Keywords: Climate change, Water quality, Species and food webs, Marine habitat, Hypoxia, Nutrient pollution, Eutrophication

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. 

Climate studies have shown that areas of the world with historically low oxygen conditions are growing in size. At the same time, new low-oxygen areas are being formed. One study estimates that the total volume of deadly low-oxygen waters has quadrupled since 1960.

As climate change results in warmer waters containing less oxygen, Puget Sound could expect to see an increase in species that can live in lower-oxygen waters but a decrease in species sensitive to the warmer conditions. Some species are likely to disappear if trends continue on their current path, according to climate researchers, but much depends on the amount of greenhouse gas emissions in the coming years.

Figuring out how various species will fare under differing conditions throughout the world has been the focus of intense research by Justin Penn, who completed his doctoral degree in oceanography from the University of Washington in 2020. One of Penn’s major findings, published last year in the journal Science, is his prediction that sea life abundance and distribution will begin to change dramatically by the end of the century if greenhouse gas emissions are not curbed soon.

Species living near the poles might not be able to meet their oxygen demands in the warmer waters of the future. Since they are already in the coolest water on Earth, they would have nowhere to go in a search of cooler water.

Tropical waters would experience the greatest loss of diversity over time, according to his findings, while species living close to the poles would face potential extinction.

Underlying Penn’s work on climate change is an understanding that most marine animals increase their respiration rates and their demand for oxygen at higher temperatures. Since warmer waters hold less oxygen, climate change produces a double-whammy for creatures trying to survive under warming conditions.

Penn’s graduate school adviser, Curtis Deutsch, has spent years studying the interrelated effects of oxygen and temperature. He developed a “metabolic index” to describe the combined temperature and oxygen requirements of a species and to compare those needs with environmental conditions in various places. Deutsch, a former UW oceanography professor, is now affiliated with Princeton University in New Jersey, where Penn is completing his post-doctoral research.

A study published in 2020 by Deutsch, Penn and Brad Seibel of the University of South Florida suggests that a wide variety of marine creatures — from vertebrates to crustaceans to mollusks — already occupy, to varying extents, all the areas that meet their “breathability” needs, described by their metabolic threshold.

Penn combined the metabolic index for various species with predictions of temperature and oxygen from climate-change models. His maps show how the animals might fare in a warming climate. For example, species living in the warm tropics appear to have traits that allow them to better cope with the warming waters compared to those in more temperate areas. But some tropical species are likely to shift their populations toward the poles to find cooler waters with more oxygen.

At the equator, warm, low-oxygen water already constrains the number of species compared to the middle latitudes between the tropical and polar regions. Further warming in the equatorial region would tend to reduce species diversity even more.

At the same time, some species living near the poles might not be able to meet their oxygen demands in the warmer waters of the future. Since they are already in the coolest water on Earth, they would have nowhere to go in a search of cooler water. If unable to adapt, their risk of extinction would be high.

Ancient climate clues to future

Projections of ongoing climate effects on marine life — assuming an accelerating rate of warming — turn out to be not so different from events that took place some 252 million years ago during the “Great Dying,” according to a 2018 study by Penn and Deutsch along with fellow researchers at Stanford University.

At the end of the Earth’s Permian Period, up to 96 percent of all marine species and 70 percent of all terrestrial species went extinct. It was a time of massive volcanic activity that raised the temperature of the air and water. Researchers have long debated the cause of the mass die-off, speculating about ocean acidity, metal and sulfide poisoning, lack of oxygen and straight-on higher temperatures.

This illustration shows the percentage of marine animals that went extinct at the end of the Permian era by latitude, based on modelling (black line) and the fossil record (blue dots).

This illustration shows the percentage of marine animals that went extinct at the end of the Permian era by latitude, based on modelling (black line) and the fossil record (blue dots). The images below the black line represent some of the 96 percent of marine species that died during the event. A greater percentage of marine animals survived in the tropics than at the poles. The color of the water shows the temperature change, with red indicating most severe warming and yellow less warming. At the top is the supercontinent Pangaea, with massive volcanic eruptions emitting carbon dioxide. Illustration: Justin Penn and Curtis Deutsch/University of Washington [Includes fossil drawings by Ernst Haeckel/Wikimedia; Blue crab photo by Wendy Kaveney/Flickr; Atlantic cod photo by Hans-Petter Fjeld/Wikimedia; Chambered nautilus photo by John White/CalPhotos]

Using models of ancient ocean conditions along with estimates of metabolic demands by prehistoric creatures, Penn was able to show that the physiological stresses caused by ocean warming and the accompanying loss of oxygen could account for more than half of the losses in marine species during the Great Dying.

