Large lakes tend to be thermally stratified, with a layer of relatively warm water near the surface, and a deeper layer of colder water underneath. The temperature of the surface water fluctuates over the year, while the temperature of the deeper water remains more stable: hence, the thermal stratification is strongest during the summer, when the surface water is warmest.
In a strongly stratified lake, the convective exchange of water between the layers is limited, and this reduces the vertical redistribution of dissolved oxygen and nutrients in the lake. In fact, the deeper water frequently receives most of its nutrients in the winter, when the stratification is weaker.
Dr Rafael Reiss, from the Department of Earth Sciences in Cambridge, has studied the dynamics of deepwater exchange in Lake Geneva, western Europe’s largest lake, with a maximum depth of around 300 m. Located between the Alps and the Jura mountains, Lake Geneva experiences strong winds, whose direction is controlled by the surrounding topography. This makes it an ideal location to investigate the impacts of wind on the transport of deep water in a lake.
Dr Reiss has combined field observations, 3D numerical modelling and particle tracking to create a model of the flows which develop in the lake in different seasons. The data suggest that wind-driven upwelling induces large vertical motion of the deep waters: cold water from a depth of 150-200 m upwells near the coastline, and after spending about one day near the surface, it sinks again and spreads over a large area at depth, leading to both vertical and horizontal exchange of the water in the basin. Inter-basin upwelling between the shallower and the deeper parts of the lake is also observed.
Recently, Dr Reiss has been investigating the impacts of different vertical modes of rotationally-modified standing internal waves (i.e., Kelvin and Poincaré waves) on the deepwater dynamics in Lake Geneva. The results of this work have been published in Communications Earth & Environment, and you can find the article here.