How much oxygen is dissolved in the water of lakes influences the biodiversity, the biochemical balance of the lake and the quality of the drinking water. A new study now shows that the oxygen content in freshwater lakes decreased significantly between 1980 and 2017. From the researchers’ point of view, the main reason for this is climate change: the warmer the water is on the surface, the less oxygen it can store. If the temperature deviates too much from that in deeper water layers, the mixing of the lake is also reduced, which means that deeper layers are also less supplied with oxygen. This threatens the sensitive lake ecosystem with its unique biodiversity and its importance for the climate and human health.
Freshwater lakes make up about three percent of the earth’s land surface. They represent largely closed ecosystems and contain an extraordinarily large variety of species. Most living things in lakes, however, depend on enough oxygen being dissolved in the water. Oxygen is absorbed from the air on the surface of the lake. The colder the water, the higher its oxygen capacity. In addition, aquatic plants, unicellular algae and cyanobacteria produce oxygen through photosynthesis. Due to the water circulation in the lake, the oxygen is distributed from the surface to the depths. There it is mainly used in the decomposition of dead animal and plant parts. The more nutrient-rich a lake is, the more oxygen is required. If there is a lack of oxygen, the ecosystem becomes unbalanced.
Surface and deep water affected
A team led by Stephen Jane from the Rensselaer Polytechnic Institute in New York has now investigated for almost 400 freshwater lakes in the temperate climate zone how their oxygen content has developed over the past few decades. To do this, they evaluated over 45,000 data points on temperature and oxygen content, which were collected between 1941 and 2017. “We found that dissolved oxygen depletion in surface and deep water habitats is widespread,” the authors write. Since 1980 in particular, the oxygen content of the examined lakes has fallen significantly: on average by 5.5 percent on the surface and by 18.6 percent in deep water layers.
In the case of surface water, the cause lies in a simple physical effect, explains Jane’s colleague Kevin Rose: “Oxygen saturation, i.e. the amount of oxygen that the water can absorb, falls when the temperature rises. This is a well-known physical relationship and explains most of the trend in surface oxygen that we see. ”In the case of deep water layers, on the other hand, the temperature hardly changed during the study period. The storage capacity for oxygen has therefore also remained the same here. But it is precisely the growing temperature difference to surface water that leads to another problem: Since warm water has a lower density than cold water, it remains on the surface and circulation in the lake stagnates. “The increasing stratification makes the mixing, in which new oxygen from the atmosphere reaches deep water layers, more difficult and less frequent and the dissolved oxygen in the depth decreases as a result,” says Rose.
More harmful algal blooms
Despite the strong influence of the surface temperature on the oxygen content of the water, the researchers also found the opposite effect in some lakes: “A large subgroup of lakes showed increases in both water temperature and dissolved oxygen concentration,” they report. This was especially true for lakes that were heavily polluted with nutrients from agricultural inflows. “The fact that we are seeing an increase in dissolved oxygen in these lakes may be an indicator of a widespread increase in algal blooms, some of which produce toxins and are harmful,” says Rose. Since the researchers have no data on the species present in the respective lake, they cannot make a definitive statement in this regard. “But nothing else we know can explain this pattern,” said Rose.
From the researchers’ point of view, the developments they have documented are a warning signal. “Lakes are indicators of environmental changes and potential threats to the environment because they respond to signals from the surrounding landscape and atmosphere. We found that these disproportionately biodiverse systems are changing rapidly, suggesting the extent to which ongoing atmospheric changes have already affected ecosystems, ”says Jane.
Sensitive balance threatened
Oxygen concentration is not only critical to biodiversity, it also regulates many other characteristics of water quality. When oxygen levels drop, bacteria that thrive in an oxygen-free environment begin to multiply, including those that produce the powerful greenhouse gas methane. This suggests that lakes may release larger amounts of methane into the atmosphere as a result of the loss of oxygen. In addition, the sediments at the bottom of the lake release more phosphorus under low-oxygen conditions, which pollutes the already oversupplied waters with additional nutrients.
“Ongoing research has shown that the oxygen levels in the world’s oceans are falling rapidly. This study now proves that the problem in freshwater is even more serious and threatens our drinking water supply and the delicate balance that enables complex freshwater ecosystems to thrive, ”says Jane’s colleague Curt Breneman. “We hope this finding will make efforts to address the progressive harmful effects of climate change more urgent.”
Source: Stephen Jane (Rensselaer Polytechnic Institute, New York) et al., Nature, doi: 10.1038 / s41586-021-03550-y