Hydrothermodynamic features of mass exchange across the sediment-water interface in shallow lakes

Long-term field observations on different shallow lakes revealed a sharp increase in concentration of dissolved inorganic nutrients (phosphorus and nitrogen) during the autumn cooling. It was found that a source of nutrients were bottom sediments. The phenomenon has been studied by applying a new approach based on the assumption of interrelation between heat and mass transfer across the sediment-water interface. During the autumn cooling, the upper sediment layer is warmer than the near-bottom water that may cause a convective instability in pore water with high nutrient concentration. Under certain conditions, the Rayleigh number (the main parameter characterizing a density convection) becomes higher than the critical one. As a result, the viscous density convection starts to develop in the pore water. In this case nutrients transfer across the sediment-water interface by thermal macrovolumes with positive buoyancy. The effectiveness of such mass transfer is several orders greater than that of molecular diffusion. This hypothesis was checked by special field and laboratory experiments which were carried out on shallow Lake Krasnoye. The mathematical model of this phenomenon was developed and verified against numerous experimental data.