Abstract: This study is designated to examine the biological activity role of carbon cycling in a tropical lagoon, especially after a natural nutrient pulse. The study took place several days after precipitation. Both physical and chemical hydrographs indicated that significant amounts of dissolved inorganic nutrients had either entered the inner lagoon through runoff drainage or regenerated from the bottom sediments of the inner lagoon. Although the variables (chlorophyll a, bacterial biomass and production, as well as dissolved and particulate organic carbon) had orders of variation, linear relationships were significant between variables. This could have been associated with high phytoplankton growth enhanced by enriched nutrients and its sequential responses. This study considered both phytoplankton and bacterial communities that are the most important components of organic carbon consumption; they accounted for more than 87% of the organic carbon consumption in this ecosystem. A large ratio of primary production to community respiration with a mean of 1.33 suggested that this lagoon was autotrophic during the study period. It also indicated that there was a carbon sink in this ecosystem, and the residual carbon was either exported into the coastal sea or preserved in lagoon sediments resulting in the hypoxic condition of the bottom water. Faster water turnover rate was suggested to improve this low oxygen condition.