Ocean deoxygenation is the loss of oxygen in different parts of the ocean caused by human activities. This happens in two main areas. First, it occurs in coastal zones where too many nutrients from human activities lead to rapid decreases in oxygen levels. These areas with very low oxygen are called dead zones. Second, deoxygenation also happens in the open ocean. In this area, oxygen levels are slowly decreasing over time. Naturally low-oxygen areas, called oxygen minimum zones (OMZs), are now growing larger because of human-caused climate change. This loss of oxygen harms marine life and people who rely on the ocean for food or work. It also affects how productive the ocean is, how nutrients and carbon move, and how ocean habitats function.
As ocean temperatures rise, oxygen loss increases. This is because warmer water increases ocean stratification, which means layers of water do not mix well. Warmer temperatures also reduce how much oxygen can dissolve in water, explaining about half of the oxygen loss in the upper ocean. Warmer water holds less oxygen and floats higher than cooler water, reducing mixing between surface and deeper water. Deeper water already has less oxygen, and warmer temperatures increase the oxygen needs of marine life, leaving less oxygen available.
Studies show that oceans have already lost 1-2% of their oxygen since the middle of the 20th century. Models predict that global ocean oxygen levels could decrease by up to 7% over the next 100 years. Scientists expect this decline to continue for many more years, possibly for thousands of years.
Terminology
Scientists use the term "ocean deoxygenation" more often because it describes the decreasing amount of oxygen in the world's oceans. Experts have discussed which term best explains this phenomenon to people who are not specialists. Some terms considered include "ocean suffocation," "marine deoxygenation," "ocean oxygen depletion," and "ocean hypoxia."
Types and mechanisms
There are two types of ocean oxygen loss that happen in different areas and have different causes: oxygen loss in coastal zones compared to oxygen loss in the open ocean and deep ocean (called oxygen minimum zones). These processes are connected but different.
Coastal areas, such as the Baltic Sea, the northern Gulf of Mexico, and the Chesapeake Bay, as well as large enclosed water bodies like Lake Erie, have experienced oxygen loss because of eutrophication. Too many nutrients enter these systems through rivers, which come from urban and agricultural runoff and are made worse by deforestation. These nutrients cause high productivity, which leads to organic material sinking to the bottom and being used by organisms. This process uses oxygen, creating low oxygen or no oxygen conditions.
In the open ocean, natural areas with low oxygen exist and are slowly growing. These areas, called oxygen minimum zones (OMZs), usually occur in the middle depths of the ocean, between 100 and 1000 meters deep. They form naturally from the use of organic material sinking from the surface. However, as ocean oxygen decreases, these zones are growing larger both vertically and horizontally. In these areas, water moves slowly, making it easier to notice small changes in oxygen, such as a drop of 1-2%. In many places, this drop does not immediately make these areas uninhabitable for fish and other marine life, but over many decades, it might, especially in the Pacific and Indian Oceans.
Oxygen enters the ocean at the surface through photosynthesis by phytoplankton and mixing with the atmosphere. Organisms, both tiny and larger, use oxygen for respiration throughout the entire ocean depth. When the oxygen supply from the surface is less than the oxygen used in deep water, oxygen loss happens.
This process is natural but is made worse by increased ocean stratification and rising ocean temperatures. Stratification happens when water layers with different temperatures and salinity form, with less dense water on top of denser water. The bigger the differences between layers, the less mixing occurs. Stratification increases when surface ocean temperatures rise or when more freshwater enters the ocean from rivers and melting ice, reducing oxygen supply. Another factor is oxygen solubility. As temperature and salinity increase, less oxygen can dissolve in water, meaning warmer and saltier water holds less oxygen.
Role of climate change
Oxygen minimum zones (OMZs) form naturally in the ocean, but human activities such as climate change and pollution from agriculture and sewage can make these zones worse. Scientists predict that global warming will cause ocean temperatures to rise and oxygen levels to drop in most of the upper ocean. Warmer water holds less oxygen, which reduces oxygen levels further. This worsens the effects of pollution that causes excessive nutrients in coastal areas.
