Mapping anthropogenic nitrogen through point sources in coral reefs using δ15N in macroalgae

To trace anthropogenic N from sewage outfalls in coral reefs of Nanwan Bay, southern Taiwan, we analyzed tissue N, C/N ratio, and delta N-15 values of macroalgae along horizontal lines parallel to the shoreline in the intertidal zone, and vertical transects perpendicular to the shoreline from the intertidal to the subtidal zone. Dominant macroalgal species in the intertidal and subtidal zones from 2 pristine islands, which were assumed to be little affected by human activities, were also collected for comparison. In total, 34 macroalgal species were collected for analysis in the wet and dry seasons to determine whether there was a seasonal effect. Low N contents and high C/N ratios were observed in macroalgae with calcareous and leathery morphologies. However, the delta N-15 values of macroalgae appeared to be little affected by the thallus forms. The N contents, C/N ratios, and delta N-15 values of macroalgae growing on the reference transects or in the subtidal zone were similar to values of the same species or genera growing around the 2 pristine islands. On the horizontal lines or along the vertical transects, the N contents and C/N ratios of macroalgae growing around the sewage-affected transects were not remarkably different (higher or lower) than the values of the same species growing around the reference transects. However, tissue delta N-15 values of macroalgae showed a clear spatial pattern, with higher values around the sewage-affected transects and lower values around the reference transects in the intertidal zone, though the pattern was less distinct in the lower intertidal zone. Furthermore, delta N-15 values in both seasons showed a clear curvilinear gradient along the sewage-affected transects, decreasing from the sewage outfalls to the subtidal zone, indicating that anthropogenic N loading was rapidly dispersed, with little evidence that the coral reefs in the subtidal zone were impacted by this nutrient source. Thus, delta N-15 values of macroalgae can be sensitive biological indicators for anthropogenic N loading. Our results do not support the hypothesis that anthropogenic N enrichment is the major cause of the prolific growth of macroalgae on the coral reefs of Nanwan Bay.