Keywords: Water quality, Species and food webs, Algae, Invertebrates, Fishes, Marine habitat, Phytoplankton, Food web, Hypoxia, Nutrient pollution

Scientists are reporting a decline in oxygen-rich waters throughout the world. Causes for the decline vary from place to place but may involve climate change and increasing discharges of tainted water. In Puget Sound, low oxygen levels can occur naturally or due to eutrophication from human-caused pollution. In this five-part series, we describe the critical nature of oxygen to Puget Sound sea life. Scientists are finding that changes in oxygen levels can lead to physiological adjustments, shifts in predator-prey relationships and other repercussions throughout the food web.

The vital nature of oxygen emerges at the cellular level, regulating the health of individuals and influencing population changes among marine and terrestrial species in every ecosystem.

Wilderness survival experts talk about a “rule of three.” A person can live three weeks without food, three days without water but only three minutes without oxygen, the essence of life. 

This “rule of three” may be over-generalized, but there is no doubt that oxygen is fundamental to life as we know it. It is a basic building block essential to the existence of plants and animals on Earth, including those in Puget Sound where the levels of oxygen can sometimes spell the difference between life and death.

Within cells, the molecular form of oxygen, O2, joins with organic molecules to engage in a series of reactions, something like a slow rate of combustion that releases sufficient energy for organisms to function.

While terrestrial creatures obtain their oxygen from the air, most aquatic organisms capture oxygen dissolved in the surrounding water. Water, for the most part, gets its oxygen delivered from the air above. Waves, currents, riffles, and waterfalls help to increase oxygen in surface waters, mixing into deeper layers.

Photosynthetic organisms, such as algae and aquatic plants, also play a role in providing dissolved oxygen. Photosynthesis, powered by the energy of sunlight, also occurs near the water’s surface — in the euphotic zone — where carbon dioxide is converted into organic compounds used for structure, operation and growth. During the process, precious oxygen gets released into the water.

The air can hold 20-40 times more oxygen than the same volume of water. That means, for the same energy needs, a marine animal must pass that much more water over its gills than a terrestrial organism passes air through its lungs — although much depends on the efficiency of uptake, in which marine animals excel by comparison.

Temperature is also a factor. Warmer water holds less oxygen than cooler water. Warmer water also results in an increased metabolic rate for cold-blooded animals, boosting the demand for oxygen. As a result, climate change strikes a double blow: Warmer temperatures increase an animal’s requirements for oxygen while warmer water provides less.

Two satellite images of Earth showing bright blue-green algal bloom in Puget Sound and Hood Canal.

Satellite images of a large bloom of microscopic phytoplankton (coccolithophores) in Hood Canal in July 2016 (left), and a closer view of the same bloom near Dabob Bay (right). Images: Jeff Schmaltz, LANCE/EOSDIS Rapid Response, NASA (left) and Jesse Allen, Earth Observatory, NASA (right).

In Puget Sound, areas of low oxygen are often the result of large plankton blooms triggered by excess nitrogen in the presence of sunlight. Sources of nitrogen vary widely, from natural production to sewage treatment plants to farm fertilizers. As the plankton die and decay, they sink into deep water, where bacterial breakdown consumes the available oxygen. In areas where mixing is slow to replenish oxygen, such as in southern Hood Canal, lower oxygen supplies can affect both the physiology and behavior of marine life.

Oxygen requirements

Tim Essington, professor of aquatic and fishery sciences at the University of Washington, says the need for oxygen can be understood in terms of human metabolism.

“When you think of a person breathing, with all the oxygen you need, your heart and muscles are working well,” he said. “If you start going up into the mountains of Colorado, you will find yourself breathing harder, your heart rate will be faster, and you will get out of breath.”

Within the scope of acclimation, a person may learn to live with less oxygen, Essington said. That’s why athletes train at higher elevations. Their bodies become more efficient at using oxygen during heavy exercise, as measured by changes in their blood.

“The brain needs a lot of energy,” he said, “and your energy supply depends on your ability to get oxygen.”

The metabolic story changes when a climber ascends Mount Everest, the highest mountain peak in the world, without supplemental oxygen. Only a few people have ever done it successfully.

“You ventilate more rapidly,” Essington said. “Your metabolism slows down. Other things happen as your body tries to adjust. But when you cross a threshold, none of these things are going to work. You either move to a better place or you will die.”

At the cellular level, a loss of oxygen leads to a deficiency of adenosine triphosphate (ATP), a compound that supplies energy to drive many cellular processes. Loss of ATP leads to a cascade of reactions, including a loss of integrity at the cell membrane, an influx of destructive enzymes, and damage to cell components. This path to cell death is the same for all vertebrates, but the time it takes varies greatly from one animal to another.

