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

Abstract

The role of transparent exopolymer particles (TEP) in the biogeochemical cycle of reactive elements in the upper ocean was studied using a thorium isotope ( 234Th) as a tracer during a spring bloom in a coastal bay. TEP concentrations were consistently higher at 0 m ( < 5 cm ) 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. 234Th: 238U activity ratios measured at 0 and 5 m over the bloom period demonstrated the transport of 234Th to the surface water. The 234Th: 238U activity ratio at 0 m was positively correlated with TEP concentration. Simultaneous enrichment of 234Th and TEP in the surface water indicated that the TEP dynamics in the upper ocean play an important role for the accumulation of 234Th. A possible cause of surface TEP and 234Th 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 234Th in the surface layer needs to be considered when the strength of the biological pump is estimated using 234Th.