The role of transparent exopolymer particles (TEP) in the transport of super(234)Th in coastal water during a spring bloom

The role of transparent exopolymer particles (TEP) in the biogeochemical cycle of reactive elements in the upper ocean was studied using a thorium isotope ( super(234)Th) as a tracer during a spring bloom in a coastal bay. TEP concentrations were consistently higher at 0 m during the bloom than at 5 m, where chlorophyll-a concentrations were highest, even though active TEP formation occurs from extracellular polysaccharides excreted by bacteria and phytoplankton. This trend is especially evident toward the end of the bloom. super(234)Th: super(238)U activity ratios measured at 0 and 5 m over the bloom period demonstrated the transport of super(234)Th to the surface water. The super(234)Th: super(238)U activity ratio at 0 m was positively correlated with TEP concentration. Simultaneous enrichment of super(234)Th and TEP in the surface water indicated that the TEP dynamics in the upper ocean play an important role for the accumulation of super(234)Th. A possible cause of surface TEP and super(234)Th enrichment is active TEP production by the subsurface phytoplankton bloom followed by their rapid transport to the surface. Our study suggests a new pathway of TEP and associated particle-reactive elements in the upper ocean biogeochemical cycles during a spring bloom in a coastal area. The observed accumulation of super(234)Th in the surface layer needs to be considered when the strength of the biological pump is estimated using super(234)Th.