Most animals can handle short periods of low oxygen, some better than others, by engaging in anaerobic respiration. This involves a breakdown of sugars to meet short-term energy needs without oxygen. Anaerobic respiration produces just a tiny fraction of the energy supplied by aerobic respiration, so neither fish nor humans can live long without oxygen.

Head on view of a fish swimming under water.

The crucian carp (Carassius carassius) of Northern Europe can survive in extreme low oxygen conditions for months in icy cold water. Photo: Jernej Furman (CC BY 2.0)

Rare exceptions include the crucian carp of Northern Europe, which can survive in extreme oxygen conditions for months in icy cold water. The carp’s internal strategy involves a dramatic drop in metabolism and severe suppression of energy-demanding pathways, such as protein synthesis, according to a report by Angela Fago a zoophysiologist of Denmark’s Aarhus University. For energy, the carp relies on a storage of glycogen in the liver. Oxygen must be restored before the glycogen runs out, or else the fish dies.

Differing levels of tolerance

Oxygen-breathing creatures have at least one thing in common when it comes to surviving in low-oxygen waters, known as hypoxic conditions: They generally do well until they find themselves in a level of hypoxia that challenges their metabolic function. Each species has its own critical tolerance for low oxygen, defined as the level below which the animal can no longer maintain its standard metabolic rate — the minimal oxygen level for self-maintenance.

Extended periods of hypoxia can be lethal for animals stuck in one place, such as geoduck clams, or those unable to move fast, such as sea stars. In Hood Canal, for example, geoducks are found in significant quantities only in the north, despite good bottom habitat for them in the south, according to Essington.

“To grossly simplify things, if you are a thing like a sea star that can’t move very well, you are generally or always absent,” he said. “But if you are a thing like an English sole, you are pretty good at just leaving the area when things get bad. Then, when things get better, you come right on back.”

Although southern Hood Canal is prone to low-oxygen conditions throughout the year, conditions tend to reach their worst during the summers, continuing into fall. In extreme conditions, a wind from the south can blow surface waters to the north, bringing hypoxic waters from the depths right up to the surface, leaving fish no place to go.

Notable fish kills — with dead sea life washing ashore — were reported in Hood Canal during the fall in 2002, 2003, 2004 and 2006, with another major event in 2010. Another bad year came in 2015, when a warm-water “blob” off the West Coast contributed to low oxygen levels all year long.

Other areas of Puget Sound also have experienced severe hypoxia at times. As with Hood Canal, the worst conditions generally occur in late summer and early fall, typically within dead-end bays, such as in South Sound and near Whidbey Island.

Side by side maps of Puget Sound with shading that indicates levels of dissolved oxygen in late spring (left map) and early fall (right map) in 2014. The late fall map shows inlets and bays with less than 2 to 3 mg/L of dissolved oxygen using shades of da

Low dissolved oxygen occurs naturally in the ocean and in Puget Sound, particularly where circulation is poor, including deep waters, and some bays and inlets. Human sources such as wastewater also contribute, and the effect is worsened by climate change. Dissolved oxygen is typically higher in late spring (left map) and lowest in late summer or early fall (right map). The above maps show an example of dissolved oxygen levels documented by the Washington State Department of Ecology in 2014. Sixteen areas of Puget Sound (labeled with point lines) identified by state regulators are within bays and inlets where human activities may further decrease dissolved oxygen. Map: PSI. Map data: Ahmed et al. Washington State Department of Ecology (2019).

While death is a dramatic outcome of low-oxygen conditions, scientists have identified sublethal effects as well. They include reduced feeding, slower growth, and potential effects on reproductive and immune systems among fish and other marine creatures.

Beyond the effects of hypoxia on individual organisms are the repercussions through the food web. For example, if fish are forced to move out of their chosen habitat because of low oxygen, they may end up congregating in a smaller area where oxygen supplies meet their needs. In such a case, increased density and competition for food may inhibit their overall growth and development. Predators may also be more successful, resulting in population effects.

In another situation, small prey species with a high tolerance for hypoxia might be able to escape into low-oxygen waters where predatory fish cannot follow. The predators, searching for food outside the hypoxic zone, might come up short in their available food supply.

Essington expected to observe something like that with herring and krill, as predator and prey, during low-oxygen conditions in Hood Canal. Krill, small shrimplike crustaceans, might be expected to survive predation by staying in waters thought to be lethal for herring, based on laboratory observations. While herring prey upon krill, herring are themselves prey for a wide variety of animals, including birds, salmon, lingcod and rockfish as well as marine mammals — so a decline in herring could create disruptions in the food web.

Instead of staying away, the herring observed in the Hood Canal study were doing quite well in the low-oxygen waters, feeding on krill as if there were no problems, Essington said. The fish may have made internal adjustments to the adverse conditions, possibly by increasing the surface area of their gills, increasing their blood circulation and modifying their tissue demand.

