Spatiotemporal Distribution Characteristics and Source Analysis of Nutrients in the Four Lakes Basin

[Objective] The Four Lakes (Changhu Lake, Sanhu Lake, Bailu Lake, and Honghu Lake) Basin, located in the hinterland of the Jianghan Plain in the middle reaches of the Yangtze River, is an essential agricultural production area and an ecologically sensitive wetland area in Hubei Province. To reveal the spatiotemporal distribution characteristics of nutrients and their pollution sources in the water bodies of the Four Lakes Basin after ecological restoration, this study systematically analyzes the variation patterns of water quality and the main driving factors based on measured monitoring data, aiming to provide a scientific basis for watershed water environment management and ecological restoration effectiveness assessment. [Methods] Based on field survey data from wet, normal, and dry seasons during 2022-2023, a total of 12 sampling sites were set up in the Four Lakes Basin, covering the upstream, midstream, and downstream areas, as well as mainstream and tributaries. Nine water quality indicators were measured, including water temperature (WT), dissolved oxygen (DO), turbidity (TUR), permanganate index (CODMn), chemical oxygen demand (COD), five-day biochemical oxygen demand (BOD5), ammonium nitrogen ( NH4+-N), total phosphorus (TP), and total nitrogen (TN). The water quality was comprehensively evaluated using the water quality index (WQI) method. Correlation analysis and principal component analysis (PCA) were combined to identify the main pollution factors and sources. The characteristics of water quality evolution were systematically revealed from temporal, spatial, and pollution source aspects through data statistics, significance tests, and graphical visualization performed using software such as Excel, SPSS, and CANOCO. [Results] Water quality indicators in the Four Lakes Basin exhibited significant differences both temporally and spatially. Temporally, WT, NH4+-N, TP, and CODMn were the highest during the flood season, while DO and TN showed opposite trends. TUR peaked during the normal season. Spatially, the concentrations of TUR, CODMn, COD, and BOD5 in the downstream water bodies were significantly higher than those in the midstream and upstream. The NH4+-N concentration was the highest in the midstream, while the TP and TN concentrations were the lowest in the upstream. Overall, the concentrations of various nutrient indicators demonstrated a pattern of “mainstream > tributaries”. The WQI values ranged from 15.61 to 32.88, indicating that the overall water quality in the Four Lakes Basin was at a “poor” to “very poor” level. Its temporal variation followed the order: normal season > dry season > wet season, and its spatial variation was characterized by upstream > midstream > downstream, and tributaries > mainstream. Correlation analysis showed that WT, DO, COD, NH4+-N, TP, and TN were the main factors affecting WQI, among which N H4+-N, TP, and TN were significantly negatively correlated with WQI (P[Conclusion] In summary, the water quality in the Four Lakes Basin exhibits significant temporal differences across the flood, normal, and dry seasons, while spatially, the upstream areas are superior to the midstream and downstream areas, and the tributaries are superior to the mainstream.. Although ecological restoration projects have been effective, the overall water quality of the basin remains at a moderate to severe pollution level. Agricultural fertilization, livestock and poultry breeding wastewater, domestic sewage, and industrial discharge are the main exogenous pollution sources, while the release of water body sediments and the decomposition of organic residues are the main endogenous pollution pathways. The innovation of this study lies in systematically revealing the spatiotemporal distribution pattern and pollution causes of nutrients in the Four Lakes Basin under the background of ecological restoration for the first time. This study also constructs a multi-level analytical framework of “WQI comprehensive evaluation-correlation analysis-PCA”, which can effectively identify key pollution factors and dominant sources, thereby providing scientific support for assessing the performance of watershed ecological restoration and implementing targeted management in the basin. The findings indicate that agricultural non-point source pollution control and sediment remediation should be further strengthened, and an integrated management system combining exogenous reduction and endogenous treatment should be established to promote the sustained improvement of the water environment and long-term restoration of ecological functions in the Four Lakes Basin.

[研究目的] 位于长江中游江汉平原腹地的四湖（长湖、三湖、白鹭湖、洪湖）流域，是湖北省重要的农业生产区与生态敏感湿地。为揭示生态修复后四湖流域水体营养物的时空分布特征及其污染源，本研究基于实地监测数据系统分析水质变化规律与主要驱动因子，旨在为流域水环境管理与生态修复成效评估提供科学依据。[研究方法] 基于2022-2023年丰水期、平水期与枯水期的野外调查数据，本研究在四湖流域共设置12个采样点位，覆盖流域上、中、下游区域以及干流与支流。共测定水温(WT)、溶解氧(DO)、浊度(TUR)、高锰酸盐指数(CODMn)、化学需氧量(COD)、五日生化需氧量(BOD₅)、氨氮(NH₄⁺-N)、总磷(TP)与总氮(TN)共9项水质指标。采用水质指数(WQI)法对水质进行综合评价，结合相关性分析与主成分分析(PCA)识别主要污染因子与污染源。通过Excel、SPSS、CANOCO等软件开展数据统计、显著性检验与图形可视化，从时间、空间与污染源维度系统揭示水质演化特征。[研究结果] 四湖流域各水质指标在时间与空间上均存在显著差异。时间维度上，水温(WT)、氨氮(NH₄⁺-N)、总磷(TP)与高锰酸盐指数(CODMn)在丰水期达到最高值，而溶解氧(DO)与总氮(TN)则呈现相反趋势；浊度(TUR)在平水期达到峰值。空间维度上，下游水体的浊度(TUR)、高锰酸盐指数(CODMn)、化学需氧量(COD)与五日生化需氧量(BOD₅)浓度显著高于中、上游区域；中游区域氨氮(NH₄⁺-N)浓度最高，上游区域总磷(TP)与总氮(TN)浓度最低。总体而言，各类营养物浓度呈现“干流>支流”的分布格局。流域WQI值介于15.61至32.88之间，表明四湖流域整体水质处于“较差”至“极差”水平。其时间变化顺序为：平水期>枯水期>丰水期；空间变化则呈现上游>中游>下游，支流>干流的特征。相关性分析结果显示，WT、DO、COD、NH₄⁺-N、TP与TN是影响WQI的主要因子，其中NH₄⁺-N、TP与TN与WQI呈显著负相关。[研究结论] 综上，四湖流域水质在丰、平、枯三期存在显著的时间差异，空间上则表现为上游区域优于中、下游区域，支流优于干流。尽管生态修复工程已初见成效，但流域整体水质仍处于中至重度污染水平。农业施肥、畜禽养殖废水、生活污水与工业排放是主要的外源污染源，而水体沉积物释放与有机残渣分解则是主要的内源污染途径。本研究的创新之处在于首次系统揭示了生态修复背景下四湖流域营养物的时空分布格局与污染成因。本研究还构建了“WQI综合评价-相关性分析-主成分分析(PCA)”的多层次分析框架，可有效识别关键污染因子与主导污染源，从而为评估流域生态修复成效与实施流域靶向管理提供科学支撑。研究结果表明，应进一步加强农业面源污染防控与沉积物修复，并建立“外源减排+内源治理”的一体化管理体系，以推动四湖流域水环境的持续改善与生态功能的长期恢复。