Groundwater Discharge Is a Major But Imprecisely Estimated Phosphorus Source to Lake Taihu

Eutrophication remains a major challenge for lakes, where phosphorus (P) control has been central to management efforts. Despite substantial reductions in surface P inputs, the role of groundwater‐derived P remains insufficiently understood. Notably, groundwater P is predominantly present as dissolved inorganic phosphorus (DIP), the most bioavailable form of P. In this study, we used radon ( 222 Rn) mass balance to quantify lake‐groundwater exchange in Lake Taihu. Results showed a clear seasonal variation in lateral groundwater discharge, with an estimated flux of 0.70 × 10 8 m 3 d −1 in the wet season and 0.18 × 10 8 m 3 d −1 in the dry season. The results of variance tests and path analysis showed that groundwater level and dissolved oxygen concentration contributed in opposite directions, promoting and inhibiting the synthesis of groundwater DIP, respectively. In addition, pH and the proportion of clay‐rich sediments promoted the enrichment of DIP during the wet and dry seasons. The annual loading of DIP from groundwater sources was estimated to be 2,304.40 (95%CI: 1821.88 to 2585.12) t yr −1 , accounting for more than 84% of the total external DIP input, approximately 7 times greater than that from surface water. Notably, about 1,954.38 t yr −1 was retained in the lake, indicating long‐term internal accumulation. These results highlight groundwater as a dominant and persistent source of bioavailable P and underscore the need to incorporate subsurface fluxes into lake nutrient budgets for effective eutrophication management.