When it comes to the health of our oceans, we mainly think about what we can see happening at the surface. Bleak predictions of the immense scale of plastic pollution often dominate the headlines, but another catastrophe is brewing below those blue crests, a potentially serious consequence of global warming that has come to light only recently: deoxygenation. Our oceans are being suffocated. The deep reserves of oxygen are disappearing, and the consequences may reach far beyond the shorelines.
Oxygen-minimum zones, AKA “dead zones,” aren’t a new phenomenon. They've been around for hundreds of millions of years and occur throughout the world every summer until plunging water temperatures, and often hurricanes, mix oxygen from the surface layers back into the depths each fall. However, today, they’re expanding rapidly, and they aren't being entirely driven by natural processes. Since the mid-20th century, dead zones in the open ocean have quadrupled in size, and those along the coast have increased by a factor of ten. That means the open ocean dead zones have expanded in size by roughly 46 percent of the area of the US. The hundreds of dead zones throughout the world cover about 8 percent of the total oceanic area. Collectively, nearly 10 million tons of biomass either moves from or dies in dead zones every year.
Deoxygenation is worsening in the open ocean mainly as a result of the increasing global temperature. As a physical rule, warmer water holds less dissolved oxygen. As surface waters warm due to climate change, the ocean loses its ability to hold oxygen, leading to an oxygen decline. Additionally, because warm water is less dense, warmer surface waters result in a more stratified (layered) water column, which reduces the likelihood of oxygen-rich surface waters mixing with oxygen-starved deeper waters. In coastal margins, deoxygenation is being driven by increasing loads of nutrients from agriculture, sewage, and industrial waste. Nitrate and phosphorous-rich pollution trigger a bloom in phytoplankton populations. When they inevitably die off in vast numbers, they sink, and the bacteria that break them down consume enormous amounts of oxygen.
Although there are extremophiles in the oceans that can live perfectly happily without oxygen, it’s necessary for most life on Earth, and it affects the physical cycles of carbon, nitrogen, and other elements. Though a complete absence of oxygen, called anoxia, is rare in the modern ocean, ocean anoxic events (OAEs) have occurred irregularly throughout geologic history. They were often associated with warmer climate conditions, rises in sea levels, and occasionally with mass extinctions. Given the current state of rising global temperatures and sea levels, an understanding of the time scales and mechanisms of past OAEs is necessary to predict the future spread of existing dead zones.
The Cretaceous Period, the last age of dinosaurs, was the stage for a 50,000-year-long deoxygenation event, known as Ocean Anoxic Event 2 (OAE2), and resulted in a major extinction event. The spinosaurs, pliosaurs, and possibly even the ichthyosaurs all died out at the same time, along with 27 percent of all marine invertebrates. Rising temperatures and volcanic activity (due to continental shifting) enhanced nutrient delivery, priming the oceans for anoxic events. Primary producer populations, such as algae, exploded in response to the high nutrient availability, in turn multiplying the numbers of oxygen-consuming heterotrophic bacteria, and leading to a progressive expansion in anoxic portions of the ocean. There was not enough oxygen left in the ocean to support all the life within it, and most of that life could not escape. In the modern oceans, marine oxygen has decreased by 2 percent over roughly the last half century, and recent models predict a continued loss of 0.5 to 3.5 percent over the next half century. Without positive human intervention, studies of ancient OAEs are likely to become practical applications in the not-so-distant future.
Besides the dependence of marine life, oxygen plays a direct role in the biogeochemical cycling of carbon, nitrogen, and many other biogeochemically important elements, such as phosphorus, iron, manganese, etc. (Biogeochemistry explores the physical, chemical, biological, and geological processes and reactions that govern the composition of and changes to the natural environment; it is the study of how chemical elements flow through living systems and their physical environments.) Dissolved oxygen is the most commonly measured property of seawater that is sensitive to biological cycles (like the carbon cycle) and is therefore the first place to look for changes in ocean biogeochemistry in a warming world. Think of it as an indicator ‘species.’
Oxygen levels also control the oceanic production of nitrous oxide, a rarely spoken about but incredibly potent greenhouse gas. Although its ‘lifespan’ is shorter than carbon dioxide, which can spend centuries in the atmosphere, nitrous oxide traps heat nearly 300 times more effectively than its more famous cousin. That makes it an incredibly effective catalyst for climate change, and as oxygen levels go down, nitrous oxide goes up, potentially amplifying global warming.
A few prediction models have been run extending hundreds, even thousands, of years into the future, assuming no positive human intervention. Due to the slow turnover of deep waters, these models show continuing deoxygenation for 1000+ years, even after carbon dioxide levels stop rising. The declines in the total dissolved oxygen levels are as large as 30%, with substantial increases in the extent of hypoxia and suboxia. Hypoxic areas are regions where oxygen deficiency is detrimental to most organisms; oxygen concentrations are roughly 70-90% lower than the mean surface concentrations. Suboxic areas are regions where oxygen is so low that most life cannot be sustained and water chemistry is significantly altered; concentrations are 98% lower than the mean surface concentrations. A 2011 study found that 1°C of warming throughout the upper ocean will result in the increase of hypoxic areas by 10% and a tripling of the volume of suboxic waters. A highly optimistic emissions scenario of rising carbon dioxide levels would lead to 1.2°C of warming by 2100. Therefore, these declines in oxygen are changes we should be prepared to see.
