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 କ 7.19 µmol L⁻¹) to dissolved inorganic phosphorus (DIP = 1.23 କ 0.57 µmol L⁻¹) was 11 କ 4 and the surface water was undersaturated with respect to dissolved oxygen. The mean value of 0.1 କ 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 CO₂ from water surface to the atmosphere. Even though it is important to compare all in and out fluxes, there is no direct link between CO₂ emission and the later CO₂ removal by biological processes.

Monthly variation of CO₂ fugacity (f CO₂ ) in surface water and related atmospheric exchanges were measured in the Hooghly estuary which is one of the most important estuaries, since it is fed by one of the world's largest rivers, the Ganges with a flow of 15 646 m³ s⁻¹ (1.6% of the world's combined river flow). Carbon dioxide fluxes averaged over the entire estuary are in the range of −2.78 to 84.4 mmol m⁻² d⁻¹. This estuary acts as a sink for CO₂ during monsoon months and seasonal variation of its flux is controlled by dilution of seawater by river water. Since the solubility of CO₂ and the disassociation of carbonic acid in estuarine water are controlled by temperature and salinity, the observed variations of CO₂ fluxes are compared with those predicted from seasonal changes in temperature, salinity and the ratio of gross primary production to community respiration using empirical equations with an explained variability of 55%.