World’s rivers, streams, lakes left gasping for oxygen; humans to blame

Rivers, streams, lakes and reservoirs are the world’s vital engines for water supply, but human activities have been suffocating them during the last century, a new global study has found.

A group of researchers, led by earth scientists of The Netherland’s Utrecht University Junjie Wang and Jack Middelburg, found that these water sources, around which life thrives across the world, are themselves running low on oxygen. The culprit is not climate change alone, but human activity.

These inland waters “breathe” oxygen, just like we do. But, we’ve been suffocating them during the last century, an era also known as the Anthropocene. The research reveals that the way oxygen is produced and used in inland waters has dramatically changed since 1900.

Oxygen, the most critical resource for life on Earth, plays an important role in other nutrient cycles such as carbon and nitrogen. Oxygen depletion in water, called hypoxia, is causing problems. They are piling up in various coastal and freshwater systems. The result? Dying fish, disrupted food webs, poor water quality and more which are already affecting freshwater ecosystems across the globe. The study shows it’s not just a local problem—it’s a planetary one.

The researchers have developed for the first time a global model that describes the entire oxygen cycle of inland waters around the world. “With this model, we offer the most complete possible understanding of this cycle on a large scale, so that one can see oxygen-related problems coming, get to know the causes, and hopefully intervene in time,” Middelburg said.

Inland waters have become much busier places when it comes to oxygen. The team found that the global “oxygen turnover”—that is how much oxygen is produced and consumed—has increased. But here’s the twist: these waters are consuming more oxygen than they produce, making them a growing sink of atmospheric oxygen, said an official statement.

“More farming, more wastewater, more dams, and a warmer climate—they all change how our freshwater ecosystems function,” says Wang. With more nutrients flowing into rivers, lakes and reservoirs, algae grow faster, but when they die and decompose, they use up huge amounts of oxygen.

“We found that the main causes lay in these direct human activities. First, it turns out that nutrient input through, for example, over-fertilization, is a major driver of this acceleration. Secondly, the longer travel time of freshwater to the sea through the construction of dams and reservoirs has proven to be just as important,” says Middelburg.

At the same time, indirect human impacts like rising temperatures make oxygen less soluble in water, transport slower vertically across the water column, and speed up processes that burn through it even faster. “Until now, the consensus in the scientific literature has always been that the rise in temperature is primarily causing this acceleration. But our model shows that warming only contributes about 10–20 per cent to this phenomenon,” Wang says.

The study showed that the modern oxygen cycle in inland waters looks nothing as it did in the early 1900s. “Even though these waters cover just a tiny fraction of Earth’s surface, they now remove nearly 1 billion tonnes of oxygen from the atmosphere each year—overall half of what the entire ocean emits back,” said Middelburg.

“We can’t ignore inland waters in global climate and oxygen budgets anymore. They’re changing faster than we thought, and they’re crucial pieces of the Earth system puzzle”, said Wang.