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 (δ15N) of coral skeletal organic material (CS-δ15N) matches that of the coral tissue and also reflects the δ15N of water column fixed N, such that CS-δ15N can be used as a proxy for spatio-temporal oceanic N isotope distributions. Strong correlations between the δ15N 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-δ15N in reconstructing coral biomass δ15N changes. However, we observed a consistent and species-specific offset between the coral tissue δ15N and the skeletal CS-δ15N. As such, the CS-δ15N is not an absolute measure of tissue δ15N. The CS-δ15N of both coral species and three species of macroalgae increased across the reef flat in response to a natural gradient in water column δ15N. There was an apparent dampening of the water column δ15N gradient recorded in the coral tissue δ15N and the CS-δ15N, possibly caused by shifting trophic status of corals across the reef flat. The CS-δ15N therefore appears to robustly reflect the δ15N of the coral tissue, and the coral system is responsive to the δ15N of the water column N pools. The distortion of the true water column δ15N by corals requires further investigation and is an important consideration for the use of coral skeletons in temporal reconstructions of water column δ15N