View from a freshwater lake. Dissolved oxygen is declining in temperate lakes

Oxygen declining in the world’s lakes

The world’s lakes and oceans are filled with dissolved oxygen (O2), which aquatic species rely on to breathe. But research published in Nature has found that oxygen levels in temperate lakes are declining rapidly.

“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,” say Kevin Rose, professor at Rensselaer Polytechnic Institute, US and author on the study. “Lakes are losing oxygen 2.75-9.3 times faster than the oceans, a decline that will have impacts throughout the ecosystem.”

The team of US researchers examined over 45,000 dissolved oxygen and temperature profiles from 393 freshwater lakes in temperate areas around the world, with data spanning from 1941 to 2017. The data came from a range of different government, university and not-for-profit sources. They found that since 1980, surface waters had recorded a 5.5% overall decrease in dissolved oxygen, while deep waters saw an 18.6% overall decrease.

The reason for the deoxygenation is straightforward in surface waters: when water heats up, it can’t hold as much O2. Surface water temperatures increased by 0.38°C per decade.

“Oxygen saturation, or the amount of oxygen that water can hold, goes down as temperatures go up. That’s a known physical relationship and it explains most of the trend in surface oxygen that we see,” says Rose.

Deep waters, however, have mostly remained stable in temperature over the past four decades. The authors theorise that the drop in deep water oxygen is due to ‘stratification’ – increased surface temperatures are creating a large difference in density between shallow and deep waters, so they don’t mix and  less oxygen is able to reach deep water.

“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.

The loss in oxygen was not the same around the globe – in some temperate lakes, oxygen levels increased even when temperatures increased. This could be due to nutrient pollution (from things like fertiliser runoff), causing blooms of oxygen-producing cyanobacteria.

“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.

The authors say this is another indicator of the effect of climate change on the world’s ecosystems.

“Lakes are indicators or ‘sentinels’ of environmental change and potential threats to the environment because they respond to signals from the surrounding landscape and atmosphere,” says Stephen Jane, lead author on the study.

“We found that these disproportionally more biodiverse systems are changing rapidly, indicating the extent to which ongoing atmospheric changes have already impacted ecosystems.”

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