Oxygen levels in the world’s lakes are declining because of climate change, according to new research published last week in the journal Nature. Global heating is increasing water temperatures, which reduces the amount of oxygen water can hold. Less oxygen may cause problems for fish and other freshwater wildlife, reports Damian Carrington for the Guardian.
The study analyzed changes in water temperature, clarity and oxygen content for 393 lakes located in temperate climates in North America, South America, Asia and Europe over time. For some lakes, the data stretched all the way back to 1941, but the majority of the records began in the 1980s, reports Kirsti Marohn for Minnesota Public Radio (MPR).
Since the 1980s, the average oxygen content of these lakes declined 5.5 percent near the surface and fell by 18.6 percent in deep waters, according to the study.
"All complex life depends on oxygen. It's the support system for aquatic food webs. And when you start losing oxygen, you have the potential to lose species," says Kevin Rose, a freshwater ecologist at Rensselaer Polytechnic Institute and the study’s senior author, in a statement. "Lakes are losing oxygen 2.75 to 9.3 times faster than the oceans, a decline that will have impacts throughout the ecosystem."
The steeper decline in oxygen at depth is an indirect consequence of the hotter, longer summers caused by climate change in many parts of the world. A longer, hotter summer creates a bigger temperature discrepancy between the water being heated up at the surface and the deeper, cooler waters. The larger the temperature difference between two layers of water the less they are inclined to mix, which results in an increase of what researchers call “stratification.”
"The increase in stratification makes the mixing or renewal of oxygen from the atmosphere to deep waters more difficult and less frequent, and deep-water dissolved oxygen drops as a result," says Rose in the statement.
Rose tells MPR that when spring comes earlier, as research has shown is happening more often under climate change, that stratification can become established earlier in the season and last longer, which cuts off deep water from oxygenated waters at the surface for a longer period of time.
In 87 of the lakes studied, surface water oxygen levels actually appeared to be increasing despite their rising temperatures. Rose says most of these lakes were near heavily developed agricultural lands and likely see elevated pollution from nutrients such as the nitrogen and phosphorus commonly found in fertilizers and manure.
"The fact that we're seeing increasing dissolved oxygen in those types of lakes is potentially an indicator of widespread increases in algal blooms, some of which produce toxins and are harmful. Absent taxonomic data, however, we can't say that definitively, but nothing else we're aware of can explain this pattern," says Rose in the statement.
Hans-Otto Poertner, an ecologist studying the effects of climate change at the Alfred-Wegener Institute who was not involved in the paper, tells the Guardian that this “new study provides a much-needed global overview of what happens in the limited freshwater stores of the planet” as a result of climate change, adding that lakes are small, isolated systems compared to the oceans and are more sensitive to changes as a result.
“Climate change, together with [agricultural pollution], threatens vulnerable freshwater systems, adding to the urgency to strongly cut emissions,” Poertner tells the Guardian.