Deoxygenation in the Global Ocean and Coastal Waters

By now we know of the serious issues caused by plastics in the world’s oceans, but there is another problem facing the planet’s oceans: deoxygenation.

By now we’re all aware of the serious issues that plastic causes in the world’s oceans. But another, growing problem faces the planet’s oceans according to new research: deoxygenation. 

What is deoxygenation?

What is deoxygenation? Quite simply, it is the reduction in the level of oxygen present in the oceans. This is serious, as life depends on oxygen in the water. Deoxygenation also affects major nutrients and carbon in the oceans.

There are already several known dead zones — where oxygen levels are at zero — and low-oxygen areas in the oceans. But new research published in Science by Denise Breitburg et al at the Smithsonian Environmental Research Center casts new light on this issue with some alarming findings.

The authors found that oxygen levels are falling in increasingly more areas of open ocean and coastal waters. Over a million square miles (several million square kilometers) of open ocean are now affected, as well as hundreds of coastal sites.  This growth has accelerated since 1950.

What is causing it?

What is spurring this increase? The research team behind the research points to two main factors: increasing nutrient levels in the water and climate change.

Although your first thought might be that more nutrients are good for ocean life, unfortunately, it’s not that simple. Too many nutrients can have extremely adverse effects on the water. In coastal areas, human action is to blame for the increase in nutrients, be it from agricultural fertilizer run-off or sewage. Both of these issues are increasing as populations grow, and we’ve seen many rivers suffer from this in the past.

How do these nutrients — mainly nitrogen and phosphorus — affect the oxygen levels in the oceans? They don’t change the ocean chemistry alone; rather, their increased levels lead to an increase of life taking advantage of them. In most cases, this results in an algal bloom. This usually involves tiny marine phytoplankton, which feed on the nutrients. Like all animals in times of abundance, they consequently reproduce much more rapidly, sucking the oxygen out of the water as part of this explosion in numbers.

Where nutrient run-off affects mainly coastal areas, climate change affects the wider ocean. Not only do increased water temperatures help make conditions better for algal blooms, but they also change the chemistry of the water itself. The amount of oxygen or gases, like nitrogen and carbon dioxide, in the water is controlled by the temperature of the water itself. The warmer the water gets the fewer the gases it can hold. Temperature affects the amount of gas a liquid can hold (called solubility), and the warmer the water the the less soluble it is. This means warmer water holds less gas.

Deoxygenation is largely a manmade problem, which means that we can reverse it with global action before it leads to more catastrophic consequences for our oceans and, by extension, humankind.