Impact of extreme hydro-meteorological events on the anoxia dynamics in a small urban lake

Urban lakes provide essential ecosystem services (hotspot of biodiversity, stormwater management, reduction of pollutant loadings…).
During late summer and early autumn 2021, two whole-lake anoxia events occurred in Lake Champs-sur-Marne (Great Paris, France).  This sandpit lake, principally fed by groundwater from the nearby Marne River, is continuously monitored by an autonomous station equipped with underwater sensors (water temperature, oxygen, Chlorophyll-a, CDOM, Nitrate). At the same point, a meteorological station is installed on a buoy, above the lake surface.
During the two anoxia events (end of August and mid-September), oxygen concentration dropped from supersaturation level corresponding to a high peak of phytoplankton biomass, to 0% within a few days. During this summer period, successive heavy rainfall events occurred, causing a flood of the Marne River and rising the watertable level to unusual values in Summer. This resulted in high water and nutrient fluxes from the river towards the lake. 
The observed whole-lake anoxia can be explained according to the following assumptions: (1) the groundwater nutrient loading, favored by the high level of the Marne River, caused a huge phytoplankton production; (2) then, the phytoplankton decline was associated to an intense mineralization of the biomass organic carbon; (3) the lake oxygen was completely exhausted, leading to a massive fish kill. 
These results highlight the severe impact of a non-extreme but high and long hydro-meteorological event on a lake ecosystem. In Lake Champs-sur-Marne, the nutrient limitation of phytoplankton production generally occurs during late summer. In 2021, the limitation was removed by the Summer exceptional nutrient loading. In temperate regions, summer algal blooms are not limited by water temperature but by nutrient availability. Climate change is expected to increase the frequency of extreme hydro-meteorological events.  Higher frequency of summer heavy rainfall may trigger repeated phytoplankton blooms, deteriorating the ecological status of lake ecosystems.