|Much of Earth’s oxygen, 70% of it, comes from the world’s oceans. It is produced by marine plants, particularly phytoplankton, kelp, and algal plankton / Photo by: Don Pablo via Shutterstock|
Much of Earth’s oxygen, 70% of it, comes from the world’s oceans. It is produced by marine plants, particularly phytoplankton, kelp, and algal plankton. Because of that, it may seem ironic that some parts of the ocean have low oxygen levels. These are called “dead zones,” where only a few organisms can manage to survive. However, these areas are not entirely dead; bacteria thrive in them.
According to ThoughtCo., a premier reference site with a 20+ year focus on expert-created education content, dead zones can be found in rivers, oceans, lakes, ponds, and even aquaria. While these can form naturally, they have been an increasing environmental concern for the past few years due to destructive human activities. These places can kill marine life. Surviving fish can suffer from reproductive problems, with low egg counts and spawn rates.
Scientists have classified dead zones depending on how long the hypoxia lasts. The first one includes permanent dead zones, where oxygen concentrations rarely exceed two milligrams per liter. This is followed by temporary dead zones where hypoxia lasts for hours or days. Seasonal dead zones, on the other hand, occur every year during the warm months. Lastly, diel cycling hypoxia occurs during warm months.
A combination of physical, chemical, and biological factors can create dead zones. However, in recent years, nutrient pollution has become the primary cause of these zones. Previous studies have shown that excess nutrients that are piped as wastewater into rivers and coasts can stimulate the overgrowth of algae, which would then sink and decompose in the water.
The Baltic Sea is a perfect example of this as it has seven of the world’s 10 largest marine dead zones. The rapid progress of the eutrophication process in the sea is driven by the increased runoff from agricultural fertilizers and sewage. The problem has intensified due to the overfishing of Baltic cod. Unfortunately, the spreading dead zones are starting to reach the cods' deep-water breeding grounds, further endangering the species.
Status of the World’s Dead Zones
Since 1950, ocean dead zones with zero oxygen have quadrupled in size, while the number of sites with extremely low oxygen levels near coasts is now ten times greater. A 2018 study projected that the oceans will continue to lose oxygen at a rapid pace as the Earth’s climate continues to warm. If this rate continues, most sea creatures would be extinct since they can’t survive in such places. Worse, this can trigger the release of dangerous chemicals from the ocean like nitrous oxide, a greenhouse gas that’s 300 times more powerful than carbon dioxide.
"The decline in ocean oxygen ranks among the most serious effects of human activities on Earth's environment,” Denise Breitburg, a marine ecologist with the Smithsonian Environmental Research Center, said.
A 2014 study by researchers Andrew Altieri of the Smithsonian Tropical Research Institute in Panama and Keryn Gedan of the University of Maryland reported that the climate will drive the expansion of dead zones across the world. According to the Smithsonian, the official journal published by the Smithsonian Institution in Washington, D.C., 94% out of more than 400 dead zones examined will experience a warming of 3.6 degrees Fahrenheit or more by the end of the century.
However, problems with dead zones can be solved through local and global efforts. Denise Breitburg from the Smithsonian Environmental Research Center cited the recoveries of the Thames river in the UK and Chesapeake Bay in the US, where better farm and sewage practices led to dead zones disappearing. Unfortunately, initiatives that would address the increasing issues with dead zones are being less tackled.
“Right now, the increasing expansion of coastal dead zones and decline in open ocean oxygen are not priority problems for governments around the world. Unfortunately, it will take severe and persistent mortality of fisheries for the seriousness of low oxygen to be realised,” Prof. Robert Diaz at the Virginia Institute of Marine Science said.
|Since 1950, ocean dead zones with zero oxygen have quadrupled in size, while the number of sites with extremely low oxygen levels near coasts is now ten times greater / Photo by: vladimir3d via Shutterstock|
Expanding Ocean Dead Zones
The expansion of ocean dead zones across the world is continuously being monitored by scientists. However, dead zones could be expanding far more quickly than previously thought. According to the Independent, a British online publisher of news, the number of known hypoxic dead zones has skyrocketed from 45 to 700 sites. Several larger and more active sea creatures such as tuna, marlins, and sharks are unable to survive in these areas.
A recent study published in the Global Biogeochemical Cycles journal emphasized the need to consider bacteria in predicting the full impacts of climate change and human activity on the marine environment. According to the researchers, organic matter that sinks to the seafloor is caused by a major contribution from bacteria that live in the dark ocean and don't just come from the sea surface. “Existing models could be missing out on a key contribution as a result of which people have underestimated the extent of the oxygen depletion we are to expect in the future, warming world,” they said.
|The expansion of ocean dead zones across the world is continuously being monitored by scientists. However, dead zones could be expanding far more quickly than previously thought / Photo by: anmo via Shutterstock|
Dr. Grethel Aguilar, the International Union for the Conservation of Nature’s (IUCN) acting director-general, stated that world leaders must commit to immediate and substantial emission cuts to control the increasing carbon dioxide in the atmosphere. Thus, world leaders are attempting to establish new international rules for emissions trading and broker systems.
“Urgent global action to overcome and reverse the effects of ocean deoxygenation is needed,” IUCN global marine and polar programme director Minna Epps said.
Threats to the world’s oceans are piling up. We can expect more of this unfortunate news as climate change worsens, which is to be expected if countries don’t immediately address these issues.