P/Ca in coral skeleton as a geochemical proxy for seawater phosphorus variation in Daya Bay, northern South China Sea

The P/Ca ratio in coral skeletons is considered to be a direct proxy for the nutrient P in seawater. We examined the reliability of this proxy by analyzing P/Ca in a Porites coral collected from a eutrophic area in the northern South China Sea. P concentrations were significantly higher compared to previously reported values from pristine and open seas, corresponding to the elevated nutrients from the study site. We compared coral P/Ca against recent in-situ records of seawater P concentrations. Our results show that P/Ca was primarily a function of TP sw rather than PO₄sw, and that the signal of skeletal P included not only phosphate, but also organic phosphorus. Besides the form of skeletal P, sub-sampling and analytical procedures and the distinctive nutrient regime were the most reasonable explanations for our results. We suggest that total P in coral skeletons may be an efficient proxy for seawater P variations and associated phytoplankton dynamics in eutrophic environments.