Hypoxia (fact sheet)

The following fact sheet provides an overview of low oxygen conditions in Puget Sound. It addresses some of the related causes and concerns that have been identified by scientists in the region. The overview was prepared in conjunction with a series of workshops on hypoxia and nutrient pollution presented by the University of Washington Puget Sound Institute. 

Massive die-offs of Dungeness crab off the Pacific Northwest Coast have been attributed to dangerously low oxygen levels. Once dead, the aquatic crabs often wash up on beaches, as seen here on Kalaloch Beach on June 14, 2022. Photo: Jenny Waddell/NOAA
Massive die-offs of Dungeness crab off the Pacific Northwest Coast have been attributed to dangerously low oxygen levels. Once dead, the aquatic crabs often wash up on beaches, as seen here on Kalaloch Beach on June 14, 2022. Photo: Jenny Waddell/NOAA

What is hypoxia?

Almost all life on Earth, on land and in the water, requires oxygen to survive. Hypoxia in a body of water refers to conditions in which the concentration of oxygen falls to levels that can harm or kill aquatic organisms. The upper threshold for hypoxia typically ranges from 2 to 5 milligrams of dissolved oxygen (sometimes abbreviated as D.O.) per liter of water. Hypoxia can be perennial, seasonal, or transient.

What are the consequences of hypoxia – why does it matter?

Hypoxia can lead to massive die-offs of aquatic life and loss of biomass and can reorganize entire food webs. Organisms that can’t move, such as shellfish, corals, and aquatic plants, can suffocate and die from lack of oxygen, as can worms and other animals that burrow in the sediment.

Benthic ecosystems, which include areas of seafloor mud and sediment, often become simplified as a consequence of hypoxia, changing from a rich array of large, slow-growing, long-lived organisms that burrow deep in the sediment, to small, short-lived, fast-growing organisms on the surface. Hypoxia alters nutrient cycling; under severe or prolonged hypoxia, little besides anaerobic bacteria can survive, and energy and nutrients are simply circulated among microbes.

Hypoxia sometimes leads to fish kills when animals become trapped in enclosed bays, for example. But mostly fish and other mobile organisms swim away from hypoxic waters. Even so, hypoxia can degrade their habitat and make food sources scarer. Fish and other swimming organisms can also become more vulnerable to predators when they flee from hypoxic conditions. And sub-lethal or intermittent exposure to hypoxia has an array of physiological effects, such as impairing growth and reproduction.

What causes hypoxia?

Hypoxia has both natural and human causes. But human activities have resulted in an increase in the duration, severity, and extent of hypoxic conditions in the world’s oceans, especially in the last several decades.

What are the natural factors that contribute to hypoxia?

A major natural contributor to hypoxic conditions is the tendency for waters of different temperature or salinity not to mix. Fresh water is less dense than salt water, and warmer water is less dense than colder water. So, fresh water coming into the sea from rivers tends to sit on top of salt water, and warmer surface waters tend to sit on top of colder bottom waters, a phenomenon known as stratification. As a result of this stratification, bottom waters can become depleted of oxygen.

The shape of ocean basins can also contribute to hypoxia. When the outlet of a water body is narrow, it can take a long time for water in these isolated areas to be refreshed by inputs of water from larger water bodies. These so-called long residence times can also lead to oxygen being depleted.

Parts of Puget Sound are naturally hypoxic due to large inputs of nutrients from the ocean carried by the California Current.

What are the human-caused factors that contribute to hypoxia?

The biggest contributor to hypoxia worldwide, however, is human caused. Nutrient pollution from agricultural runoff, fossil-fuel burning, and wastewater discharges, triggers a cascade of events that lead to hypoxia.

Hypoxia is thus a common symptom of eutrophication – an overabundance of nutrients such as nitrogen and phosphorus in a water body.

These nutrients, such as nitrogen and phosphorous, are necessary for aquatic life. But an excess of nutrients triggers an overgrowth of cynanobacteria and algae. When these blooms of microscopic organisms die, they fall to the bottom. There, they are broken down by bacteria, a process that requires oxygen and thus leads to depletion of oxygen in bottom waters.

“There is an intimate connection” between algae blooms at the surface of the sea and hypoxia at the bottom, says Jacob Carstensen, a marine ecologist at Aarhus University in Denmark.

What parts of the global ocean are affected by hypoxia?

