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

Abstract

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 (Kd) of the two radionuclides varied considerably (from 104 to 106 L kg−1), and higher Kd 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.