Lakes need to contain a certain level of dissolved oxygen to maintain water quality and preserve their ecosystems. While the upper layers of a lake are typically rich in oxygen, that’s not the case for deeper layers; in most lakes, oxygenation of these layers occurs primarily through a process called convective cooling that takes place during the cooler autumn and winter periods. For deep lakes in temperate climates, like Lake Geneva, winters are often not cold enough for this process to occur on an adequate scale, meaning the very deep waters are not affected. The last full-depth convective cooling in Lake Geneva took place in 2012 during a severe cold spell (CIPEL).
Understanding other deepwater renewal mechanisms
“With climate change, there are more and more winters when the conditions needed for this process are not met,” says Rafael Reiss, a recent PhD graduate at EPFL’s Ecological Engineering Laboratory (ECOL). “So we need to understand other mechanisms that could enable the oxygenation of a lake’s deeper layers.” He studied two alternative deepwater renewal mechanisms as part of his PhD thesis, both of which are induced by wind: interbasin exchange, where water is exchanged between the shallow Petit Lac and the deeper Grand Lac basin, and coastal upwelling. “Unlike convective cooling that’s triggered by cold air temperatures, the mechanisms we studied are less sensitive to climate change because they are wind-driven. They occur in Lake Geneva several times each winter and could therefore play an increasingly important role in renewing and aerating the deeper layers,” says Reiss.
Rafael Reiss and his ECOL colleague Htet Kyi Wynn during the deployment of field equipment for measuring profiles of current velocity and water temperature in December 2017.© ECOL/EPFL
Water in these deep layers is usually cold, oxygen-poor and nutrient-rich. The upper layers, on the other hand, are warmer with higher concentrations of oxygen and lower concentrations of nutrients. The two layers barely mix for most of the year due to their different densities – warm water is less dense than cold water, leading to a so-called stable stratification. But once the air temperature drops during autumn and winter, the surface waters cool and the stable stratification is gradually eroded from the top downwards. If the winter is cold enough, the waters near the surface reach the same temperature, and consequently the same density, as the deeper waters. The result is a complete overturning of the water column, whereby oxygen from the upper layers is brought to the bottom and nutrients from the lower layers rise to the surface.
Deepwater renewal several times a winter
Reiss’ study showed that, under the effect of the earth’s rotation, the strong winter winds that frequently blow across Lake Geneva from the southwest push coastal waters at the northern shore of the Grand Lac towards the center of the lake, with these waters being replaced by the rising of deeper waters. The same winds push the surface waters of the Petit Lac towards the Grand Lac, causing deeper waters from the Grand Lac to take their place. These two complex exchange mechanisms cause the oxygen-poor, nutrient-rich lower layers to rise upwards, sometimes from depths of over 200 meters (Lake Geneva has a maximum depth of 309 meters). These upwelled, deep waters can remain close to the surface for several days (or even reach the surface) before descending back to great depths, allowing them to be enriched with oxygen through exchange with the upper layers and the atmosphere.
To carry out this study, Reiss and his team first spent two winters collecting data in the field, measuring current velocities and water temperatures. They then employed a 3D hydrodynamic model and combined it with a modeling technique called particle tracking in order to analyze the pathways of the upwelling waters in great detail. “Our findings show just how complex these mechanisms are,” says Reiss. “They take place in 3D, meaning they can’t be described using the one-dimensional models that are frequently used to predict the impact of climate change on lakes. These mechanisms deserve further attention when assessing deepwater renewal in large, deep lakes.”