Unusually high 210Po activities in the surface water of the Zhubi Coral Reef Lagoon in the South China Sea Original Research Article

Recent researches revealed the exciting application of 210Po in tracing carbon and nitrogen cycling in the coral reef system. In order to quantify the recycling of particulate organic nitrogen (PON), both 210Po and 210Pb were examined at both high and low tides in the Zhubi Coral Reef lagoon, the South China Sea. Unusually, much higher 210Po activities and 210Po/210Pb ratios, in comparison with those found in the open seawater and the lagoon subsurface water, showed additional input of 210Po besides production from in situ 210Pb in the lagoon surface water. Statistical analysis identified that the reef flat seawater was the additional 210Po source. Based on a mass balance model, the input rates of 210Po varied from 0.04 Bq m−3 year−1 to 8.41 Bq m−3 year−1. On average, the additional 210Po contributed more than 60% of the total 210Po. The particulate 210Po significantly correlated with the concentrations of PON, indicating that diffusion of 210Po from sediment could be used to quantify the recycling of nitrogen. The average input rate of nitrogen was 16 mmol m−3 year−1, which can support up to 11% of the primary production rate. These results suggested that the unusual behavior of 210Po could provide new insight into the nitrogen recycling in the coral reef system.