Over the past 50 years, the area of the open ocean with no oxygen has grown by more than 1.7 million square miles. In coastal waters, the area with low oxygen has increased ten times. Studies of oxygen levels in ocean water over the last 50 years show a clear decrease in oxygen. This decline is linked to larger areas of low oxygen, deeper layers of the ocean affected, and longer periods of low oxygen in all ocean regions. Evidence from ancient ocean records shows that OMZs have expanded over time, and this expansion is connected to ocean warming and reduced mixing of deep ocean waters.
Scientists are trying to model how OMZs might change due to rising temperatures and human activities. This is difficult because many factors influence OMZs, and some are hard to measure. These factors include how rising temperatures affect oxygen solubility, and changes in the balance between respiration and photosynthesis near OMZs. Many studies show that OMZs are growing in many places, but scientists still do not fully understand how modern OMZs change. Models of Earth’s systems predict that climate change will cause large reductions in ocean oxygen and other ocean conditions, which could harm ecosystems and people.
The global loss of oxygen in the ocean is a clear and growing trend, happening faster than natural changes. This loss became more obvious after the 1980s. The rate and total amount of oxygen loss vary by region, with the North Pacific showing a major loss of oxygen because its deep waters have not been refreshed for a long time and use oxygen quickly. Scientists estimate that the ocean has lost between 119 and 680 teramoles of oxygen each decade since the 1950s. This loss represents about 2% of the ocean’s total oxygen.
When gas hydrates in the ocean floor melt, they may release methane from sediments. This methane is then used in a process that consumes oxygen. Another effect of climate change is changes in ocean circulation. Warmer surface waters may reduce ocean mixing, slowing circulation and further lowering oxygen levels.
Mathematical models suggest that ocean oxygen loss will continue to increase, reaching about 125 teramoles per year by 2100. This is due to ongoing warming, less mixing of deep waters, higher oxygen use by marine life, and the spread of OMZs into shallower areas.
Variations
Several areas of the open ocean naturally have low oxygen levels because the amount of oxygen used by living things is greater than the amount of oxygen added through physical movement of water, mixing with air, or photosynthesis. These areas are called oxygen minimum zones (OMZs), and many parts of the open ocean, such as areas where deep water rises to the surface (upwelling zones), deep parts of enclosed seas, and the centers of certain ocean currents, experience these naturally low oxygen conditions.
Ocean deoxygenation has caused very low, low, and no oxygen conditions in both coastal waters and the open ocean. Since 1950, more than 500 locations in coastal waters have reported oxygen levels below 2 milligrams per liter, which is the generally accepted level for hypoxic conditions.
The size of OMZs has grown larger in tropical oceans over the past 50 years.
Oxygen-poor waters in coastal and open ocean areas have mostly been studied separately. Researchers have focused on low oxygen caused by too many nutrients in coastal waters and naturally occurring low oxygen in open ocean OMZs. However, these oxygen-poor areas are connected, and both have seen more intense, widespread, and longer-lasting low oxygen conditions.
The size of low oxygen areas can vary greatly. In coastal waters, these areas can range from less than one square kilometer to thousands of square kilometers. OMZs exist in all ocean basins and also vary in size; about 8% of the global ocean volume is within OMZs. The largest OMZ is in the eastern tropical north Pacific and covers 41% of the global OMZ volume, while the smallest OMZ is in the eastern tropical North Atlantic and covers only 5% of the global OMZ volume.
The depth of low oxygen conditions also varies. Areas with long-term low oxygen levels have yearly changes in the upper and lower boundaries of oxygen-poor waters. OMZs usually occur between 200 and 1,000 meters deep. The upper edge of OMZs has a sharp change in oxygen levels, called the oxycline. The depth of the oxycline differs between OMZs and is mainly influenced by physical processes such as oxygen mixing between air and water and movement of water in the thermocline layer. The lower edge of OMZs is where biological oxygen use decreases because most organic matter is used up in the top 1,000 meters of the ocean. In shallow coastal areas, oxygen-poor waters may reach the ocean floor, harming bottom-dwelling life.
