Oceans and freshwater bodies are losing oxygen at an alarming rate, and several recent studies warn that this phenomenon is pushing the planet toward a critical state from which it may be difficult to recover. The scientific community has been warning about aquatic deoxygenation for years, but now two independent studies have focused on its global consequences and the need to treat it as a major planetary threat, on par with climate change or biodiversity loss.
A review led by the Scripps Institution of Oceanography in the United States proposes incorporating dissolved oxygen levels into the Planetary Boundaries framework, a system that, since 2009, has identified the critical processes for maintaining Earth's stability. According to its authors, the rapid decline of oxygen in seas, rivers, and lakes is interacting with other boundaries, such as acidification and biogeochemical cycles , generating cascading effects that could be irreversible on human timescales. The study, published in the journal Limnology and Oceanography, emphasizes that deoxygenation does not act in isolation: global warming, marine pollution , and changes in ocean ventilation are its main drivers.
A planet on the verge of collapse due to lack of oxygen
The Scripps research, involving scientists from several universities, analyzes how oxygen loss alters the biological and chemical processes that regulate the climate. From microscopic organisms to large marine predators, all depend on dissolved oxygen , and its decline threatens aquatic life and, by extension, the health of the planet. The authors point out that deoxygenation is already affecting coastal ecosystems, rivers, and lakes, and that its impacts are compounded by other planetary boundaries, such as biodiversity loss and altered nitrogen and phosphorus flows.
Erica Ferrer, lead author of the study, explained that the idea arose after attending COP25 in Madrid, and that they hope this review will help policymakers consider deoxygenation as a key factor in the stability of the Earth system . The proposal to add dissolved oxygen to the Planetary Boundaries framework aims to raise awareness and encourage mitigation measures that help maintain biodiversity and climate.
The Quitralco Fjord: a natural laboratory of anoxia
Meanwhile, in southern Chile, an interdisciplinary team of scientists has documented an extreme case of oxygen depletion in the Quitralco Fjord, in the Aysén Region. This is the first evidence of a euxinic fjord in the area, meaning one with a total absence of dissolved oxygen and high concentrations of hydrogen sulfide , a compound toxic to most organisms. The study, led by the I~mar center at the University of Los Lagos, determined that this condition is due to natural factors: limited water circulation, long residence times, and the influence of volcanic activity from the nearby Liquiñe-Ofqui Fault and the Mate Grande and Hudson volcanoes.
The researchers, who worked between 2022 and 2025, found that the anoxic layer begins between 90 and 120 meters deep and extends to the seabed at about 160 meters. Surface waters maintain normal oxygen levels thanks to exchange with the atmosphere , but at depth, the water chemistry changes completely: nitrate disappears, ammonium, phosphate, methane, and hydrogen sulfide increase, and the microbial community becomes dominated by anaerobic bacteria. Although there are salmon farms in the fjord, the authors conclude that the euxinic conditions are primarily natural, making Quitralco a unique laboratory for studying how volcanic activity and restricted circulation can transform a marine ecosystem.
Lessons from the past: The Great Dying
To better understand what might happen if oxygen loss accelerates, scientists are looking to the past. Some 252 million years ago, during the Permian-Triassic mass extinction, known as the Great Dying, 96% of marine species and 70% of terrestrial species disappeared . A Stanford University study, published in the Proceedings of the National Academy of Sciences, has shown that the main trigger was a combination of extreme heat waves and a drop in ocean oxygen levels, caused by massive volcanic eruptions.
Researchers conducted experiments with living organisms to compare the vulnerability of different groups. They discovered that animals with slow metabolisms and limited mobility, such as brachiopods, were much more sensitive to rising temperatures and oxygen depletion , while mollusks, fish, and echinoderms, with their greater muscular and gill capacity, managed to survive and thrive. This biological shift was irreversible: before the extinction, brachiopods dominated the seas; today, bivalves outnumber them. The study warns that, although current warming projections are lower than those of the Permian-Triassic period, the trend is worrisome, and we still have time to act.
The combination of these three fronts—the proposal to include oxygen in planetary boundaries, the natural case of Quitralco, and the lesson of the Great Dying—paints a picture in which aquatic deoxygenation is emerging as one of the major environmental challenges of the 21st century . Scientists insist that mitigating its causes, primarily global warming and nutrient pollution, is fundamental to preventing the planet from crossing a threshold from which it will be difficult to recover. The health of the oceans and freshwater is no small matter: the stability of the entire Earth system depends on it.