Global oxygen drain: Inland waters now consume more oxygen than they produce, researchers raise red flag

Rivers, lakes, and reservoirs have always been Earth's hidden lungs — breathing life through the oxygen they cycle. But over the last century, that breath has grown shallow. A new study led by researchers at Utrecht University and published in Science Advances reveals a startling transformation: the oxygen cycle in inland waters has been dramatically altered since 1900, choked by human influence during what's now known as the Anthropocene.

Oxygen is central to life — not just for breathing, but for powering the vital cycles of carbon and nitrogen. Yet, oxygen levels in inland waters are dropping fast. This condition, known as hypoxia, is now a global crisis. It’s already manifesting in dead fish, collapsing food chains, and plummeting water quality.

Utrecht earth scientists Junjie Wang and Jack Middelburg have built the first-ever global model tracking oxygen's full journey through rivers, lakes, and reservoirs. “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 told phys.org.

Their findings are clear: inland waters now consume more oxygen than they produce. As a result, they’ve become massive sinks of atmospheric oxygen — an unexpected twist in Earth's oxygen balance.

“More farming, more wastewater, more dams, and a warmer climate — they all change how our freshwater ecosystems function,” says Wang. With excess nutrients flowing in, algae blooms thrive — only to die off and suck up oxygen as they decay.

According to Middelburg, two main human activities are accelerating this crisis: nutrient runoff from over-fertilization and the slowing of freshwater flows caused by dams and reservoirs. These changes have quietly but profoundly altered the way oxygen moves and vanishes in freshwater.

Rising temperatures, while a factor, are less to blame than once thought. “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% to this phenomenon,” Wang explains.

The scale is staggering. Though they cover only a small slice of Earth’s surface, inland waters now pull nearly 1 billion tonnes of oxygen from the atmosphere every year — half as much as the entire ocean emits back.

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