Many OMZs have grown thicker over the past 50 years because their upper boundary has become shallower and their lower boundary has moved deeper.
The time span of low oxygen conditions can vary from seasons, to years, or even decades. In coastal areas like the Gulf of Mexico, low oxygen is often linked to river water flow, water layer separation caused by temperature and salt differences, wind patterns, and ocean currents near the shore. These factors create seasonal and yearly patterns in the start, duration, and end of intense low oxygen conditions. In the open ocean and near the edges of coastal and open ocean areas, oxygen levels can change in intensity, size, and time span due to long-term changes in climate patterns.
Coastal areas have also seen larger areas and longer-lasting low oxygen conditions because of increased human-caused nutrient pollution and changes in ocean currents. Some areas that once had normal oxygen levels, like the Oregon coast in the United States, have recently developed seasonal low oxygen conditions.
Impacts
Ocean deoxygenation affects ocean productivity, nutrient cycling, carbon cycling, and marine habitats. Studies show that oceans have already lost 1-2% of their oxygen since the middle of the 20th century. Models predict that global ocean oxygen levels could decrease by up to 7% over the next 100 years. Scientists expect this decline to continue for more than 1,000 years.
Ocean deoxygenation causes oxygen minimum zones to expand. These zones form when the balance between oxygen sources and sinks in water is disrupted. This change has happened quickly and threatens fish and other marine life, as well as people who rely on ocean life for food or work.
As low-oxygen zones move closer to the ocean surface, they can harm coastal upwelling systems, such as the California Current off the coast of Oregon, United States. These systems depend on seasonal winds that push surface water away from the shore, bringing deeper water up along the continental shelf. When deeper, low-oxygen water becomes shallower, it can reach the continental shelf, causing low oxygen levels near the coast and leading to fish deaths. These events could greatly harm the aquaculture industry.
Changes in ocean chemistry are disrupting the survival of species throughout the ocean food web. As the ocean warms, mixing between water layers decreases, reducing the availability of oxygen and nutrients for marine life.
Short-term effects include sudden, life-threatening situations for marine organisms. Other effects include reduced reproductive success, slower growth, and increased disease rates. These issues occur because stress from low oxygen levels can harm other aspects of an organism’s life. Warmer water holds less oxygen and increases the metabolic rates of marine organisms, causing them to use oxygen faster and lowering oxygen levels even more. Some species may also lose habitat as oxygen-poor zones shrink the areas where they can live.
Long-term effects include changes in biodiversity and food web structures. Habitat changes can alter predator-prey relationships. For example, when species are forced into smaller, oxygen-rich areas, predator-prey encounters increase, which may strain prey populations. Overall, ecosystem diversity is expected to decline due to lower oxygen levels.
The expansion of tropical oxygen minimum zones has reduced the space between these zones and the ocean surface. This change may affect species that live near the surface, such as fish. Research is ongoing to understand how these changes impact food webs in the tropical Pacific and Atlantic. Studies have found that reduced oxygen levels have harmed fish populations and fisheries, likely due to habitat loss when oxygen minimum zones moved to shallower depths.
Fish behavior in response to low oxygen levels depends on their ability to tolerate oxygen-poor conditions. Species that cannot handle low oxygen often move closer to the ocean surface, where oxygen is higher. Some fish, like billfish, may grow faster if their prey also moves to similar areas, making prey easier to catch. Other species, such as jumbo squid and lanternfish, can remain active in low-oxygen environments, avoiding predators and accessing resources that others cannot.
Zooplankton respond differently to low oxygen zones depending on their species and life stage. Some gelatinous zooplankton grow more slowly in low-oxygen conditions, while others use these areas to find food without changes in growth. Some zooplankton may survive in low-oxygen zones by storing oxygen in special parts of their bodies. Changes in zooplankton movement due to deoxygenation can affect fisheries, nitrogen cycling, and food web relationships. These changes may lead to economic and environmental challenges, such as overfishing or disrupted ecosystems.