The ²¹⁰Po / ²¹⁰Pb disequilibrium in a spring-blooming marginal sea, the Southern Yellow Sea

The Southern Yellow Sea (SYS) is suffering from the increasing environment problems, such as the recurrent algal bloom. The ²¹⁰Po/²¹⁰Pb disequilibrium is very useful for assessing particulate organic matter dynamics during phytoplankton blooming. In this study, 23 surface samples were collected from the SYS after the 2009 spring bloom, to investigate the disequilibrium between these two radionuclides. The dissolved ²¹⁰Pb and particulate ²¹⁰Pb activities (dpm 100 L⁻¹) in the SYS surface waters varied within a wide range, with values of 2.28–17.82 (average 7.63 ± 4.25, n = 23) and 1.08–13.56 (average: 4.72 ± 2.84, n = 23). A deficiency of ²¹⁰Po relative to ²¹⁰Pb in the seawater was observed. The distribution coefficients (K_d) of the two radionuclides varied considerably (from 10⁴ to 10⁶ L kg⁻¹), and higher K_d values of ²¹⁰Po relative to ²¹⁰Pb generally increased with POC/TSM (when above 10%). The negative correlation (R = 0.97, P = 0.012) between ²¹⁰Po/²¹⁰Pb activity ratios and primary productivities in all four seasons implies that marine biological processes may enhance the disequilibrium between ²¹⁰Po and ²¹⁰Pb. The residence times of ²¹⁰Po and ²¹⁰Pb were estimated to be 7–206 days and 14–105 days, respectively. The longer ²¹⁰Po residence times might be connected with several processes, e.g., ²¹⁰Po uptake by marine particles or plankton, and recycling of fine-grained particles in the surface water. These short residence times of ²¹⁰Po and ²¹⁰Pb might indicate the existence of efficient scavenging processes, causing heavy metals and pollutants to deposit into the Yellow Sea (YS) bottom sediments.