Tracing nitrate sources using the isotopic composition of skeletal-bound organic matter from the calcareous green algae Halimeda

The nitrogen (N) isotopic composition (δ 15 N) of water column nitrate (NO 3 − ) reveals important information about the source(s) of N, and/or the transformation processes occurring during N transport, to a particular environment. However the physical collection of samples for δ 15 N–NO 3 − determination can be logistically challenging and does not address questions about historical δ 15 N–NO 3 − . Here we adapt the persulphate/denitrifier protocol, previously developed for scleractinian corals, to analyse the δ 15 N of organic material in the skeletons of the calcareous macroalgae Halimeda (hereafter referred to as δ 15 N-SOM). Using the newly established protocol, we show that the Halimeda δ 15 N-SOM can be used as a proxy for the δ 15 N of water column NO 3 − . Based on aquarium and field measurements we show that the maximum offset between the Halimeda δ 15 N-SOM and the δ 15 N of water column NO 3 − , as a result of fractionation during uptake, is 1‰. However this was only tested at low NO 3 − concentrations (< 10 μmol L −1 ). Our results demonstrate that in low-nutrient environments, Halimeda skeletal material, including historical material obtained from sediment cores, can provide a reliable proxy record of δ 15 N–NO 3 − at the base of the food web. This technique holds much promise for reconstructing spatial and temporal patterns of N supply to coral reef systems worldwide.