How do recreational swimmers affect lake-atmosphere and lake-sediment transport?

Natural waters in urban and suburban areas experience growing stresses from active water use and recreational activities, such as leisure boating, fishing, and swimming. Among other factors, leisure swimmers are considered to have a minor effect on the large-scale dynamics of water bodies. Certain physical processes are however very sensitive to disturbances of boundary layers - a thin diffusive layer at the lake surface and the upper layer of sediment. During hot summers, a high concentration of swimmers in small lakes and ponds may disrupt the boundary layers, intensifying the vertical heat and mass exchange and producing localized outbursts of methane into the atmosphere and(or) release of the dissolved nutrients from sediment to the water column. We performed a series of experiments in the suburban area of Berlin, Germany to estimate the potential swimmer effects on the vertical heat and mass transport. Monitoring of physical and water quality parameters in small (~2 km²) Lake Mellensee revealed a consistent  increase of turbidity and decrease of transparency in the vicinity of the beach actively visited by weekend swimmers from Berlin and surroundings. The measured concentrations of the dissolved methane and the methane fluxes at the lake surface, while indicating potential increase due to localized swimming activities, were heterogeneous and depended strongly on variations in sediment composition and on wind conditions. To quantify the effects on the lake-atmosphere fluxes, we performed estimations of the surface temperature disturbance by swimmers in a large mesocosm facility using UAV-based multispectral and infrared cameras. A manifold increase of the surface heat flux derived from the root-mean-square temperature fluctuations results from the diffusive layer disruption and implies a proportional increase of the dissolved gas release.