A process-based dynamic modelling study of the impact of discharge water from shrimp culture on riverine nitrogen cycling: A case study

Shrimp is the primary aquaculture product exported from India in the global market. The shrimp industry contributes to the economic boom in the coastal provinces of India. However, shrimp culture imposes several externalities on the local ecosystem. One of the vital concerns is the nutrient-rich, chemically manipulated culture water discharge, which causes violations of sustainable and cleaner shrimp culture protocols. This study aims to understand the impacts of culture water on the nitrogen cycle of the river. The water is periodically released back into the river through a canal from the shrimp farms. This study was conducted by building a process-based system dynamic model following modelling protocols. The model was simulated, calibrated, and validated with the observed data collected from periodic sampling from the river, shrimp farms, and canals. Seven state variables were considered, and sediment nitrogen was treated as a single state variable. The interrelationship between different state variables was established by the inflows and outflows, incorporating several parameters. Study results indicate that NOx (nitrate and nitrite) is the main nutrient influx from the culture water. The model is a good predictor for the water column nitrogen components. The ecological functionality by different rate parameters of biotic components (Zooplankton and Phytoplankton) are not sensitive for this model's nitrogenous state variables. By the model outcomes, no direct relation between shrimp culture and the abundance of biotic components of this model can be established. Sediment nitrogen acts as the sink for the water column nitrogen components. This modelling study focuses on one of the river's vital regulatory ecosystem services, i.e., the nitrogen cycle. This model can be implemented to predict the impacts of any water discharge from such production on the nitrogen cycle of a lotic system. The study is important for establishing viable production protocols to achieve a sustainable balance between the shrimp production industry and the ecosystem.