Generation and dispersion of chemical and biological gradients in a large-deep multi-basin lake (Lake Como, north Italy): The joint effect of external drivers and internal wave motions.

The formation of chemical and biological heterogeneity in lakes can be favored by physical and morphometric constrains. This study describes the results of four whole-lake field campaigns carried out in Lake Como (north Italy) during thermal stratification. The aim was to analyze the distribution of chemical-biological variables in a multi-basin lake as a result of internal and external physical drivers and constrains. Lake Como has a y-like shape encompassing three main sub-basins: northern, south-eastern, and south-western. Field data underlined: the presence of chemical-biological gradients between the south-western basin and the rest of the lake and the propagation of a fresher water-plume (formed by the two main northern inflows) into the northern basin and then into the south-western closed arm. The use of a three-dimensional hydrodynamic model showed a periodic movement of this plume that tends to enter the south-western basin first and then to return toward north, moving forward and back through the junction of the three arms. The entrance into the eastern basin, instead, occurs only secondarily. Wavelet transform analysis revealed common periodicity between the plume movement and the action of different external and internal lake-stressors, including: the discharge of the main inflow (period centered at 1, 3.3, and 6.5 day), the wind intensity (0.5 and 1 day) and the two main basin-scale internal wave motions: (3.3 day and 7.1 day). The periodic movement of the fresher water-plume enhances the water exchange and reduces the chemical-biological gradients between the western closed basin and the main lake, playing a crucial role in distributing both inorganic and organic materials at the lake basin-scale.