Colloidal organic carbon and colloidal 234Th dynamics during a coastal phytoplankton bloom

To investigate the association of 234Th with organic colloids, colloidal 234Th and colloidal organic carbon (COC) were measured during a coastal phytoplankton bloom. Bacterial and viral abundance were measured in addition to chlorophyll a so that changes in COC and 234Th could be related to some basic microbial parameters during the course of the bloom. Two blooms, separated by a storm event, were observed at the 5 m sampling depth during the study. COC and colloidal 234Th increased over the development of the first bloom, presumably due to the release of exudates by phytoplankton and the complexation of 234Th with the exudates. Over the second bloom, 234Th appeared to be tracking specific components of the COC and not the bulk COC. Exopolymers were not measured in this study; however, they were probably a major component of the phytoplankton exudates since the bloom was dominated by diatoms. It is proposed that exopolymers may play a significant role in the transfer of 234Th from solution to particles and that the relative importance of exudate complexation and particles sorption to 234Th transport needs to be determined.