To investigate the association of 2 3 4 Th with organic colloids, colloidal 2 3 4 Th 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 2 3 4 Th 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 2 3 4 Th increased over the development of the first bloom, presumably due to the release of exudates by phytoplankton and the complexation of 2 3 4 Th with the exudates. Over the second bloom, 2 3 4 Th 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 2 3 4 Th from solution to particles and that the relative importance of exudate complexation and particles sorption to 2 3 4 Th transport needs to be determined.