Hypoxia is now seen in most developed coastal areas around the world. In temperate latitudes, areas of coastal hypoxia are often worst in the summer months because of the combination of high nutrient loading and high stratification. More than 400 hypoxic zones have been identified along the world’s coastlines, affecting more than 245,000 square kilometers. These areas have been expanding rapidly in recent decades.

Some well-studied areas of coastal hypoxia include:

  • The northern Gulf of Mexico, where the United States’ largest dead zone, covering as much as 22,000 square kilometers, forms each summer due to nutrient runoff and large inputs of fresh water from the Mississippi River.
  • The Chesapeake Bay, the largest estuary in the United States, where summer hypoxia affects up to 40% of the area and 5% of the volume.
  • The Black Sea, which is naturally hypoxic due to restricted exchange of water with the Mediterranean Sea. In addition to perennially hypoxic depths, the northwestern coastal waters experience seasonal hypoxia due to nutrient inputs from large river systems such as the Danube and Dnieper.
  • The Changjiang (Yangtze) and Zhujiang (Pearl) River estuaries in China, the site of rapid expansion of agriculture and development in recent decades.
  • The Baltic Sea, which is naturally prone to hypoxia because of the narrow entrance to the North Sea through the Danish Straits and has seen a worsening of both perennial and seasonal hypoxia due to nutrient inputs over the last century.

What are the concerns with hypoxia in Puget Sound?

Hypoxia affects a number of inlets and embayments in Puget Sound, including Bellingham Bay in northern Puget Sound; Port Susan and Penn Cove in central Puget Sound; Liberty Bay, Dyes Inlet, and Sinclair Inlet on the west side of Puget Sound; Lynch Cove and other areas of Hood Canal; and Carr Inlet, Case Inlet, Eld Inlet, and Budd Inlet in southern Puget Sound.

Hypoxia in these areas of Puget Sound is typically worst during the summer months because warm weather increases stratification of the water, reducing the availability of oxygen to the bottom layers.

Is hypoxia in Puget Sound natural or human-caused?

The amount of oxygen in Puget Sound has naturally varied over the past 400 years. Dissolved oxygen has been declining in Puget Sound, particularly in the upper layers of the water, since around the year 2000, but it’s not clear whether this trend is due to natural or human causes.

Several natural factors make Puget Sound, especially certain parts of it, naturally prone to hypoxia, such as inputs of large amounts of nutrients from ocean waters carried by the California Current, fjord-like inlets and embayments that have long residence times of water and strong stratification. In such areas increased inputs of nutrients from wastewater treatment plants and other human sources can worsen hypoxia.

How is climate change expected to influence hypoxia?

Climate change is expected to worsen hypoxia in the ocean via several mechanisms. Oxygen is less soluble in warmer water, so less oxygen enters the water from the atmosphere as the temperature warms. Higher temperatures increase the metabolic rates of marine organisms, so they consume more oxygen. Higher temperatures also increase stratification, meaning less replenishment of oxygen-poor bottom waters via mixing with oxygen-rich surface layers.

Scientists predict that seasonal hypoxia will happen more often, be more severe, and affect more and larger areas as the climate warms.

What’s more, climate change can increase nutrient inputs into the ocean. More frequent and intense storms and greater precipitation mean more nutrient-laden runoff from land. These factors also mean more freshwater input into coastal waters, which in turn increases stratification.

Finally, warmer temperatures and ocean acidification that come along with climate change can stress marine organisms – perhaps magnifying the impacts of low oxygen on their physiology.

What can be done about hypoxia?

One strategy to reduce hypoxia is to reduce the amount of nutrients entering the sea from land. This can be done by upgrading water treatment plants to remove nutrients from wastewater; planting cover crops in agricultural fields to reduce erosion; limiting the number of livestock per area of land; and limiting fertilizer application to croplands.

In areas where the residence time of water is long, recovery from hypoxia-prone conditions can take decades. But there are success stories. Upgrading wastewater treatment plants has reduced nutrient loading and hypoxia episodes in Narragansett Bay, Rhode Island. And scientists have calculated that without efforts to reduce nutrients in the Baltic Sea, the extent of hypoxia would be twice as large as it currently is.

Funding for this fact sheet was provided by King County as part of a series of online workshops addressing scientific uncertainities around nutrient pollution and hypoxia. 

About the Author: 
Sarah DeWeerdt is a Seattle-based freelance science writer specializing in biology, medicine, and the environment. Her work has appeared in publications including Nature, Conservation, and Nautilus.