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)^(210)radionuclides were influenced by particle content effect and particle composition based on distribution coefficient(Kd)vs.total suspended matter(TSM)content and Kd 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(FPo/Pb)(>1)and fractionation factor of^(210)Po and^(234)Th(FPo/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.