A Tale of Two Years: Changes in dynamics of physical and biogeochemical processes in an oligotrophic lake.

Climate change, in combination with reduced external nutrient loading are important drivers of phytoplankton changes in lakes undergoing re-oligotrophication. Understanding and forecasting future changes in primary production in response to local and global forcing have become a major challenge for developing sustainable lake management. Aquatic ecosystem models serve as an efficient tool to analyse and interpret in-situ data, and help to gain insights into dynamics of physical and biogeochemical processes. The objective of this study is to apply a coupled 1D physical-biogeochemical model system to investigate large differences in observed nutrient concentrations and primary production rates in Lake Geneva between two consecutive years (2012 and 2013).

The 1D General Ocean Turbulence Model (GOTM) is coupled with a biogeochemical model on the framework of aquatic biogeochemical models (FABM) interface. We calibrated GOTM, by tweaking a few physical parameters, with observed temperature profiles. In the next step, a data assimilation method is applied to minimise data-model misfits and estimate optimised parameter values for the biological parameters related to phytoplankton growth dynamics.

The winter of 2012 was exceptionally cold, measured air temperatures values dipped to -10°C in the month of February. Based on estimates of net heat flux, mixed layer depth, Monin–Obukhov length and turbulent kinetic energy, GOTM suggests that there was a deep mixing event, driven by convection, in the winter of 2012. This is supported by homogeneity in the observed temperature profiles. We found significant differences in internal phosphorus loads in the epilimnion between the two years, the estimate for 2012 is higher. The biogeochemical model predicts weak nutrient limitation on phytoplankton in 2012, which indicates luxury uptake of nutrients by them, and thus stimulation of growth rates. Apparently, high primary production rates that were observed in the spring and summer of 2012 could be a response to high turnover of nutrients, particularly dissolved inorganic phosphorus (DIP), due to the deep mixing event. On the contrary, DIP gets fully depleted by late spring in 2013. This may have limited the nitrate uptake by phytoplankton and hence dissolved inorganic nitrogen concentrations remained high in the epilimnion.