Stratification, mixing and transport processes in Lake Kivu
This chapter summarizes the knowledge on mixing and transport processes in Lake Kivu. Seasonal mixing, which varies in intensity from year to year, influences the top â¼65 m. Below, the lake is permanently stratified, with density increasing stepwise from â¼998 kg mâ3 at the surface to â¼1,002 kg mâ3 at the maximum depth of 485 m. The permanently stratified deep water is divided into two distinctly different zones by a main gradient layer. This gradient is maintained by a strong inflow of relatively fresh and cool water entering at â¼250 m depth which is the most important of several subaquatic springs affecting the density stratification. The springs drive a slow upwelling of the whole water column with a depth-dependent rate of 0.15â0.9 m yearâ1. This upwelling is the main driver of internal nutrient recycling and upward transport of dissolved gases. Diffusive transport in the deep water is dominated by double-diffusive convection, which manifests in a spectacular staircase of more than 300 steps and mixed layers. Double diffusion allows heat to be removed from the deep zone faster than dissolved substances, supporting the stable stratification and the accumulation of nutrients and gases over hundreds of years. The stratification in the lake seems to be near steady-state conditions, except for a warming trend of â¼0.01Â°C yearâ1.