While much of the world's attention has been on record-breaking heat this summer, a quieter and less visible crisis has been building beneath the surface.

A new review published in Limnology and Oceanography, led by scientists at the University of California San Diego's Scripps Institution of Oceanography, warns that oxygen is disappearing from oceans, lakes, rivers and coastal waters at an alarming rate.

The researchers argue that oxygen loss in our waters is now serious enough to rank alongside better-known threats such as climate change, biodiversity loss and other major environmental challenges. They also say that it's closely linked to almost all of them.

The causes are familiar. Warmer water holds less oxygen, while rising temperatures also reduce water circulation, preventing oxygen from reaching deeper layers. Pollution from agricultural fertilisers, industrial discharge and other sources feeds rapid growths of algae. As the algae die and decompose, oxygen is rapidly used up.

The knock-on effects, however, are wide-ranging. Oxygen loss disrupts food chains, weakens the ocean's ability to regulate the climate and puts many species of all sizes at risk.

The authors conclude that aquatic deoxygenation threatens not only wildlife in our waters, but human communities and the Earth's long-term stability.