There are areas in the sea where oxygen is so severely depleted that most forms of life can’t survive.

These ocean “dead zones” are growing and scientists warn that they will continue to increase unless we are able to slow the advance of climate change, which is fuelling this alarming shift in ocean chemistry.

Even outside these almost lifeless ocean regions, rising global temperatures and the influx of nutrient pollution are throttling
oxygen levels in the open ocean and in coastal areas, threatening communities of sea life around the world.

South Africa has three such areas – near St Helena Bay in the Western Cape, near Port Elizabeth in the Eastern Cape and near Kosi Bay in KwaZulu-Natal.

This sobering view of the “suffocating” ocean was described in a study published online on Thursday in the journal Science.

The study is the first to present such a comprehensive evaluation of ocean oxygen depletion and its causes.

Less oxygen in the ocean doesn’t just spell trouble for marine plants and animals, it could have serious repercussions on life on land.

While water molecules contain oxygen atoms, water must also contain dissolved oxygen for fish and other organisms to breathe.

Oxygen-deprived dead zones were first identified in estuaries in the mid-19th century and their oxygen depletion was linked to the presence of urban sewage in the water, said the study’s lead author, Denise Breitburg, a marine ecologist with the Smithsonian Environmental Research Centre.

Since then, growth of industrial and agricultural activity has disrupted the ocean’s chemical balance, with regions in many areas worldwide becoming infused with pollutants and nutrients that starve the water of oxygen.

Meanwhile, rising global temperatures hamper oxygen’s solubility in water and restrict its distribution into the deeper ocean.

In some cases, evidence shows that the damage can be reversed, and areas where nutrients and sewage have sucked oxygen from the water may recover if the flow of contaminants is removed, Breitburg said.

For example, parts of the Thames Estuary in London and the Delaware River Estuary – both of which endured long periods with no oxygen and no fish – “are now much improved and host vibrant fish communities”, Breitburg said.

However, when it comes to oxygen loss in the open ocean, “we’re in uncharted territory. We don’t know how long it will take for the ocean to respond.” – Live Science