If you want a glimpse of the near future, just take a look at the dead zone in our own Gulf. Although oxygen depletion occurs naturally in some parts of the ocean, such as fjords and deep basins, the Gulf of Mexico’s dead zone is caused by humans. Every spring, the Mississippi River funnels a deluge of nitrates, phosphorus, and other nutrients into the Gulf, fueling an explosion of phytoplankton growth. The bloom of life for the algae is short lived, and they sink into the depths, where their corpses give rise to a burst in bacterial growth. The bacteria rapidly consume not only the plankton, but also all of the oxygen. Simultaneously, the incoming river water forms a layer on the surface of the Gulf (another form of stratification), preventing new oxygen from dissolving and mixing with the depleted waters below. Creatures that do not flee suffocate, as do all stationary species and plant life.
The nutrients are primarily from agriculture – excessive amounts of manure and fertilizer drain into the streams and rivers that eventually feed into the Gulf. The USGS operates more than 3,000 real-time stream gauges, 60 real-time nitrate sensors, and tracks trends in nutrient loads and concentrations throughout the Mississippi-Atchafalaya watershed, which drains parts or all of 31 states. In May 2017, the USGS estimated that 165,000 metric tons of nitrate – about 2,800 train cars of fertilizer – and 22,600 metric tons of phosphorus flowed down the Mississippi and Atchafalaya rivers into the Gulf of Mexico. The heavy May stream flows, which were about 34 percent above the long-term average, coupled with the massive nutrient load resulted in the largest dead zone ever recorded. Every summer, the National Oceanic and Atmospheric Administration commissions scientists to venture out into the Gulf to measure our recurring dead zone. They determined in August 2017 that the Gulf of Mexico dead zone was 8,776 square miles, an area about the size of New Jersey. It is the largest measured, by almost 50 percent, since dead zone mapping began in 1985. Even though the measurements broke records, they fell short of the actual size. The team of scientists said the entire area of the dead zone couldn't be mapped due to an insufficient number of workable days on the ship. There was more hypoxia to the west, so the measured size would have been larger if there had been more time. Though the oxygen loss is temporary, the effects can be permanent, and there is already evidence that it is chronically affecting the reproduction of some species.
Significant oxygen declines have also been reported along the western shelf of North America. Off the central Oregon coast, a wide region was exposed to severe hypoxia in 2002, resulting in massive fish and crab die-offs, and surveys in 2006 revealed the widespread occurrence of not just hypoxia but also anoxia. Such occurrences of anoxia and widespread hypoxia are unprecedented in the previous 50 years. Declines were also noted from 1984 to 2004 over an array of stations off the coast of Southern California. The only benefit out of all this is that fish escaping the underwater apocalypse end up swimming to the surface layers, making them easier to catch, just as the Pacific cod have done along the Japanese continental slope and blue marlin and tuna have done in the tropical Atlantic. Of course, it’s only a short-term benefit, and not really worth the cost.
The science of ocean deoxygenation is still in its early stages. Nevertheless, there is a convergence of evidence suggesting that significant changes are in store. Together, warming, acidification, and deoxygenation present a triple threat to marine life. The oxygen requirements of marine organisms vary with temperature. Low-oxygen waters also typically have low pH (high acidity), meaning that marine organisms may need to use more energy to maintain acid-base equilibrium, except that their metabolism may be limited by the low oxygen conditions. These interacting effects make it necessary to study the system as a whole, which requires a unified research agenda across the full range of oceanographic disciplines, including physical, biological, and chemical.
Dead zones can be reversed with time and effort. Since climate change is the driving cause of ocean deoxygenation, reducing carbon dioxide emissions is the only real solution, BUT certain other actions, especially at a local level, can help protect oxygen-sensitive marine resources. Chesapeake Bay had a similar dead zone problem. In 2010, despite fierce objections from farmers, the federal government set mandatory limits on nutrient pollution entering the bay, accomplished by making major advances in wastewater treatment, sediment and storm water controls, soil management practices, and more selective and precise applications of fertilizer. State governments spent billions of dollars to meet those targets. Now pollution in the bay is down, and some wildlife in the Chesapeake is starting to recover.
To mitigate the Gulf’s dead zone, federal and state agencies are encouraging Midwestern farmers to try to keep nutrients from washing away by doing such things as planting wide grassy strips along streams to trap fertilizer runoff. New initiatives such as the Runoff Risk Advisory Forecast are designed to help farmers apply fertilizer at optimum times to limit nutrient runoff. A study of crop management systems released by Iowa State University showed that rotating corn and soybean crops, despite using nearly 90 percent less fertilizer, resulted in increased yields, improved soil quality, and 25 percent decreased soil erosion, all without decreasing profitability. However, even if enough farmers jumped on the bandwagon – possibly through such tactics as policy changes, financial incentives, and technical assistance – and runoff is substantially reduced, it could be 10 to 20 years before the Gulf shows visible improvement.
We rely on our oceans more than most people realize. Not only are they an incredible carbon sink (removing carbon dioxide from the atmosphere, thereby preventing a disastrously warm world), they also produce half of all the oxygen in the world (thank you, phytoplankton). Plus… sushi. We’ll never completely halt climate change, and we’ll probably never completely stamp out pollution, but we can stall both.
When we try to pick out anything by itself, we find it hitched to everything else in the Universe.
Where I learned about dead zones, and you can too!
National Oceanic & Atmospheric Administration
US Environmental Protection Agency
United Nations Educational, Scientific, & Cultural Organization
Ocean Scientists for Informed Policy
The Washington Post