Anoxic marine basins, also known as oxygen minimum zones (OMZs), are areas of the ocean with extremely low levels of dissolved oxygen. They occur naturally, mainly on the eastern edges of ocean basins at depths of about 300-1000 meters. The level of dissolved oxygen in OMZs drops below 0.5 ml/L, compared to typical ocean values of 4-5 ml/L.
Key Features
Formation
- Caused by high nutrient runoff from land that stimulates phytoplankton blooms
- Bacterial decomposition of dead phytoplankton depletes oxygen
- Slow replacement of oxygen from atmosphere due to depth and ocean circulation
Location
- Found along eastern boundaries of oceans where wind-driven surface circulation draws deep, nutrient-rich waters to the surface
- Major OMZs occur off Peru/Chile, West Africa, and the Arabian Sea
Effects
- Supports unique, adapted ecosystems
- May exacerbate climate change through production of nitrous oxide, a potent greenhouse gas
- Can encroach on coastal habitats, threatening fisheries and aquaculture
Regional OMZs
Arabian Sea
- The world’s thickest and most intense OMZ
- Caused by monsoon-driven upwelling and high primary productivity
- Oxygen levels can approach anoxia (total depletion)
- Supports sulfide-oxidizing bacteria, giant foraminiferans, fin whales
Bay of Bengal
- Less severe than the Arabian Sea OMZ
- Linked to seasonal changes in monsoon winds and river runoff
- Harbors diverse microbial taxa, many new to science
Eastern Tropical North Pacific
- Found off Mexico and parts of Central America
- Encroaching on coastal ecosystems and fisheries due to climate change and nutrient pollution
- Significant emissions of nitrous oxide, a powerful greenhouse gas
Eastern Tropical South Pacific
- Located off Peru and Chile
- One of the more permanent, stable OMZs
- Associated with the Humboldt Current upwelling system
Effects and Issues
On Marine Life
- Most animals cannot survive sustained oxygen levels below 0.5 ml/L
- OMZs support sulfate-reducing bacteria, giant protozoans, jellyfish, andcephalopods
- Can force coastal species into shallower waters
Climate Change
- Expansion of OMZs observed in recent decades
- May release nitrogen and greenhouse gases, causing feedback loops
- Interacts with ocean warming and acidification
Water Column Biogeochemistry
- Various redox reactions occur, cycling nitrogen, sulfur, metals
- Includes denitrification, loss of fixed nitrogen as N2 gas
- Affects carbon cycling and sequestration in the ocean
Key Statistics and Data
| Statistic/Data | Arabian Sea OMZ | Eastern Tropical North Pacific OMZ |
| Minimum Oxygen Concentration | ~0 ml/L | ~0.1 ml/L |
| Approximate Surface Area | ~1.1 million km2 | ~1.8 million km2 |
| Seasonal Variation | High due to monsoons | Lower due to more consistent upwelling |
| Key Microbial Types | Thioploca, Beggiatoa, Giant Foraminiferans | Nitrifying and Anammox Bacteria |
| Nitrous Oxide Production | Lower | Higher |
| Effect on Fisheries | Variable | Negative Impacts Observed |
Recent Developments
- Recent studies published in early 2023 indicate oxygen minimum zones are expanding faster than predicted, driven by factors like climate change and nutrient pollution from agriculture. Up to seven percent more ocean area may become oxygen deficient by 2100 according to the latest models.
- A major study released in June 2022 provided strong evidence that oxygen minimum zones are significant sources of both carbon dioxide and the powerful greenhouse gas nitrous oxide, which has a global warming potential 300 times greater than carbon dioxide. This represents an unaccounted feedback loop between ocean deoxygenation and climate change.
- In December 2022, the UN Decade of Ocean Science published a report on declining ocean oxygen levels that called for improved sewage treatment and sustainable agriculture practices to reduce nutrient runoff. It also recommended policies to reduce greenhouse gas emissions in order to limit ocean warming trends that reduce oxygen solubility.
- Fisheries located near oxygen minimum zones are being increasingly impacted. A paper in early February 2023 noted catches of squid, tuna, and other marketable fish may decline by over 25% off the coasts of Chile and Peru if current deoxygenation trends continue. Lower dissolved oxygen is forcing species migrations and altering food chains.
- Researchers discovered in August 2022 new adaptations that allow “extremophilic” microorganisms like giant sulfur bacteria to thrive in almost completely oxygen-depleted conditions within the Arabian Sea dead zone, raising questions about just how little oxygen deep water species can survive on. This points to the need for further study of these still poorly understood ecosystems.
Anoxic basins occur naturally but may be expanded by climate change.
- Support unique organisms yet can encroach on coastal habitats
- More study needed of biogeochemistry and greenhouse gas production
- Changes to OMZs have unclear, complex effects on cycling of elements.