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Dead Zones Are a Global Water Pollution Challenge — But With Sustained Effort They Can Come Back to Life
By Donald Scavia
Scientists have identified a dead zone as large as Florida in the Gulf of Oman, which connects the Arabian Sea to the Persian Gulf. Around the world there are more than 400 current dead zones in oceans and lakes, where water contains so little oxygen that aquatic life can't survive.
Dead zones form when aquatic organisms consume dissolved oxygen faster than it can be supplied. This typically happens when warmer water sits on top of colder water, or freshwater sits on top of saltier water—for example, where a river meets the sea. In either case the water on top is less dense and floats. The layers don't mix much, so very little oxygen from the atmosphere reaches the lower layers.
Blooms of algae, like this growth in 2015 in Lake St. Clair between Michigan and Ontario, promote the formation of dead zones.NASA Goddard Space Flight Center / Flickr / CC BY 2.0
The next ingredient is organic matter in the water. It can come from untreated sewage, or from blooms of algae, along with dead plankton and fish. This material eventually sinks into the bottom layer, where bacteria decompose it, using oxygen as fuel. This process can consume most or all of the oxygen from the water.
Temperature is also a factor. Higher temperatures promote faster algae growth, enhance formation of layers in the water, and reduce the amount of dissolved oxygen that the water can hold. Climate change is tending to increase temperatures and make dead zones worse.
But the biggest driver is nutrient pollution—excess inputs of nitrogen and phosphorus. These nutrients stimulate algae growth. They come from municipal and industrial wastewater treatment plants, and increasingly from fertilizer runoff from industrial-scale agriculture.
A recent global-scale analysis shows that oxygen-depleted zones in the open ocean have expanded by several million square kilometers since the mid-20th century, and oxygen concentrations at hundreds of coastal sites like the Gulf of Mexico are now low enough to limit the distribution and abundance of fish. These impacts are also being felt in estuaries and the Great Lakes.
As my research has shown, large-scale dead zones are resistant to change. But nutrient reductions in the Chesapeake Bay are starting to improve conditions there. Communities around Lake Erie dramatically reduced its dead zone and toxic algae blooms in the 1970s by reducing phosphorus inputs. Now, however, these issues are resurfacing there—evidence that this problem is an ongoing challenge.
Reposted with permission from our media associate The Conversation.
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Poverty and violence in Central America are major factors driving migration to the United States. But there's another force that's often overlooked: climate change.
Retired Lt. Cmdr. Oliver Leighton Barrett is with the Center for Climate and Security. He says that in Guatemala, Honduras and El Salvador, crime and poor economic conditions have long led to instability.
"And when you combine that with protracted drought," he says, "it's just a stressor that makes everything worse."
Barrett says that with crops failing, many people have fled their homes.
"These folks are leaving not because they're opportunists," he says, "but because they are in survival mode. You have people that are legitimate refugees."
So Barrett supports allocating foreign aid to programs that help people in drought-ridden areas adapt to climate change.
"There are nonprofits that are operating in those countries that have great ideas in terms of teaching farmers to use the land better, to harvest water better, to use different variety of crops that are more resilient to drought conditions," he says. "Those are the kinds of programs I think are needed."
So he says the best way to reduce the number of climate change migrants is to help people thrive in their home countries.
Reporting credit: Deborah Jian Lee / ChavoBart Digital Media.
Reposted with permission from Yale Climate Connections.
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