Controls on the nitrogen isotopic composition of shallow water corals across a tropical reef flat transect

We tested the hypothesis that the nitrogen (N) isotopic signature (δ 15 N) of coral skeletal organic material (CS-δ 15 N) matches that of the coral tissue and also reflects the δ 15 N of water column fixed N, such that CS-δ 15 N can be used as a proxy for spatio-temporal oceanic N isotope distributions. Strong correlations between the δ 15 N of skeletal organic material and that of coral tissue in two scleractinian corals ( Porites lutea and Favia stelligera ) across a tropical coral reef flat support the use of CS-δ 15 N in reconstructing coral biomass δ 15 N changes. However, we observed a consistent and species-specific offset between the coral tissue δ 15 N and the skeletal CS-δ 15 N. As such, the CS-δ 15 N is not an absolute measure of tissue δ 15 N. The CS-δ 15 N of both coral species and three species of macroalgae increased across the reef flat in response to a natural gradient in water column δ 15 N. There was an apparent dampening of the water column δ 15 N gradient recorded in the coral tissue δ 15 N and the CS-δ 15 N, possibly caused by shifting trophic status of corals across the reef flat. The CS-δ 15 N therefore appears to robustly reflect the δ 15 N of the coral tissue, and the coral system is responsive to the δ 15 N of the water column N pools. The distortion of the true water column δ 15 N by corals requires further investigation and is an important consideration for the use of coral skeletons in temporal reconstructions of water column δ 15 N.