High frequency monitoring of biogeochemical processes in a small and shallow lake

National audienceSmall and shallow inland water bodies are important providers of ecosystem services, e.g. in metropolitan areas. Their presence is massive in most landscapes, and they contribute substantially to global biogeochemical cycles. The dynamics of their ecological functioning is fast (e.g., algal blooms can appear and recede within a few days) and its spatial variability can be high. Understanding these processes requires high-frequency measurements. Within the ANR ANSWER and the AquaREA project, biogeochemical processes are monitored and modelled in a small and shallow lake, both in the water column and at the sediment-water interface. The study site is a 0.12 km2 and 2.3 m deep lake in the Greater Paris. Physical-chemical variables (water temperature, conductivity, pH, dissolved oxygen), chlorophyll-a fluorescence, a proxy of total phytoplankton biomass, and phycocyanin fluorescence, a proxy of cyanobacteria biomass, have been monitored since 2015 at high frequency (every 10 min) at three different points and two depths. Nitrate concentration is also continuously monitored by UV and visible light spectroscopy.An innovative system, based on the atmospheric technique Relaxed Eddy Accumulation, is under development and will enable to quantify the fluxes of nutrients and greenhouse gases at the water-sediment interface. This continuous monitoring dataset is used for phytoplankton modelling and bloom forecast