Improving the understanding of the functioning of water bodies in the urban critical zoneAn observation platform of an urban lake in the Greater Paris region

In urban areas, water bodies provide a number of ecosystem services that are particularly crucial: flood control, preservation of biodiversity, formation of cool islands, recreational activities, landscape quality, etc.

Lakes and ponds are part of the urban critical zone. Many of them have been created in the recent decades as sand-pit lakes or retention ponds. Sand-pit lakes are the result of sand and gravel extraction for the construction of towns. Retention ponds have been implemented to limit flooding risks and to reduce the pollution peaks associated with heavy rainfall. Actually, in the context of climate change, urbanisation which is associated with the imperviousness of soils, increases the run-off processes.

Moreover, the large interface between the aquatic and terrestrial environments makes these urban lakes fundamental ecosystems for maintaining biodiversity in the city.

The hydrodynamics, ecological functioning and fate of contaminants in the water column of these lakes are very important environmental issues. In order to better understand the physical and biogeochemical processes at stake and to which extent they may be affected by climate change, autonomous monitoring stations can provide long-term, high-frequency, reliable datasets. These data are also very useful for the calibration of numerical model parameters.

The monitoring station implemented in Lake Creteil, in a highly urbanised area of the Greater Paris region (France) is presented. The surface area of the lake is 0.4 km², average depth 4 m, maximum depth 6 m. The lake is fed by groundwater flowing from the Marne to the Seine and by the stormwater network of an urban catchment (1 km²). This observation platform is part of an OSU (Observatoire des Sciences de l’Univers) and is also associated to the French SNO OBSERVIL (Service National d’Observation) network.

The instrumented buoy is equipped with underwater probes to measure physical and biogeochemical parameters and a weather station. Underwater measurements are performed every 15 minutes and meteorological measurements every 10 minutes. Temperature probes (CS225 Campbell) are deployed at five different depths: 0.5 m, 1.5 m, 2.5 m, 3.5 m and 4.5 m. At 1.5 m depth, a multiparameter probe (YSI Exo3) measures oxygen, conductivity, chlorophyll-a and phycocyanin. The weather station measures the wind speed and direction, air temperature, relative humidity, atmospheric pressure, rainfall height, short and longwave radiations.

The lake data are exported to a local database via a GSM protocol. The data is visualized on a web dashboard using the open-source Grafana software. On the dashboard, the timeseries of the underwater and the meteorological measurements are displayed in a panel and the short-term (2 days) forecast of the variables obtained by a neural network model are plotted as gauge charts.

The results of the timeseries analysis are presented to illustrate how some physical and biogeochemical processes occurring in the lake (e.g. thermal stratification, peak of phytoplankton biomass, anoxia of the deep layers…) have been quantified. The use of the data for parameter calibration and validation of hydro-ecological numerical models is also presented.