The Po / Pb 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/Pb 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 and particulate Pb activities (dpm 100 L) 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 relative to Pb in the seawater was observed. The distribution coefficients (K) of the two radionuclides varied considerably (from 10 to 10 L kg), and higher K values of Po relative to Pb generally increased with POC/TSM (when above 10%). The negative correlation (R = 0.97, P = 0.012) between Po/Pb activity ratios and primary productivities in all four seasons implies that marine biological processes may enhance the disequilibrium between Po and Pb. The residence times of Po and Pb were estimated to be 7-206 days and 14-105 days, respectively. The longer Po residence times might be connected with several processes, e.g., Po uptake by marine particles or plankton, and recycling of fine-grained particles in the surface water. These short residence times of Po and Pb might indicate the existence of efficient scavenging processes, causing heavy metals and pollutants to deposit into the Yellow Sea (YS) bottom sediments.