High-resolution spatiotemporal heterogeneities of water optical properties in a large lake to infer physical and biogeochemical drivers of primary production

Measuring primary production (PP) is of major importance to evaluate how lakes are recovering from eutrophication, and better constrain their role in global biogeochemical cycles. The classical sampling method involving radiolabelled carbon incubated in bottles at selected depths is associated with heavy logistics and prevents scientists from sampling the short-term dynamics of PP. Therefore, we investigate alternative methods based on oxygen sensors and optical properties.

To explore the interplay between biogeochemical and physical processes and how they are related to PP in large lakes, we have deployed since October 2018 a sophisticated autonomous profiler in Lake Geneva, Switzerland. The so-called Thetis profiler measures with a centimeter resolution backscattering and fluorescence at discrete wavelengths, hyperspectral absorption and attenuation, hyperspectral reflectance, along with temperature, dissolved oxygen and conductivity. The profiler was deployed over contrasted seasons and recorded these parameters every three hours over the top 50 m of the water column.

Observations revealed large spatiotemporal heterogeneities of optical properties as a result of seasonal and short-term (weekly to sub-daily) physical and biogeochemical processes. Diel cycles in the optical properties representing phytoplankton abundance resulted from night-time convection and grazing and production during the day. Short-lived but strong upwelling events triggered sudden disruptive shifts in the phytoplankton community composition over the fall to winter transition period which, in turn, impacted the oxygen budgets, opening the door to a better understanding of the vertical heterogeneity of PP directly from inherent optical properties. The data also features advected riverine and resuspended sediments during storms and snowmelt, and during upwelling events, respectively.

This in-situ data is now being combined with remotely sensed water quality parameters (OLCI products from Sentinel 3A and 3B) and a three-dimensional hydrodynamic model of Lake Geneva (www.meteolakes.ch) to upscale PP estimates from local to basin scale.