Trends in water quality parameters and trophic level of Lake Xingkai/Khanka

Understanding the drivers of eutrophication is essential for mitigating pollution in transboundary lakes. This study investigates seasonal and interannual variations in key water quality parameters in Lake Xingkai/Khanka, a transboundary water body between China and Russia, from 2016 to 2023. Key parameters included suspended solids, dissolved oxygen (DO), biochemical oxygen demand (BOD₅), dissolved inorganic phosphorus (DIP) and nitrogen (DIN), iron (Fe), and the Trophic Level Index (TRIX). The temperate monsoon climate governed strong seasonal patterns, with peak precipitation in summer and higher DO in winter. The latter resulted from underwater photosynthesis under ice cover, sustaining biota despite concurrent organic matter decomposition. Dissolved inorganic phosphorus (DIP) exhibited significant seasonal variability, increasing from spring to autumn, a pattern attributed to agricultural runoff from phosphorus-based fertilizers used in soybean cultivation. DIN, composed primarily of nitrate and nitrite, displayed seasonal dynamics. Nitrite concentrations correlated with DIP across all seasons except autumn, whereas nitrate correlated with DIP only in winter. The Trophic Level Index (TRIX) revealed a significant trend of increasing eutrophication, particularly pronounced in spring. Maximum Permissible Concentrations were exceeded for nitrites, iron, and suspended solids, particularly near wastewater discharges from rivers in the southern sector. Nevertheless, the lake currently maintains resilient ecological functioning, evidenced by persistent oxygen supersaturation (> 140%), the absence of hypoxia, and efficient nutrient assimilation. This resilience buffers the current anthropogenic nutrient loads, though ongoing monitoring is crucial to mitigate emerging eutrophication risks.