Many lakes around the world have too little oxygen in their water during the summer months. This not only harms the animals and plants living there in the short term, but also threatens the ecosystem for many years, as a new study shows. Accordingly, a one-time lack of oxygen in a lake sets off a downward spiral. As global warming increases, it also rotates faster and faster.
With climate change, more and more lakes and their inhabitants are suffering from a lack of oxygen. The rising temperatures and the ever earlier onset of spring are making it more difficult for water to exchange between warm, near-surface layers and cold, deep layers in the waters. This means that natural biological degradation processes in deep water have more time to completely use up the limited oxygen supply. As a result, oxygen deficiency occurs more frequently, especially in the lower water layers, as numerous studies have shown.
This is also promoted by algae, which die when there is a lack of oxygen and are decomposed by bacteria at the bottom of the lakes, whereby oxygen is also consumed. The rising temperatures in turn promote algae growth and thus fuel the process. It is well known that global warming is damaging lakes and that a lack of oxygen is causing further loss of oxygen. However, it was previously unclear how long the waters would suffer from the consequences of individual years with a lack of oxygen.
Hundreds of oxygen-deficient lakes examined
In order to find out how different parameters in the lakes influence each other and in the long term, a research team led by Abigail Lewis from the Virginia Polytechnic Institute and State University has now evaluated long-term data from 656 lakes and reservoirs for the first time - primarily in North America and Europe, where there are particularly many Lakes already suffer from a lack of oxygen. The data comes from more than 100,000 independent measurement campaigns from the GLEON network (Global Lake Observatory Network). In addition to data on the oxygen content in the water, they also contain information on the water temperature, the retention of nutrients in the sediment in the form of phosphorus and the development of planktonic algae, measured via the chlorophyll content.
The result: “If a lake has fallen below a critical oxygen level in one year, there is a very high probability that it will be affected by an even more intense lack of oxygen in the following year,” explains co-author Maximilian Lau from the TU Bergakademie Freiberg. Accordingly, lakes that were once affected by a lack of oxygen in the deep water are usually affected again the following summer. As a result, the living conditions for fish and invertebrates continue to deteriorate, greenhouse gases are increasingly released and nutrient cycles are changed. More nutrients such as phosphorus are released from the sediments, allowing plant plankton such as small algae to spread more easily. The intensity of the individual consequences in the study differed depending on the size and depth of the lakes, but the downward spiral described occurred in all ecosystems examined.
Findings help protect lakes from further crises
“Although this problematic vicious circle can be theoretically derived from previous knowledge of nutrient dynamics, thanks to the large sample, our team was now able to decipher the effect of the processes involved for the first time,” says Lau. Thanks to the detailed findings, the team can now more accurately predict the susceptibility of the waters to further oxygen crises. The results can help to better understand the health status of lakes and improve it through targeted nutrient management. This also benefits our drinking water supply.
Source: Abigail Lewis (Virginia Tech) et al., Global Change Biology, doi: 10.1111/gcb.17046