Carbon cycling from mangrove litter to the adjacent Hooghly estuary, India – A modelling study

Carbon cycle has a fundamental role in changing climate of the earth. Recent models of the estuary-atmosphere system demonstrate the potentially great importance of estuarine environment to the regulation of atmospheric carbon dioxide. Carbon, the key element of life is available in organic and inorganic forms as different compounds in air, water and soil. Our study site, Hooghly - Matla estuarine system receives a large load of nutrients throughout the year. This is mainly due to high litterfall from the adjacent luxuriant mangroves and yield of huge suspended sediment carried along with the river Ganges which is flowing through the western part of the Sundarban mangrove ecosystem.Keeping in view the crucial role of carbon, a seven compartment model has been proposed to study the dynamics of carbon in this estuarine system. Different forms of carbon present in soil (as soil organic carbon (SOC), soil inorganic carbon (SIC)) and in water (as dissolved inorganic carbon (DIC), dissolved carbon dioxide (DCO2), dissolved bicarbonate (DBC), dissolved organic carbon (DOC) and particulate organic carbon (POC)) are taken as state variables. Litter biomass, dissolved oxygen, primary productivity, community respiration, temperature of water, pH of water, pH of soil, air-water exchange of carbon dioxide and conversion rates among different forms of carbon are considered as graph time functions. The data used in the present model are collected for over two years from our field works and experiments. Other sensitive rate parameters which are not possible to collect from survey or experiment, calibrated following standard procedure. Sensitivity analysis is performed along with calibration. Model simulation results are validated with observed data. Results show seasonal variations of litterfall and which is the main source of SOC pool and ultimately transported to the estuary. Other than litterfall, death of organisms in soil and water enriches the SOC and POC respectively. pH of water is governing factor and depending on this factor, DIC is converted to DCO2 and DBC, which are taken up by phytoplankton during photosynthesis. Mineralization rate of SOC to SIC and uptake rate of DCO2 and DBC are the sensitive parameters.