Ocean Dead Zones: The silent killers of marine life are expanding and have increased exponentially since the 1960s
Dive into the Gulf of Mexico between May and September, chances are that you might find yourself in a dead zone. Appearing cloudy and desolate, the dead zone breaks the harmony of the oceans and threatens marine life.
You might not see fish, as most would have swam away. In certain instances, you can find dead animals floating up to the surface, Robert J. Diaz, Virginia Institute of Marine Science told MEA Worldwide (MEAWW).
The dead zone result from the loss of oxygen. There are now 700 of these sites around the world, including the Gulf of Mexico -- the second largest dead zone in the world. Besides Gulf of Mexico, Chesapeake Bay and Hood Canal are two other well-known dead zones in the US. "We have many large and small tributaries: about 47% (307 of 647) are stressed by the loss of oxygen," says Dr Diaz.
And these zones are expanding due to human-induced activities. In the 1960's, the number of dead zones were only 45.
"As a "dead zone" grows larger, more mobile animals -- fish, crabs, lobsters, and the like -- migrate out of the affected area. Immobile animals like corals undergo behavioral and sometimes physiological changes to try and cope with the lack of oxygen, but eventually experience death," Dr. Kersey Sturdivant from Duke University, told MEAWW.
This does not mean that the loss of oxygen will affect all marine life, according to a recent report from The International Union for Conservation of Nature (IUCN). Instead, it will end up breaking the normal balance of life, favoring organisms such as microbes, jellyfish and some squid that thrive in low-oxygen conditions, at the expense of many marine species, including most fish that prefer normal oxygen levels.
Of them, tuna, marlin and sharks are particularly sensitive to low oxygen levels because of their large size and energy demands. These species are starting to be driven into increasingly shallow surface layers of oxygen-rich water, making them more vulnerable to over-fishing, says the report.
These effects extend to humans too, but it is not well-established. Scientists are trying to study the impact of low-oxygen in the oceans on human health, social, and economic consequences.
"But there is a growing body of literature pointing to increasingly more dire consequences for humans, primarily though negative impacts on fisheries and aquaculture," says Dr. Diaz.
The oceans feed more than 500 million people, especially in poorer nations, and provide jobs for 350 million people, reveals an analysis. For instance, low-oxygen levels slow or arrest the growth of shrimps. This makes it harder to find bigger shrimp, inflating the price of large shrimp for consumers.
Why are dead zones on the rise?
The major drivers of ocean oxygen loss are climate change and nutrient pollution, with the latter affecting coastal areas, says the IUCN report.
Coastal regions like the Gulf of Mexico have the Mississippi River watershed to blame. They carry excess nutrients -- phosphorus and nitrogen -- from fertilizer, sewage, animal and aquaculture waste. The River carries these pollutants as it drains into the Gulf, which, in turn, spurs the overgrowth of algae, in what is called Eutrophication. When they die, the algae sinks into the ocean, where it decomposes. The decomposition process consumes oxygen, cutting off the supply to the healthy marine life, according to the National Oceanic and Atmospheric Administration (NOAA).
Burning fossil fuels has also driven the loss of oxygen: warmer waters hold less oxygen. “Major extinction events in Earth’s history have been associated with warm climates and oxygen-deficient oceans.” Denise Breitburg, at the Smithsonian Environmental Research Center in the US told The Guardian.
The future trajectory
"If we continue to expand our population, farm the land and sea, and manage our living resources as usual, our coastal areas will continue to suffer and so will humanity," Dr Diaz told MEAWW.
By 2100, the ocean is expected to lose 3–4% of its oxygen inventory globally under a business-as-usual scenario, according to the IUCN report. But some regions are expected to be hit more severely, especially regions in the mid to high latitudes. Most of the losses are predicted to be concentrated in the upper 1000 m of the water column, which is richest in marine biodiversity.
The road to recovery
Over 900 areas around the world have been identified as experiencing the effects of eutrophication. Of these, over 700 are dead zones, says Dr Diaz. But about 70 are recovering, he adds, through proper management of nutrient and organic loading.
The same approach can be applied to other regions as well. "The necessary action required to address the issue is simple — reduce nutrient loading," Dr. Sturdivant told MEAWW. "When nutrient loading is reduced, water quality increases, chronic oxygen-levels diminish, and many of the associated problems that arise with them are mitigated."
Though the approach looks simple, the implementation of nutrient-reduction policies is not going to be easy. "This is especially true in a system where the watershed is impacted by multiple, separately governed states, and/or where a large component of the nutrient (nitrogen) input to the system comes from atmospheric deposition," Dr. Sturdivant explains.
The importance of maintaining adequate levels of oxygen in our coastal systems is best summarized by the motto of the American Lung Association: “if you can’t breathe, nothing else matters,” says Dr Diaz.