Simulating phytoplankton growth during the spring thermal bar in a deep lake

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Phytoplankton growth in Lake Baikal was simulated during thermal bar events using meteorological data from June 1–30, 2017. Dynamics of the thermal bar and the plankton population were reproduced by the coupled physical–biological model that describes hydrodynamic processes and interactions of the components of the nutrient–phytoplankton–zooplankton–detritus system. The key point of the model is consideration of atmospheric impact on the lake surface, taking into account the intraday variability of wind forcing and fluxes of shortwave and longwave radiation and latent and sensible heat. Numerical modeling showed that a combination of cabbeling instability, associated with the thermal bar, and wind-driven currents lead to sinking of the phytoplankton biomass from the surface area to the lower layers of the lake. The increase in phytoplankton results in a degradation of the nutrients in the corresponding water environment. Simulations demonstrated that the maximum concentrations of phytoplankton are localized in the inshore side of the thermal bar. At the same time, the thermal bar acted as a barrier preventing the horizontal propagation of the biological substances into open waters. A comparative analysis of the calculated temperature and phytoplankton distributions with available field data was performed. • Plankton dynamics during the spring thermal bar in a deep lake were simulated. • The thermal bar acts as a barrier to horizontal propagation of the plankton biomass. • Wind forcing plays a significant role in the time-space distribution of the phytoplankton. • Wind contributes to sinking of the phytoplankton to the lower layers of the lake.