Contrasting behaviors of 210 Po, 210 Pb and 234 Th in the East China Sea during a severe red tide: Enhanced scavenging and promoted fractionation

210 Po, 210 Pb, and 234 Th were determined in water columns of the East China Sea (ECS) to investigate their biogeochemical behaviors during a severe red tide event. Dissolved 210 Po, 210 Pb, and 234 Th accounted for large fractions of the total phases. The abnormally high concentrations of dissolved 210 Pb were observed. Partition behaviors of these radionuclides were influenced by particle content effect and particle composition based on distribution coefficient ( K d ) vs. total suspended matter (TSM) content and K d vs. ratios of particulate organic carbon and total suspended matter contents (POC/TSM content ratios). The peaks of mass specific activities of 210 Po, 210 Pb, and 234 Th indicated that degraded particles could have an intensified enrichment ability for radionuclides compared with the surficial suspended matters. Fractionation factor of 210 Po and 210 Pb ( F Po/Pb ) (>1) and fractionation factor of 210 Po and 234 Th ( F Po/Th ) (>1) were much higher at algal blooming regions than that at non-blooming stations, indicating that algal blooms promoted the fractionation of 210 Po against 210 Pb and 234 Th, and proving that 210 Po exhibited a stronger affinity for biogenic particles than 210 Pb and 234 Th when POC content increased in the sea. POC/ 210 Po, POC/ 210 Pb and POC/ 234 Th ratios (content/activity ratios) sharply decreased with depth in both algal bloom and non-bloom stations. The outbreak of algal bloom promoted the complexity of suspended particles and increased the variability of POC/tracer ratios (content/activity ratios) in the different depth of the shallow seas. More considerations should be taken to the difficulty of the selection of export interface and the suitable tracers when algal blooming occurs.