Dissolved oxygen reserves in water bodies around the world are rapidly depleting, and scientists say this poses one of the biggest risks to Earth’s life-support system.

Just as atmospheric oxygen is essential for animals, dissolved oxygen (DO) in water is essential for healthy aquatic ecosystems, both freshwater and marine. With billions of people dependent on marine and freshwater habitats for food and income, oxygen in these ecosystems is being severely and rapidly depleted.

A group of scientists is proposing to add water deoxygenation to the list of “planetary boundaries,” which in their latest form define nine areas that set thresholds “beyond which humanity can continue to thrive and prosper for future generations.”

The planet’s current frontiers are climate change, ocean acidification, stratospheric ozone depletion, interference with global phosphorus and nitrogen cycles, rates of biodiversity loss, global freshwater use, land system change, aerosol loading and chemical pollution.

The team, led by freshwater ecologist Kevin Rose of Rensselaer Polytechnic Institute in the US, fears that this list misses one of Earth’s most important boundaries.

“The deoxygenation observed in Earth’s freshwater and marine ecosystems represents an additional planetary boundary process, one that is critical to the integrity of Earth’s ecological and social systems and also regulates and responds to ongoing changes in other planetary boundary processes,” the authors write.

“The relevant critical oxygen thresholds are approaching at rates comparable to other planetary boundary processes.”

The concentration of dissolved oxygen in water decreases for a variety of reasons. For example, warmer waters cannot hold as much dissolved oxygen, and as greenhouse gas emissions continue to raise air and water temperatures above long-term averages, surface waters are becoming less able to hold this vital element.

Dissolved oxygen can be depleted by aquatic organisms faster than it can be replenished by ecosystem producers. Algal blooms and bacterial blooms triggered by the influx of organic matter and nutrients in the form of agricultural and municipal fertilizers, sewage, and industrial wastes rapidly absorb available dissolved oxygen.

In the worst cases, oxygen becomes so low that microbes suffocate and die, often taking larger species with them. Oxygen-independent microbe populations feed on dead organic matter and grow to densities that reduce light and limit photosynthesis, drawing the entire water body into a vicious, suffocating cycle called eutrophication.

Deoxygenation of water also results from an increase in the density difference between layers of the water column, which can be explained by the fact that surface waters warm more rapidly than deep waters and that melting ice reduces the surface salinity of the ocean.

The more clearly defined these layers are, the less movement occurs between these layers of the water column on which the vertical layers of aquatic life depend. These density fluctuations contribute to the movement of oxygenated surface waters towards depth, and without this temperature load, ventilation at shallower depths of the aquatic environment would cease.

All of this is damaging aquatic ecosystems, which are the source of food, water, income and well-being for many of our species.

The report’s authors call for a concerted global effort to monitor and investigate the deoxygenation of the “blue” parts of our planet, as well as policy efforts to prevent the rapid deoxygenation and related challenges we are already beginning to face.

“Reducing greenhouse gas emissions, nutrient runoff, and organic carbon inputs (such as untreated sewage) will slow or potentially stop deoxygenation,” they write.

“Expansion of the Planetary Boundary Framework to Include Deoxygenation as a Boundary” [допоможе] Focus on those efforts.”

This article was published in the journal Nature Ecology and Evolution.