Researchers from the University of Montreal and the University of Basil investigated the impact of climate change on lake environments. Remarkably, they found out that, despite the temperature of the water at the surface increases, water near the bottom of the lake may cool down.
A lake generally begins to warm at its surface. This triggers a procedure of algae production and increases turbidity effects of the water resulting in isolating the bottom layers which consequently experience a temperature decrease. “The surface-water warming and the loss of transparency have the effect that more heat is trapped in the upper layers of the lakes, leaving the deeper waters thermally isolated. Under some circumstances, this can even lead to the cooling of water masses near the bottom of the lake,” Dr. Maciej Bartosiewicz, lead author of the study from the University of Basel, said.
The temperature of the water is a critical factor that affects the emission processes of a lake. In general, a temperature rise increases the carbon dioxide and methane emissions and simultaneously, decreases the lake's carbon storage capacity.
The fact that the water temperature is reduced in near-bottom water is of high importance and can reveal unexpected effects. According to the authors, the study challenges the perception that global warming increases greenhouse gas emissions in lakes through direct temperature effects. Water cooling leads to a decrease in carbon dioxide production and a rise in carbon burial capacity of a lake. Researchers showed that this phenomenon is exacerbated in relatively small and shallow lakes.
“We don’t want to question the fundamentals of thermodynamics. There is no doubt that the rates of respiratory metabolic processes in lakes are generally higher at increased water temperatures. However, climate change will not cause every lake to warm up everywhere,” Moritz Lehmann, Professor at the Department of Environmental Sciences at the University of Basel, stated.
Nevertheless, the temperature differences between the water layers in a lake triggers another effect. Deeper water layers are cooler and do not mix with those on top. Therefore, they are not properly ventilated and, as a result, methane production by anaerobic microorganisms is increased. “All in all, global warming increases the greenhouse gas potential of lakes, as expected. However, this has less to do with the warming directly, and more to do with increased oxygen depletion at the bottom of these lakes,” Dr. Bartosiewicz, added.
Source: University of Basel