Oxygen is essential for sophisticated life on Earth, especially aquatic life such as fish and zooplankton. However, according to a new study published in the journal Nature, oxygen concentrations in lakes worldwide are decreasing, posing a threat to the organisms that dwell in those bodies of water (and the rest of us, too).
“A lot of monitoring wasn’t going on until more recent decades,” says senior author Kevin Rose, a biology professor at Rensselaer Polytechnic Institute. “And now we have enough information to tell a storey.”
Data were collected from lakes and reservoirs across the United States, from Lake George in New York to Trout Bog in Wisconsin and some lakes in other temperate nations like Canada, Japan, and Sweden, by an international team of researchers. It studied nearly 400 lakes, all of which had at least 15 years of dissolved oxygen (the oxygen present in water) and temperature data.
In a statement, lead author Stephen Jane, who recently finished his Ph.D. at Rensselaer Polytechnic Institute, said, “Lakes are indicators or sentinels of environmental change.” “We discovered that these disproportionally more biodiverse systems are changing at a faster rate, demonstrating the extent to which ongoing atmospheric changes have already damaged ecosystems,” says the researcher.
“Stratification” is the process of dividing a massive body of water into layers. Surface water becomes less dense than the water below it as it heats up due to solar radiation and air temperature, forming an upper and lower layer. In their investigation, the scientists separated surface waters from deep waters. They found drops in oxygen levels between 1980 and 2017—a 5.5 percent drop in surface waters and a 19 percent drop in deep waters, a rate roughly three to nine times faster than oceans.
According to Rose, the decreases in surface waters are explained mainly because when water temperatures rise, the water can carry less oxygen. Deep waters, on the other hand, where many creatures that require cooler temperatures (such as cold-water fish like trout) take refuge, the temperature remained relatively constant over time. “However, oxygen concentrations continue to decline.” According to the researchers, the key impact was likely an increase in the strength of water stratification, which lowers oxygen’s capacity to seep down into the water’s bottom layer.
“As the temperature of the surface water rises over time, [its] density drops,” Rose notes. “The larger the density difference between layers in a water column, the greater the barrier to oxygen mixing from the atmosphere down to deeper seas.” That means that, even if the water temperature remains technically ideal, species that rely on colder temperatures may eventually be unable to survive due to a lack of oxygen.
There were reductions in certain surface waterways, but not all. Despite the warmer temperatures, surface water oxygen levels increased in nearly a fifth of the lakes. However, this was not necessarily good news. “What we believe is happening is that the warmer temperatures favoured algae development, particularly cyanobacterial growth,” according to Rose, which can lead to toxic algal blooms that endanger people and wildlife. These poisonous blooms have the potential to raise surface oxygen levels.
In an email to Popular Science, Sapna Sharma, an associate professor of biology at the University of York in Canada, said, “I think this is a very important paper.” “They showed how oxygen levels have been falling over time in many temperate lakes throughout the water column,” Sharma said, pointing out that deep waters are critical for freshwater fish and invertebrates.
“It doesn’t mean that, you know, we’re going to have fishless lakes everywhere in a few years,” Rose adds, even though this isn’t an encouraging trend. Because no two lakes are alike, more data from more lakes around the world is required. There’s also a lot that it can do to slow the loss of oxygen, from reducing global emissions to bettering local lake management.
“To observe these losses in oxygen in our aquatic ecosystems is a concerning aspect for the future of biodiversity and chemical reactions in these systems, as well as overall trends toward potentially poor water quality in the future,” Rose adds.