Simulation model of phosphorus dynamics of an eutrophic impoundment – East Calcutta Wetlands, a Ramsar site in India

Abstract

The East Calcutta Wetlands is one of the largest wastewater-fed aquaculture in the world. It is the aggregation of salt marshes, salt meadows and wastewater-fed ponds that receive wastewater from various small and large scale industries and urban settlements. The available nutrient supports the grazing food chain of the wetland ecosystem and agriculture around it. Eutrophication is caused by the imbalance between nutrient load and utilization by the autotrophs in the system. In this wetland, there has been a gradual decrease in the fish production for the last few decades. The physical and chemical conditions of the system are deteriorating continuously. Therefore, modelling of the system is felt to be the need of the hour to mitigate the problem of eutrophication and identify the important parameters that govern the system dynamics.Here, a dynamic model is constructed to study the phosphorus dynamics of the system. The five state variable includes; inorganic phosphorus (orthophosphate), phytoplankton, zooplankton, organic phosphorus and sediment. Important processes, like mineralization, uptake, grazing, predation, settling, resuspension and sorption are considered in the model. Field data are collected over two years and incorporated in the model as graph time function. Sensitivity analysis is performed and the model is calibrated using first set data followed by the validation process. Results indicate that the wetland is in hypereutrophic condition. The model result shows that the inorganic phosphorus of the system is mainly controlled by desorption of phosphorus from the sediments and mineralization during the premonsoon. The settling of organic phosphorus and adsorption of inorganic phosphorus are major regulating processes in the postmonsoon. In monsoon, resuspension process has important role in governing the phosphorus dynamics of the wetland. Moreover, allochthonous input of phosphorus is dominant over autochthonus input and controls the system dynamics. The input rate of inorganic phosphorus from outside is the system sensitive parameter. The paper discusses the present condition of the East Calcutta Wetlands and outlines some of the important management strategies that can be applied to alleviate the eutrophication problem of the wetland ecosystem.