Decay Dynamics of Hydrophyte and Nutrient Release in a Lake System: Experimental and Predicting Modeling

This research focuses on Nansi Lake, an important regulating reservoir in China's South-to-North Water Diversion Project. It studies the natural decay process of five major hydrophytes in the lake and analyzes the impact of this process on water quality. The results show that the decay patterns of the five plants are similar, each being divided into three stages. In the initial stage, easily decomposable organic matter rapidly breaks down, releasing large amounts of nitrogen and phosphorus into the overlying water, quickly consuming dissolved oxygen and significantly lowering the pH. In the middle stage, the decomposition rate of organic matter slows down, and the nitrogen and phosphorus content in the water gradually decreases while dissolved oxygen and pH slowly rise. In the final stage, the organic matter within the plants is completely consumed, leaving primarily lignin, and the concentrations of various substances in the water stabilize, reaching a steady state. Additionally, this study establishes a Monod-type kinetic model to simulate changes in ammonia nitrogen in water during the decay process. By adjusting the parameters K1, K2, and K3, we obtained fitted curves consistent with the decay data trends for five plants. This helps to understand the impact of plant decay on water quality in Nansi Lake and provides a basis for plant disposal strategies and water quality management across different seasons in Nansi Lake.