The Sundarban mangrove forest (4,264 km2) constitutes about 3% of the total area of the world mangrove. We measured diurnal and seasonal variations of air-water CO2 exchange in relation to the occurrence of phytoplankton during January-December 2001. Diurnal variations of airflows showed that the minimum and maximum CO2 flux of -16.2 μmol m -2 h-1 and 49.9 μmol m-2 h-1, respectively, occurred during the higher sea breeze. The average ratio of dissolved inorganic nitrogen (DIN = 13.85 ± 7.19 μmol L-1) to dissolved inorganic phosphorus (DIP = 1.23 ± 0.57 μmol L -1) was 11 ± 4 and the surface water was undersaturated with respect to dissolved oxygen. The mean value of 0.1 ± 0.08 for the ratio of phytoplankton production (P) to community respiration (R) indicated that the ecosystem was heterotrophic. The saturation of dissolved carbon dioxide with respect to the atmosphere varied seasonally between 59% and 156%, with minimum levels in postmonsoon and maximum levels in premonsoon/early monsoon (June/July). Out of the 36 genera of diatoms, 1 blue green alga, and 3 dinoflagellates that occurred throughout the year, only 6 reached bloom proportions in postmonsoon, when mangrove water was a sink of atmospheric CO2. Although 59.3% of the emitted CO2 was removed from the atmosphere by biological processes, on an annual basis, the Sundarban mangrove forest supplies 13.8 kg C ha-1 yr-1 of CO 2 from water surface to the atmosphere. Even though it is important to compare all in and out fluxes, there is no direct link between CO 2 emission and the later CO2 removal by biological processes.