Said Essington, “They were undergoing a lot of physiological acclimation that was enabling them to thrive, despite having oxygen conditions in which an acute exposure might actually kill them.”

These findings with herring and krill demonstrate that laboratory tests don’t always predict conditions in the real world. Like an athlete training at high altitude, acclimation could allow a fish in the wild to adjust to conditions of less oxygen, whereas a fish in the laboratory might not be given that chance. Sometimes a wild fish might require more oxygen for swimming than a laboratory fish tested in a resting state. It all depends on conditions.

While acclimation is the ability of an individual to alter its oxygen needs, some researchers say populations can adapt to harsh conditions by making genetic modifications for permanent change. Adaptations of this kind typically occur over multiple generations with genetic changes that respond to adverse conditions. Genetic changes are then passed down to offspring. If Hood Canal herring are a unique population, could they be endowed with traits that help them survive low-oxygen conditions? These questions are yet to be answered.

Risk-based management

As policymakers consider actions to limit human sources of nitrogen in Puget Sound, major questions are being raised about the effects of hypoxia on individual species. Although various species differ in their tolerance for hypoxia, each has a natural limit, which generally depends on temperature and other water conditions.

“Oxygen is a limiting factor,” Essington said. “When there is plenty of oxygen, changing the amount of oxygen doesn’t really matter. It’s when oxygen is relatively low anyway that you worry about changes in oxygen.”

Because of low-oxygen thresholds, the goal of protecting species from the risks of hypoxia might be compared to hiking in the mountains near a dangerous cliff, he said. “If I think I am pretty far away from the cliff, I know I am not at risk, and I don’t need to be careful with every single step. If I don’t know where I am to the cliff, I am going to perceive that as a pretty risky situation.”

Puget Sound Institute is undertaking a project to compile information about low-oxygen thresholds for Puget Sound species and determine where sensitive species are likely to confront dangerous levels of oxygen, now and in the future. The effort, led by PSI’s lead ecologist Tessa Francis, will start with survey data and take account of the conditions where various species are living — and not living —as an indicator of low-oxygen thresholds.

Rather than conduct expensive and time-consuming studies of hypoxia tolerance for Puget Sound species, researchers might be able to borrow from existing studies, such as those that address similar species in other areas, said Essington, who is involved in the project. 

Another idea, Francis said, would be to take advantage of existing computer models of food webs and water quality to create maps showing where low-oxygen conditions may significantly affect sea life. Funding for the work, which is just getting underway, comes from the federal Environmental Protection Agency, through the state’s Puget Sound Partnership, as part of Puget Sound recovery efforts.

In estimating risk, it is essential to consider the entire life history of an animal. For example, crab larvae appear to be more sensitive to low oxygen than adult crabs. When crab larvae get caught in lethal levels of low oxygen, the loss in numbers can reduce the future population. Fortunately, as some observers have noted, the larval stage typically occurs earlier in the year than the worst hypoxic conditions.

This article was funded in part by King County in conjunction with a series of online workshops exploring Puget Sound water quality.

Up next: Our series continues with a look at how Dungeness crabs respond to low oxygen conditions in Hood Canal.

View the entire series.


A person holding a rope attached to a wire cage holding recently captured Dungeness crabs.

As observed in Hood Canal, low-oxygen conditions can upend the lives of Dungeness crabs trying to stay alive. Levels of dissolved oxygen can alter predator-prey relationships for a multitude of species, affecting populations throughout the food web. Part two of our series "Oxygen for life" examines a crab case study.


View from underwater of bubbles rising to the surface of the ocean with sunlight above.

In time, lower dissolved oxygen worsened by climate change could increase the abundance of rare species in Puget Sound while putting populations of more common species into a tailspin. Part three of our series "Oxygen for life" looks at how warmer waters will gradually make it harder for many sea creatures to breathe. 


A crab pot (circular mesh cage) with an oxygen sensor (a white tube inside the cage) is held off the side of a boat as it is about to be dropped into the water.

The search goes on for a set of definitions and thresholds to represent low-oxygen concentrations that threaten various aquatic creatures. Over the years, ecologists have relocated, reshaped and revised the word “hypoxia” to describe these conditions. In part four of our series "Oxygen for life" we look at how scientists determine whether oxygen levels are low enough to be considered harmful to sea life. 


View of Puget Sound with red-orange water near the shoreline and blue sky with clouds above land in the distant background.

How do excess nutrients trigger low oxygen conditions in Puget Sound and what do those conditions mean for the species that live here?

About the author: Christopher Dunagan is a senior writer at the Puget Sound Institute.

Oxygen for life: The biological impacts of low dissolved oxygen

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