Quantcast
Environmental News for a Healthier Planet and Life

Dead Zones Are a Global Water Pollution Challenge — But With Sustained Effort They Can Come Back to Life

Climate
Dead Zones Are a Global Water Pollution Challenge — But With Sustained Effort They Can Come Back to Life
A massive dead zone forms in the Gulf of Mexico every year, fed by farm runoff that washes down the Mississippi River. EPA

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.

A group of climate activists that have been cycling from the North of the country in stages to draw attention to the climate case are arriving to the Court of Justice on the day that the climate lawsuit against Shell starts in The Hague, on December 1st, 2020. Romy Arroyo Fernandez / NurPhoto / Getty Images

By Julia Conley

Representing more than 17,000 claimants who support climate action, the international organization Friends of the Earth on Tuesday opened its case against fossil fuel giant Shell at The Hague by demanding that a judge order the corporation to significantly reduce its carbon emissions in the next decade.

Read More Show Less

EcoWatch Daily Newsletter

Eat Just, Inc. announced that its cultured chicken has been approved for sale in Singapore as an ingredient in chicken bites. The company has developed other cultured chicken formats as well. Eat Just

As concern mounts over the environmental impacts of animal agriculture, Singapore has issued the world's first regulatory approval for lab-grown meat.

Read More Show Less

Trending

Wildfires are seen burning out of control on November 30, 2020 on Fraser Island, Australia. Queensland Fire and Emergency Services / Getty Images

The world's largest sand island has been on fire for the past six weeks due to a campfire, and Australia's firefighters have yet to prevent flames from destroying the fragile ecosystem.

Read More Show Less
A plane sprays pesticide over the Wynwood neighborhood in the hope of controlling and reducing the number of mosquitos, some of which may be capable of spreading the Zika virus on Aug. 6, 2016 in Miami, Florida. Joe Raedle / Getty Images

By Jessica Corbett

A national nonprofit revealed Tuesday that testing commissioned by the group as well as separate analysis conducted by Massachusetts officials show samples of an aerially sprayed pesticide used by the commonwealth and at least 25 other states to control mosquito-borne illnesses contain toxic substances that critics call "forever chemicals."

Read More Show Less
Prime Minister Jacinda Ardern plants a tree as part of Trees That Count, a project to help New Zealand make a positive impact on climate change, on June 30, 2019 in Wellington, New Zealand. Hagen Hopkins / Getty Images

The government of New Zealand declared a climate emergency on Wednesday, a symbolic step recognizing the Intergovernmental Panel on Climate Change (IPCC) predictions of substantial global warming if emissions do not fall.

Read More Show Less