Lake ecosystems play an important role in maintaining regional ecosystem balance, regulating regional climate, supplementing groundwater sources, regulating and storing excess floodwater, and protecting biodiversity and rare species resources (Wang et al., 2021). Moreover, lake ecosystems play a significant role in regional ecological security. The most active social and economic development activities associated with lake systems are typically distributed in the lakeside zone, which is characterized by robust urban-rural construction and represents the development area in lake basins (Wang et al., 2020). Urbanization, industrial development, rapid population growth, and agricultural production activities often lead to an increase in point source and non-point source (NPS) pollutants (Li et al., 2010; Liu et al., 2021b). The issues of territorial ecological space occupation, watershed ecosystem degradation, water quality deterioration, and water surface reduction are becoming increasingly severe, resulting in a significant reduction in the ecological self-regulation and restoration function of watershed ecosystems (Li et al., 2021b). After years of ongoing ecological rectification and administration, the water environment quality of lakes in China continues to improve; however, these improvement trends show significant fluctuation (Wang et al., 2012). As a result, considerable attention has been paid to the control of industrial pollution. However, the improvement of aquatic environment quality by point source emissions reduction has encountered a bottleneck. Therefore, it is imperative to explore and establish new methods for systematic and long-term lake management (Efstratiadis and Hadjibiros, 2011).
The correlation between land use and lake water quality changes has a complex spatial and temporal relationship and is also influenced by additional factors (Nielsen et al., 2012). The development and application of landscape ecology provide new perspectives and ideas for environment science research (Post, 1996; Turner and Gardner, 2015). The landscape is a heterogeneous or patchy spatial unit consortium based on land use and is composed of various ecosystem types, representing a composite mosaic that can reflect the comprehensive characteristics of meteorology, climate, ecological processes, and the social economy (Turner, 2005; Wu, 2013). The reclassification of landscape types from the perspective of source and sink is widely recognized by researchers and allows for the clarification of all the functions and properties of land use in the process of pollution mitigation. Moreover, the impact of land use scale and spatial patterns on lake water quality can be determined based on source-sink relationships and used to identify landscape ecosystems susceptible to pollutants in the basin and to control nutrient loss and transmission processes (Chang et al., 2021; Li et al., 2017; Ouyang et al., 2010b).
The NPS pollution exhibited characteristics of multiple governing factors, complex and changeable processes, spatial heterogeneity, and temporal fluctuation (Zou et al., 2020). At present, the basic premise of simulating NPS pollution migration and transmission processes is a constant influence of environmental variables on the process of NPS pollution. However, in the landscape mosaic, which is affected by topography, climate, vegetation conditions, and human activities, the energy flow and logistics state between heterogeneous patches are complex and changeable (Varekar et al., 2021). Moreover, the impact of landscape patterns on lake water quality by changing processes such as material exchange, hydrological processes, and soil erosion between landscapes (Guo et al., 2021; Liu et al., 2011), which are the primary processes involved in the formation and development of watershed pollution (Li et al., 2008). Landscape metrics describe the landscape spatial structure and are the primary methods used to study the impact of landscape patterns on water quality (Xu et al., 2020; Yuan et al., 2015). Ouyang et al. used patch density, edge density, fractal distribution index, and other indicators to express the landscape pattern and determined that various landscape types produced different effects on the nitrogen and phosphorus loads of water systems (Ouyang et al., 2010a). Moreover, Xia et al. reported that water quality is susceptible to changes in landscape patterns, with construction and cultivated land showing a positive correlation with the water pollution index and higher forest coverage correlating to better water quality (Xia et al., 2012). Although landscape metrics are widely used in the analysis of landscape patterns, most researchers consider the type, quantity, and spatial allocation of landscape features as independent variables when analyzing the effects of landscapes on watershed pollution without considering the influence of process mechanisms or the coupling relationship between landscape scale and spatial allocation (Li et al., 2021a). The watershed pollution process is a multiscale and nonlinear spatially explicit process. The conventional landscape metric lacks consideration of scale pattern relationships and process pattern relationships; thus, it is challenging to accurately describe the migration and diffusion processes of watershed pollutants (Winslow, 2014).
The spatially explicit landscape model typically divides the landscape into different geospatial units according to the landscape function and establishes the relationship between the landscape and the watershed pollution process by describing the characteristics of the units and the ecological flow process between the units (Nobre et al., 2020; Nowosad and Stepinski, 2019). Because this model fully incorporates the characteristics of ecosystems, topography, as well as other aspects, it can be used to accurately describe the energy and material exchange processes between spatial units and simulate the watershed pollution process. Existing research on material migration and energy flow has provided theoretical support and laid a good foundation for establishing ecological process-based models (Sampson et al., 2006). According to the functions of different landscape types in landscape ecology, landscapes can be divided into “source” and “sink.” The source landscapes are landscape units (patches) that positively promote the pollution process, whereas the sink landscapes refer to the landscape units (patches) that negatively inhibit or delay the occurrence of the pollution process or the diffusion of pollutants. According to the source-sink theory, the fundamental cause of watershed pollution and pollutant diffusion is the unbalanced profit and loss of nutrients in the landscape unit. When the amount of pollutants generated or received in the landscape unit or patch exceeds its functional threshold, pollutant diffusion and loss will occur. Source-sink landscape models are based on the source-sink landscape function and integrate the coupling relationship between landscape spatial characteristics and ecological processes to establish the internal relationship between landscape variables and watershed pollution processes.
In this study, we aimed to reveal the response law of watershed pollutant load to land use composition and pattern, and determine the influencing factors and response law of lake water quality change. Based on our findings, a lake water quality simulation model was proposed to solve the problem of insufficient expression of spatial processes in the existing models. To achieve these aims, the following procedures were followed: (1) obtaining the land use/cover vector data in the study area using remote sensing data interpretation and analyzing the temporal and spatial differences of land use/cover; (2) calculating the amount of pollutants entering the lake, and analyzing the composition and spatial structure of the pollution source; (3) identifying the response relationship between pollutant emissions and pollutant inflow into the lake; and (4) proposing an estimation method for pollutant load loss rate.