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Dead zones: growing areas of aquatic hypoxia are threatening our oceans and rivers

During the early days of the COVID-19 pandemic, physicians were baffled by patients who had no problems breathing—they could still chat away on their cell phone, even though their blood oxygen levels were so low that they should have been unconscious. The condition was dubbed “silent hypoxia.” The oxygen level in these coronavirus-afflicted people was less than 90%, the medical definition of hypoxia, much lower than the 95-100% seen in a healthy person. Even without outward signs of respiratory problems, silent hypoxia is both a problem in of itself and a signal that the many other manifestations of this deadly disease are on their way.

“Hypoxia” has a similar interpretation in ecology, although the oxygen level defining it is lower. The word is used to describe habitats with less than 2 milligrams of oxygen per liter, or roughly 25% of fully-oxygenated water. The term favored in the popular media for these habitats is “dead zone.” Whether hypoxia or a dead zone, these waters have less oxygen than the “death zone” at Mount Everest where the gas is 40% of the concentration at sea level and mountaineers perish on their way to the summit. But the percentages don’t give a complete picture of how oxygen-poor a dead zone really is. Water can hold much less oxygen, 40 times less than in the same volume of air. So, even small declines in dissolved oxygen can stress out aquatic organisms like Atlantic rock crab or salmon, and most are quickly killed when oxygen drops to dead zone levels.

A lake, sea, or coastal ocean turns into a dead zone when the supply of oxygen from the atmosphere and photosynthesis is overwhelmed by the use of oxygen during organic material degradation. In the past and in some regions today without adequate wastewater treatment, the organic material comes from sewage. In many current dead zones like the Gulf of Mexico and the Baltic Sea, the organics come mainly from excessive growth of algae, fueled by fertilizers leaching from croplands. As the algal organics are degraded, oxygen disappears.

“Dead zones are everywhere around the globe, totaling perhaps as many as a thousand.”

Dead zones in the Gulf and Baltic receive the lion’s share of attention because of their size. But dead zones are everywhere around the globe, totaling perhaps as many as a thousand. Although the number of oxygen-poor aquatic habitats may no longer be increasing, the size of many is expanding, oxygen is dropping to even lower levels, and the number of months they linger is lengthening. Some dead zones like in the Baltic Sea now last throughout the year.

There is nothing silent about the hypoxia in some of these habitats. Dead fish litter beaches and the harvest of bottom-dwelling fish, shrimp, and crabs declines. Hypoxic waters can release phosphorus nutrients and promote harmful algal blooms, which produce chemicals that are toxic to aquatic organisms and animals onshore.

But often hypoxia is silent to the casual observer.  In dead zones like the Gulf of Mexico and the Baltic Sea, the hypoxic water hugs the bottom, out of contact with atmospheric oxygen and out of sight except to the specialist. Meanwhile, surface waters can teem with life. Along with a dead zone, the northern Gulf is home to the Fertile Fisheries Crescent, where commercial and sport anglers go after redfish, yellowfin tuna, amberjack, and speckled trout, to name a few of the over 1440 species of finfish in the Gulf. Yet below this cornucopia of marine life, hypoxic waters force fish to flee and devastate the bottom-dwelling invertebrate community, leaving behind only a few opportunistic, small animals tolerant of oxygen deprivation.

Dead fish (carp) float to the surface of the water in this polluted channel.
Image by Jason Mintzer

The hypoxia in the open oceans is even more silent. Open oceans far from land and hard to study are losing oxygen. Known to oceanographers as oxygen minimum zones (OMZs), regions like in the equatorial Pacific Ocean always have had low oxygen. What’s new is the fact that OMZs have been increasing in size and becoming even more oxygen deficient. Over the last few decades, the volume of the Pacific OMZ has expanded by about 7% and oxygen concentrations have dropped by as much as 50% in some regions. Arguably even more troubling, all parts of the ocean are losing oxygen, because of an even bigger problem.

The bigger problem is climate change. A big reason why the oceans are losing oxygen is because they are warming. As temperatures rise, water holds less oxygen and other gases. Along with damaging coral reefs and threatening other marine life, ocean warming exacerbates the problems caused by low oxygen. As the oceans warm, some marine animals require even more oxygen and are even more stressed by low oxygen levels.

Just as silent hypoxia in people cannot be ignored, we need to pay attention to hypoxia in aquatic habitats. Even if the effects are not readily seen, the loss of oxygen signals the declining health of the biosphere and the threat of more trouble to come.

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