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.