Nutrient distribution across the insular shelf of La Parguera, Puerto Rico: assessment by algal tissue nitrogen
Ballantine, David L.
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A survey was conducted to investigate possible presence of an inshore to shelf edge gradient in % nitrogen (%N) and stable nitrogen isotope ratio (δ 15 N; ‰ vs. air) in tissues of the rhodophyte Acanthophora spicifera and the ocrophytes Lobophora variegata and Dictyota bartayresiana in coastal waters of La Parguera, southwest Puerto Rico. Average %N (1.26 ± 0.08 SD to 3.25 ± 0.10) and δ 15 N (2.06 ± 0.04 to 8.16 ± 0.14‰) in Acanthophora spicifera was highly variable along the shoreline. The highest inshore values of δ 15 N occurred at two stations influenced by secondary sewage input and a bird rookery and lower values (0.81 ± 0.06‰) were observed at mid-shelf locations. Dictyota bartayresiana and L. variegata did not display clear trends in %N across the insular shelf; however, δ 15 N for Dictyota bartayresiana was significantly higher inshore(3.48 ± 0.09‰) versus mid-shelf and shelf edge (0.00 ± 0.08‰) and δ 15 N for L. variegata was significantly higher at mid-shelf (2.13 ± 0.25‰) versus shelf edge locations (0.34 ± 0.24‰). These results indicate that anthropogenic effects on tissue nutrients are spatially distributed across the shelf, being higher near shore and not evident at the shelf edge.