Urban lakes provide essential ecosystem services (biodiversity, flood attenuation, recreational activities …). Results presented here relate to anoxia events which occurred during late summer and autumn 2021, in a small urban lake. This sandpit lake located in the Great Paris region is principally fed by groundwater from the nearby Marne River. It is equipped with underwater continuous sensors (Temperature, Oxygen, Chlorophyll-a, Nitrate). A meteorological station is implemented on a buoy, above the sensor chain. Three whole-lake anoxia were observed from end of August to October. Oxygen dropped from supersaturation to 0% within a few days, leading to massive fish kills. The anoxia events occurred following high peaks of phytoplankton biomass. Over the studied period, the level of the Marne River was higher than usually in late summer. We propose the following scenario to explain the observed anoxia events: (1) a high level of the Marne River sustained a massive phytoplankton production through the groundwater nutrient loading; (2) anoxia was then caused by the oxygen uptake for the mineralization of the phytoplankton organic carbon. This hypothesis is tested using a one-dimensional thermal-biogeochemical model. These results highlight that, beside the impact caused by warmer water temperature and increased thermal stratification, hydrological change can also affect drastically lake ecosystems. More chronic occurrences of this type of hydrological episodes would impair conservation or restoration actions of the ecological status of lake ecosystems.