Spatiotemporal modeling of wetland environments’ hydrodynamics and water quality characteristics is key to understanding and managing these ecologically important areas’ physical and environmental properties. We developed a two-dimensional numerical model based on the MIKE 21 module to analyze flow and pollution dynamics in the island-dominated Sanyang wetland of eastern China. Three simulation periods representing annual precipitation cycles were used to model freshwater discharge and water quality in the wetland. The results showed that the flow velocity in the study area had hydrodynamic characteristics typical of such a setting, with an average monthly flow velocity ranging from 0.01 to 0.04 m/s, contributing to an increased risk of serious eutrophication. The water quality problems (represented by ammonia nitrogen, NH3-N, and total phosphorus, TP, levels) peaked during the early summer peak rain season, followed by a gradual decline during a later flood period and the lowest values during the fall/winter dry period. Moreover, the spatial distribution of NH3-N and TP levels decreased from northwest to east, reflecting the influence of a highly polluted source. Our results provide a useful context for restoration efforts in the Sanyang wetland and other similar areas.

The calculation of water environmental capacity of Puzhehei lake is of great significance for preventing water pollution and protecting water ecological environment of Puzhehei Lake Basin. Based on the lack of hydrological and water quality data in Puzhehei Lake Basin, a large number of basic data were collected through pollution source investigation and water quality monitoring. On this basis, a twodimensional hydrodynamic water quality model of Puzhehei lake was established by using Mike21 model to simulate the migration and diffusion of pollutants into the lake. The current situation of pollution load in Puzhehei lake was analyzed, and the characteristics of water flow, hydrodynamic force and the migration and diffusion law of pollutants in Puzhehei Lake were analyzed. The results show that: ①the annual loads of COD, TN, TP and NH3-N in puzhehai Lake in 2018 are 4090.0t, 401.3t, 34.4t and 122.6t; ②Puzhehei lake is mainly non-point source pollution, and the difference of water environmental capacity between non rainy season and rainy season is very significant.

Transient storage refers to a vital process affecting nutrient retention dynamics. To explore how streambed topography, discharge, and nutrient ambient concentration impact the transient storage and nitrogen retention of the headwater stream, tracer experiments were performed in five reaches of the Yanqi watershed. The one-dimensional transport with inflow and storage model (OTIS) was employed to simulate the solute injection. Transient storage potential was evaluated by transient storage metrics. Moreover, the material balance method was adopted to obtain the amount of nitrogen retention in the respective reach, and the nutrient spiraling metrics were determined to assess the nitrogen retention potential. As revealed from this study, improving transient storage under low flow conditions effectively facilitated nitrogen retention (except for the case of low-head weirs reach), while the erosion of high discharge destroyed the transient storage zone, thereby reducing the nitrogen retention potential. Ecological restoration in the Yanqi watershed could rescue nitrogen retention potential to a natural state under low flow, whereas limitations were identified in nitrogen retention potential under high flow and high ambient concentration.