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 Po-210/Pb-210 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 Pb-210 and particulate Pb-210 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 Po-210 relative to Pb-210 in the seawater was observed. The distribution coefficients (K-d) of the two radionuclides varied considerably (from 10(4) to 10(6) L kg(-1)), and higher Kd values of Po-210 relative to Pb-210 generally increased with POC/TSM (when above 10%). The negative correlation (R = 0.97, P = 0.012) between Po-210/Pb-210 activity ratios and primary productivities in all four seasons implies that marine biological processes may enhance the disequilibrium between Po-210 and Pb-210. The residence times of Po-210 and Pb-210 were estimated to be 7-206 days and 14-105 days, respectively. The longer Po-210 residence times might be connected with several processes, e.g., Po-210 uptake by marine particles or plankton, and recycling of fine-grained particles in the surface water. These short residence times of Po-210 and Pb-210 might indicate the existence of efficient scavenging processes, causing heavy metals and pollutants to deposit into the Yellow Sea (YS) bottom sediments.