Seasonal dynamics of closed lakes nutrient status controlled by lacustrine groundwater discharge

Abstract. Lacustrine groundwater discharge (LGD) and its associated nitrogen (N) and phosphorus (P) inputs are increasingly recognized as the critical drivers of lake eutrophication. However, the intermonthly variability in LGD and its influence on lake nutrient dynamics remain poorly understood. In this study, high-frequency monitoring and hydrochemical analyses were conducted over a full hydrological year to investigate LGD-related nutrient fluxes and their effects in a typical oxbow lake in the central Yangtze Basin. Water level data and ²²²Rn tracing revealed a seasonal LGD pattern characterized by an increase followed by a decline, ranging from 35.36 to 51.71 mm·d^-1. This pattern was regulated by monthly net precipitation, which controlled the lake level fluctuations and LGD rates. The corresponding N and P loads varied synchronously with LGD and showed seasonal synchrony with lake N and P concentrations. Moreover, variations in the N/P ratio carried by LGD regulate the lake water N/P ratio, thereby influencing its relationship with the dynamic changes in chlorophyll-a. From a global perspective, in closed lakes, LGD is typically governed by climatic factors such as precipitation and evaporation, thereby serving as a key process regulating the lake’s trophic status. This study provides the first evidence that groundwater-driven nutrient loading influences lake nutrient status on an intermonthly scale offering new insights and management strategies for eutrophication control in shallow, closed lake systems worldwide.