Oxygen is becoming increasingly scarce in the deeper water layers of lakes and reservoirs - caused by high nutrient inputs and climate change. As a result, the living conditions for fish and invertebrates deteriorate, greenhouse gases are increasingly released and nutrient cycles intensify. In addition, hydrogen sulphide, which is toxic to higher organisms, can accumulate. The treatment of oxygen-free deep water to produce drinking water is associated with high costs. Researchers at the TU Bergakademie Freiberg and the Leibniz Institute of Freshwater Ecology and Inland Fisheries (IGB) have now developed a calculation method with which the ecological consequences of oxygen deficiency can be predicted for each lake based on a few characteristic values.
The formula is familiar from school: If the temperature rises, the water can absorb less oxygen. If there are many nutrients or degradable biomass in the water, the little oxygen is consumed more quickly by the metabolism of the microorganisms. In addition, climate change increases the duration and stability of the thermal stratification of lakes, so that the oxygen supply to the deep water is interrupted for longer and longer periods.
"In times of warmer water temperatures and poor ecological status of many water bodies, oxygen deficiency is a problem for many lakes and small stillwaters," notes Richard LaBrie. He conducts research at the TU Bergakademie Freiberg and is the lead author of the study.
Lack of oxygen leads to the formation of pollutants and the intensification of material cycles
Fish and invertebrates react sensitively to a lack of oxygen. But there are other negative consequences: Oxygen-poor waters produce more greenhouse gases, including methane, which is particularly harmful to the climate. In addition, a lack of oxygen produces pollutants such as ammonium or toxins such as methylmercury and oxygen-sensitive hydrogen sulphide. The availability of nutrients for plant growth is affected by these changes in complex ways.
"In order to better understand and predict the effects of climate change on phosphorus and algae growth, data from as many lakes as possible are currently being included in a nationwide project. However, long-term climate monitoring is expensive and can therefore only be carried out for a limited number of lakes, so alternative tools are urgently needed," says IGB researcher Michael Hupfer, co-author of the study.
Oxygen profiles and topography can be used to estimate the consequences of anoxia
The research team from TU Bergakademie Freiberg and IGB has now developed an easy-to-use method to estimate the consequences of oxygen deficiency on the deep water of lakes. To do this, the researchers use temporally and spatially highly resolved information about the duration of the anoxia. The team also shows that this information can be calculated relatively easily from a few measurements and the topography of the water bottom using mathematical models. The extent of substance formation and release depends largely on the duration of the oxygen deficiency as well as the availability and type of parent substances.
"Data on oxygen profiles of lakes are available worldwide or can be easily determined. Our approach is therefore also easily applicable for other researchers and authorities. For the first time, a method is now available to predict the ecological consequences of temporally and spatially increasing anoxia in lakes with different nutrient availability," says Maximilian Lau, who is a junior professor at the TU Bergakademie Freiberg and at the IGB and led the study.