Modelling of the contribution of dissolved inorganic nitrogen (DIN) from litterfall of adjacent mangrove forest to Hooghly–Matla estuary, India

Abstract

Hooghly–Matla estuarine ecosystem of India is very rich in natural resources and receives large amount of nutrients through litterfall from adjacent mangrove forest. Nitrogen as an important nutrient occurs in various forms and plays a crucial role in the regulation of productivity in this estuarine system. Modelling of nitrogen dynamics from mangrove litterfall and particularly the release of dissolved inorganic nitrogen in this estuarine system is important because of its role in augmenting growth of phytoplankton and other higher plants and all other biological components of grazing food chain. Considering the importance of nitrogen, a dynamic model with seven-dimension differential equation is proposed. In this model, nitrogen of the mangrove litterfall is considered as source and its conversion into different organic and inorganic forms (soil total nitrogen, soil organic nitrogen, soil inorganic nitrogen, total organic nitrogen of water, dissolved organic nitrogen of water, particulate organic nitrogen of water and dissolved inorganic nitrogen of water) are considered as state variables. Some physical and chemical factors and also rate parameters such as nitrogen fraction of litter biomass, litter biomass, redox potential, soil temperature, conversion rates of nitrogen of one form to another form, phytoplankton uptake rate of dissolved inorganic nitrogen, dissolved oxygen, water temperature and water pH are considered as graph-time functions in this model. These data are collected over two years from our field works and experiments. Other rate parameters are calibrated following standard procedure. Sensitivity analysis is performed before calibration. Model simulation results are properly validated with observed data. Sensitivity analysis reveals that the leaching rate of soil organic nitrogen to total organic nitrogen of water and loss rate of soil organic nitrogen as humic acid and fulvic acids are very sensitive parameters in this system. Redox potential plays an important role in the conversion of soil total nitrogen to soil inorganic nitrogen whereas soil temperature is considered to be key factor regulating the microbial activity for the conversion of soil organic nitrogen to soil inorganic nitrogen. Similarly in water, total organic nitrogen of water including dissolved organic nitrogen and particulate organic nitrogen is dependent on soil organic nitrogen. The dissolved organic nitrogen, particulate organic nitrogen and soil inorganic nitrogen ultimately get converted into dissolved inorganic nitrogen and accumulated in water which is taken by phytoplankton and microflora as minerals.