Aquatic Deoxygenation Could Be Earth’s Next Planetary Boundary

Oxygen dissolved in lakes, rivers and seas worldwide is declining at speed, and researchers warn that this may rank among the most serious threats to Earth’s life-support system.

Much as oxygen in the atmosphere is indispensable to animals including humans, dissolved oxygen (DO) is fundamental to flourishing aquatic ecosystems in both fresh and salt water. Billions of people depend on marine and freshwater environments for food and livelihoods, making the marked and rapid fall in their oxygen levels particularly alarming.

Aquatic deoxygenation and planetary boundaries

A group of scientists has proposed adding aquatic deoxygenation to the ‘planetary boundaries’ framework. In its current version, the framework identifies nine domains with limits "within which humanity can continue to develop and thrive for generations to come."

At present, the planetary boundaries comprise climate change, ocean acidification, stratospheric ozone depletion, disruption of the global phosphorus and nitrogen cycles, the rate 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, argues that this framework leaves out one of the planet’s most significant constraints.

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

"Relevant, critical oxygen thresholds are being approached at rates comparable to other planetary boundary processes."

Why dissolved oxygen is falling in water

Dissolved oxygen concentrations can fall for several causes. Warmer water, for example, holds less dissolved oxygen. As greenhouse gas emissions continue to push air and water temperatures above their long-term averages, surface waters are increasingly unable to retain this essential element.

Aquatic organisms may also consume dissolved oxygen more rapidly than the ecosystem’s producers can restore it. Blooms of algae and surges in bacteria, caused by incoming organic matter and nutrients from agricultural and household fertilisers, sewage and industrial waste, rapidly use up the available dissolved oxygen.

In the most severe situations, oxygen levels decline so far that microbes suffocate and die, often bringing larger species down with them. Microbial populations that do not require oxygen then consume the abundance of dead organic matter. Their growth can become dense enough to reduce light and restrict photosynthesis, locking the whole body of water into a destructive, suffocating process known as eutrophication.

Aquatic deoxygenation is further fuelled when the density contrast between layers of the water column grows. Surface water warming more quickly than deeper water, along with melting ice lowering ocean-surface salinity, can create this stronger separation.

The more clearly these layers are divided, the less water moves between them, despite underwater life across the vertical strata depending on that exchange. Changes in density drive oxygen-rich surface water downwards; without this temperature-driven transport, ventilation in deeper aquatic environments comes to a standstill.

Global monitoring and action

These changes have disrupted aquatic ecosystems, many of which people themselves depend on for food, water, livelihoods and wellbeing.

The paper’s authors urge a coordinated worldwide programme of monitoring and research into deoxygenation across the planet’s ‘blue’ environments. They also call for policies that can prevent rapid oxygen loss and address the related difficulties that are already beginning to emerge.

"Reducing greenhouse gas emissions, nutrient runoff and organic carbon inputs (for example, raw sewage loading) would slow or potentially reverse deoxygenation," they write.

"The expansion of the planetary boundaries framework to include deoxygenation as a boundary [will help] to focus those efforts."

The paper appeared as a Perspective in Nature Ecology & Evolution.