FAQ: Ocean Deoxygenation

Why is oxygen important for the ocean, and what is ocean deoxygenation?

Oxygen is important in the ocean for the same reasons it is important on land; the plants and animals that live there require oxygen to survive. Deoxygenation is the overall decline in the oxygen content of oceanic and coastal waters. Deoxygenation occurs when oxygen consumption (e.g. from respiration, or breathing) is greater than oxygen replenishment through photosynthesis, ventilation, mixing. Oxygen has two main ways of entering the ocean: surface mixing where air meets the water through wind and waves and subsequent vertical mixing into the ocean interior, and photosynthesis by microscopic phytoplankton or macroalgae that produce oxygen. In contrast, oxygen gets used by the organisms that live in the ocean in the process of respiration.

What causes ocean deoxygenation and how much has occurred?

Human activities are the primary cause of ocean deoxygenation in both coastal environments and in the open ocean. The burning of fossil fuels, deforestation, agriculture, and other activities cause an increase in greenhouse gases like CO_2, which causes the earth to warm. The ocean absorbs more than 93% of the earth’s warming from climate change. Warm water holds less oxygen than cold water, because the higher the temperature, the less soluble oxygen becomes.

Warm surface layers in the ocean prevent oxygen from mixing deeper into the ocean. Mixing is crucial because the ways in which oxygen enters the ocean all occur in the upper layers. In addition to warming, nutrient inputs (like nitrogen and phosphorus) from agriculture or raw wastewater are a major cause of oxygen loss in coastal areas. Excess nutrients cause an increase of phytoplankton, followed by a massive decrease of oxygen by microbes after the nutrients are gone and the phytoplankton die. This is called eutrophication. Warming and excess nutrients also increase the microbial consumption of oxygen.

Globally, about 2% of the oxygen content in the ocean has been lost since the 1960s. The area of low oxygen water in the open ocean has increased by 4.5 million km² and over 500 low-oxygen sites have been identified in coastal waters, including estuaries.

Is ocean deoxygenation occurring everywhere?

The specific drivers of deoxygenation for different regions can vary, and deoxygenation is not uniform across the ocean. Some regions are experiencing oxygen loss at much greater rates than the global 2% total. For example, some areas have already shown oxygen declines of 20-50%; these areas often also have naturally low oxygen that can be exacerbated by deoxygenation (such as Eastern Boundary Upwelling Systems, like California). Oxygen minimum zones occur naturally in midwater areas of the ocean,generally from 100-1,000 m depth, and can also occur in partially enclosed areas such as the Black and Baltic Seas. Ocean deoxygenation is causing these midwater areas to expand. Areas with excess nutrient input to the ocean (eutrophication) also increase deoxygenation. The resulting algal blooms are subject to decay by microbes which consume oxygen, causing hypoxia (oxygen shortage) and generating coastal dead zones,such as in the Gulf of Mexico. This process is common in warm seasons. The number, intensity, and duration of these hypoxic zones are exacerbated by warming temperatures.

How does ocean deoxygenation impact marine life and people?

Deoxygenation is causing a wide range of effects on marine life, including reducing the quality and quantity of suitable habitat, also known as habitat compression, reducing growth rate, changing visual function, interfering with reproduction, and increasing disease susceptibility. Different animals have varied responses to low oxygen. These differences can cause ecosystem-wide changes, by altering the composition, diversity, abundance, and distribution of marine microbes and animals. The abundance and diversity of species can change. Even very small declines of oxygen can affect biodiversity, especially in locations that may be close to physiological thresholds, such as oxygen minimum zones. Mobile animals can sometimes move away from low oxygen areas, however this compression can still have large implications. Fished species have already shown habitat compression as a result of avoiding low oxygen, and locations for aquaculture may be limited due to coastal oxygen losses. Aquaculture animals in net pens may be unable to escape exposure to low oxygen, and aquaculture can also increase deoxygenation. Habitat compression may cause species moving towards the surface to be at higher risk for predation or fisheries capture. Most animals face multiple stressors, and may additionally be affected by warming temperatures, ocean acidification, or pressure from overfishing in addition to ocean deoxygenation.

How do scientists study ocean deoxygenation? 

In order to measure changes in oxygen content on both large and small spatial scales, there must be high quality oxygen measurements. The most common way to measure oxygen is by using an optical sensor, however electrochemical sensors and manual measurements using a chemical titration (Winkler method) can also be used. Oxygen sensors can be mounted on a variety of platforms, including stationary logging moorings, cabled observatories, and profiling CTD instruments that measure conductivity, temperature, pressure, oxygen, fluorescence, and other parameters; CTDs are typically deployed off an oceanographic research vessel. More recently, oxygen sensors have been placed on remotely deployed instruments, including floats (such as through the Argo program), gliders, landers, and even migratory, pelagic animals such as mammals and sharks. These remotely deployed instruments typically give data with high spatial and temporal resolution, and are paired with other important biogeochemical sensors to give scientists a comprehensive view of changing ocean conditions.

