Thermal use of lakes and rivers needs more environmental research

Lake Lucerne could provide heating and cooling for the whole region, but what impact on aquatic life? Keystone

Lakes, rivers and other bodies of water have great potential for heating or cooling buildings or industrial processes, but the impact on aquatic life is under-researched, say Swiss scientists.

Climate warming is not the only cause of temperature change in bodies of water but also their increasing use for heating and cooling, say researchers at the Swiss Federal Institute of Aquatic Science and TechnologyExternal link (EAWAG) and the University of Bern. For heating purposes warmth is extracted from the water and discharged back into a lake or river at a lower temperature. The opposite applies when bodies of water are used for cooling.

One example is the Mühleberg nuclear power plant near Bern, which is cooled by water from the River Aare. Earlier studies have shown that the river water is warmed by 0.7 degrees Celsius (1.3 degrees Fahrenheit) in summer and up to 2.4 degrees Celsius (4.3 degrees Fahrenheit) in winter. Cases with such high impact are known as “thermal pollution”. 

EAWAG research in 2017 for the Swiss environment ministry found that Lake Lucerne could theoretically provide the whole surrounding region with heating and cooling. But what impact does temperature change have on aquatic ecosystems? This is a vital question, say the researchers. They have therefore carried out a review of literature to assess the physical, chemical and ecological effects on lakes and rivers.        

“Temperature change affects a multitude of processes in waterbodies, including water circulation and chemical reactions”, says Adrien Gaudard of EAWAG, who was involved in the review. Daphnia, snails, fish and other organisms react to temperature fluctuations, altering their eating habits or adjusting their metabolic rate. 

The researchers were not able to deduce the extent of temperature change that can be borne by an ecosystem from the available data. Nevertheless, the literature review did allow the researchers to draw certain conclusions. For example, they say the quantity of thermal energy that is extracted from a body of water is an all-important factor, as is the manner in which this happens. “In the context of climate change also, cooling the water generally has a lesser effect than warming it,” notes Gaudard.