The pattern of loss — more toward the poles — is predicted by computer models used in the study and confirmed in the fossil record, which reveals a before-and-after picture of species found in the ocean.

The pattern is similar to what he has predicted for today’s world. Assuming current trends in emissions, ocean warming would reach 20 percent of that in the late Permian by the year 2100 and between 35 and 50 percent by 2300, according to Penn. Based on his analysis, if things don’t change, it is quite possible that the Earth will experience another mass extinction, based on human actions instead of volcanic activity.

The metabolic index may be helpful in identifying habitats with enough oxygen for certain species to survive. But it does not describe current population distributions, how populations may shift in response to warming, nor how quickly one species may replace another based on their tolerance for low-oxygen conditions. Some researchers have speculated that a reported increase in jellyfish in Hood Canal may be attributed in part to their ability to survive in low-oxygen waters and out-compete less tolerant fish, such as herring.

A view of the ocean’s surface showing a large number of white jellyfish floating in the water.

Jellyfish can survive in low-oxygen waters. A large congregation of moon jellies pictured here were seen in Puget Sound's Sinclair Inlet in August 2021. Photo: Haila Schultz/University of Washington

Which species will dominate?

Because of the complex nature of food webs, it is hard to predict winners and losers, said oceanographer Evan Howard, a UW graduate who now works for Curtis Deutsch at Princeton. According to Howard, a particular but important challenge is to understand primary productivity at the base of the food web. Despite robust predictions of future temperature and oxygen, he said unknown factors make it difficult to describe the type and extent of future phytoplankton growth. Ultimately, phytoplankton are a key to future conditions affecting growth, reproduction, competition and predation for much of the food web.

Upwelling of ocean waters along the West Coast brings nutrient-rich waters from the depths, increasing plankton growth and boosting natural production. The rate of upwelling is driven by natural oceanic cycles influenced by the added and increasing effects of climate change.

“We have this teeter-totter of natural climate variability that sometimes gets outside of the norm and into conditions that we have never seen before,” Howard said.

His studies of anchovies and oxygen conditions along the West Coast led him to predict a complete loss of anchovy habitat in the southern part of their range by the end of this century. That would include waters off the Mexican coast reaching into Southern California.

During warm-water “heat waves,” such as the so-called “blob” from 2014 to 2016, a greater number of anchovies were observed in Puget Sound, but the cause is not well understood. Howard wonders how much those observations were the result of migration of anchovies into Puget Sound from other areas versus a movement of fish from deeper to shallower waters to get more oxygen.

“Sometimes you see an abundance in animals in one place because they are stressed somewhere else,” he said, noting that the overall effect of warming is to reduce the size of existing areas with sufficient oxygen for a given species. At the same time, populations may move northward along the West Coast to find breathable habitat in colder waters.

While a steady rate of warming can be expected to shrink populations of cold-water fish such as salmon off the West Coast, sudden heat waves — such as “the blob” — could greatly increase the population losses, according to a study led by William Cheung of the University of British Columbia. Climate-change models predict at least four additional “blobs” before the end of the century, he said, although nobody can predict when exactly they will occur.

In the North Pacific, Cheung said pelagic (open water) fish are expected to be most affected by these sudden changes in temperature, with somewhat lesser effects on salmon and bottom fish.

“Our results underscore the need for a reduction of anthropogenic greenhouse gas emissions – the fundamental driver of ocean warming — to limit challenges from marine heat waves on fish stocks and fisheries,” Cheung said.

As for what will happen in Puget Sound or any localized area, it would be largely speculation at this point, according to Howard. “One thing we can say,” he noted, “is that there will be ecological disruption no matter who wins or loses.”

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

Up Next: A look at how scientists have interpreted thresholds for hypoxia and dissolved oxygen. How low is too low for aquatic species? 

View the entire series.


A purple sea star attached to a rock covered with mussels and seaweed.

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.


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.


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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