseira amentacea. Variation of delta N-15 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 delta N-15 enrichment in the Gulf of Gaeta was observed. The spatial distribution of delta N-15 enrichment indicated that different macro-areas in the Gulf were affected by N inputs from different origins. Comparison of the delta N-15 values of fragments taken from the same transplanted frond avoided bias arising from natural isotopic variability. (C) 2014 The Authors. Published by Elsevier Ltd.