The temporal and spatial scale of the data required, in addition to the platform the sensors are mounted on, are specific to the question scientists want to ask. For example, measuring long-term trends in ocean deoxygenation requires a long history of repeated measurements, but these can be completed every few months. In contrast, measuring daily to seasonal changes in oxygen or for specific events, such as eutrophication, requires more frequent measurements for a shorter time period.

Moving forward, having long-term monitoring programs in multiple locations around the world, as well as combining oxygen measurements with other biogeochemical sensors will allow scientists to determine patterns of ocean change and predict the corresponding biological response to multiple stressors.

What can we do to help reduce deoxygenation?

Reducing greenhouse gas emissions and controlling nutrient runoff to the ocean are the most important ways to reduce ocean deoxygenation. Other actions that eliminate or mitigate climate change will also help reduce the extent of ocean deoxygenation. Incorporating science into management and policy is essential.

What are science priorities for ocean deoxygenation?

Leading experts at Scripps suggest that scientists should focus on increasing the availability of high resolution oxygen measurements in a diversity of marine environments, and increase the ability of models to predict current and future locations and effects of low oxygen. Multiple stressor experiments are critical to determine how deoxygenation, warming, and acidification interact to impact marine life. Assessments should detail the effects of deoxygenation on human economies and societies to prevent oxygen declines further affecting fisheries, aquaculture, and livelihoods.

Summary of Deoxygenation Effects:

1. Increasing temperatures will reduce the capacity of the ocean to hold oxygen in the future;
2. Oxygen deficiency is predicted to worsen in estuaries, coastal areas, and oxygen minimum zones in the open ocean;
3. Habitat loss is expected to worsen, leading to vertical and horizontal migration of species;
4. Oxygen deficiency will alter biogeochemical cycles and food webs;
5. Lower oxygen concentrations are projected to result in a decrease in reproductive capacity and biodiversity loss;
6. There are important local decreases of commercially important species and aquaculture production;
7. Harmful algal blooms might benefit from nutrients released in bottom waters due to hypoxia (e.g. in the Baltic Sea);
8. Reduced ocean oxygen concentrations will lead to an increase in greenhouse gas emissions, thereby initiating feedbacks on climate change;
9. An insufficient number of high-resolution oxygen measurements are being made globally;
10. Future scenarios for oxygen depend on a combination of drivers related to global environmental change and land-use, including warming, growing human population, and extensive coastal agricultural practices, which, in turn, act together in affecting marine ecosystems – thus, a multi-stressor approach is important.

Expert Reviewers:

• Lilly McCormick, Biological Oceanographer and Postdoctoral Fellow 
• Lisa Levin, Biological Oceanographer and Marine Ecologist

Recent Press Releases:

• Deep sea fish community structure strongly affected by oxygen and temperature
• El Niño impacts on Southern California estuaries reveal potential for more frequent closures
• IUCN to release report on ocean deoxygenation 
• Low oxygen levels could temporarily blind marine invertebrates 
• In waters nearly oxygen-free, researchers find thriving fish populations

Additional Resources:

• Changing Waters Podcast: Deoxygenation and the Oceans in the UN with Dr. Lisa Levin
• Birch Aquarium at Scripps Perspectives on Ocean Science Lecture: Oxygen minimum zones in a warming climate 
• Birch Aquarium at Scripps Perspectives on Ocean Science Lecture: Biological impacts of oxygen in the ocean

References:

1. Breitburg et al., “And on top of all that…: Coping with ocean acidification in the midst of many stressors.”
2. Breitburg et al., “Declining oxygen in the global ocean and coastal waters.”
3. Diaz et al., “Spreading dead zones and consequences for marine ecosystems.”
4. Keeling et al., “Ocean deoxygenation in a warming world.”
5. Levin, L.A., “Manifestation, Drivers, and Emergence of Open Ocean Deoxygenation.”
6. Pörtner, H.-O., “Oxygen-and capacity-limitation of thermal tolerance: a matrix for integrating climate-related stressor effects in marine ecosystems.”
7. Rabalais et al., “Eutrophication-driven deoxygenation in the coastal ocean.”
8. Rabalais et al., “Dynamics and distribution of natural and human-caused coastal hypoxia.”
9. Roman et al., “Impacts of hypoxia on zooplankton spatial distributions in the northern Gulf of Mexico.”
10. Schmidtko et al., “Decline in global oceanic oxygen content during the past five decades.”
11. Sperling et al., “Biodiversity response to natural gradients of multiple stressors on continental margins.”
12. Stramma et al., “Ocean oxygen minima expansions and their biological impacts.”
13. Sydeman et al., “Climate change and wind intensification in coastal upwelling ecosystems.”
14. Vaquer-Sunyer and Duarte. “Temperature effects on oxygen thresholds for hypoxia in marine benthic organisms.”