Dead Zones
The Birth of Death:
How do dead zones form?
Dead zones form naturally – they have for years. But they also form because of human activity. This is worrisome.
Natural dead zones
The science: A dead zone forms naturally because of coastal upwelling when currents bring cooler, salty, and dense waters to the surface. These waters are rich with nutrients, so plankton thrives and grows profusely. Eventually, these tiny plants die and sink to the bottom. Bacteria eat away at them. It's these bacteria that use up the oxygen in the water, creating a low-oxygen or "dead" zone. So areas with coastal upwelling, such as in Mexico, California, Peru, Chile and western Africa, tend to have naturally occurring low-oxygen waters.
B.C. dead zone: Like these areas, the growing dead zone in Saanich Inlet was born naturally. It was discovered in 1960. Over the years, the mid-depth waters of the inlet have become increasingly hypoxic, shrinking the habitat for many marine animals. In short, this dead zone is growing.
Human-induced dead zones
Changing climate, changing currents
Climate change is altering global ocean circulation patterns. The winds and currents that usually oxygenate waters are changing, sometimes preventing this ocean mixing from happening or introducing influxes of nutrients. So now we're seeing dead zones in areas that we usually don't.
One example close to home is the coast of Oregon. Scientists have recorded a dead zone every summer since 2002 in an unusual place – in shallow waters, close to shore. The culprit is a change in the California current that now delivers nutrient-rich and oxygen poor waters to Oregon's inner coast.
Fertilizing oceans to death
Nutrient pollution by humans is another culprit. It often originates from agricultural fertilizer but also from industry and human waste, that cause the tiny plants in the ocean to grow like crazy (just as these chemicals do to plants on land). These plants grow, then die and fall to the bottom of the ocean. This becomes food for bacteria, which feast on the dead plankton, multiply, and in doing so, use up all the oxygen in this part of the ocean. If fresh water, with its ready supply of oxygen, does not rush in and mix with this water, then a vast area with low oxygen develops – the dead zone.
The Gulf of Mexico has the largest dead zone in the world. It stretches for over 22,000 square kilometres. That's 3,000 square kilometres larger than Lake Ontario. The main culprit is 1.6 million metric tons of nutrient run-off from agricultural operations each year. The run-off flows into the Mississippi River and eventually empties out into the Gulf of Mexico. The result? The loss of more than 212,000 metric tons of fish and other animals that feed marine animals and humans – or the equivalent weight of more than two million refrigerators.
Mystery swirls with big questions
Two questions to throw at scientists: Can we stop dead zones by altering human behaviour? Can we heal the dead zones that already exist? Find out their answers.
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