Oxygen Minimum Zones

Declining Oxygen in the World’s Ocean and Coastal Waters

Source for the following material unless otherwise noted:   Breitburg, D., Grégoire, M. and Isensee, K. (eds.). Global Ocean Oxygen Network 2018. The ocean is losing its breath: Declining oxygen in the world’s ocean and coastal waters. IOC-UNESCO, IOC Technical Series, No. 137 40pp.  The Ocean is Losing its Breath

Significance:

  1. Insufficient oxygen reduces growth, increases disease, alters behaviour and increases mortality of marine animals, including finfish and shellfish. The quality and quantity of habitat for economically and ecologically important species is reduced as oxygen declines.
  2. Finfish and crustacean aquaculture can be particularly susceptible to deoxygenation because animals are constrained in nets or other structures and cannot escape to highly-oxygenated water masses.
  3. Deoxygenation affects marine biogeochemical cycles; phosphorus availability, hydrogen sulphide production and micronutrients are affected.
  4. Deoxygenation may also contribute to climate change through its effects on the nitrogen cycle. When oxygen is insufficient for aerobic respiration, microbes conduct denitrification to obtain energy. This produces N2O – a powerful greenhouse gas – as well as N, which is inert and makes up most of the earth’s atmosphere.

Impacts of Excess Nutrients (eutrophication) on ocean oxygen

Nutrients – primarily nitrogen and phosphorus – from human waste, agriculture and industry, fertilize coastal waters. In a process called eutrophication, these nutrients stimulate photosynthesis, which increases the growth of algae and other organisms (See figure below). This results in more organic material sinking into deep water and to the sediment. Increased respiration by animals and many microbes eating or decomposing this organic material uses oxygen. The consequence can be oxygen concentrations that are far lower than those that would occur without human influence, and in some cases a complete lack of oxygen in bottom waters. Strong density differences between surface and bottom waters (referred to as ‘stratification’), due to temperature and salinity, can isolate bottom waters from the atmosphere and reduce or prevent re- oxygenation through ventilation. Semi-enclosed seas (e.g. the Black and Baltic Sea) can be sensitive to eutrophication and related deoxygenation because of their characteristic limited water exchange with the open ocean, and low ventilation rates.

Eutrophication:  nutrient flux of nitrogen and phosphorus from human waste, agriculture and industry, fertilize coastal waters


Impacts of excess nutrients (eutrophication) on ocean oxygen. (Figure modified from https://upload.wikimedia.org/ wikipedia/commons/d/dd/Scheme_eutrophication-en.svg)

Deoxygenation Facts

Oxygen Minimum Zones (OMZs)

OMZs are places in the world ocean where oxygen saturation in the water column is at its lowest, are shown in blue.  Areas with coastal hypoxia are shown in red.   Hypoxic conditions are often defined as 2 mg/L O2.

The number of water bodies in which hypoxia associated with eutrophication has been reported has increased exponentially since the 1960s; hundreds of systems worldwide have been reported with oxygen concentrations <2 mg L-1, lasting from hours to years. The increasing severity and prevalence of this problem reflects the invention and increasing use of synthetic fertilizers and the growing human population. Global fertilizer use shown includes data of fertilizers with nitrogen, phosphate and potash.

World’s two largest dead zones

Source:  European Environment Agency Ocean Oxygen Content Indicator Assessment 


Distribution of oxygen-depleted ‘dead zones’ in European seas

Oxygen-depleted zones in the Baltic Sea have increased more than 10-fold, from 5 000 to 60 000 km2, since 1900, with most of the increase happening after 1950. The Baltic Sea now has the largest dead zone in the world. Oxygen depletion has also been observed in other European seas in recent decades.

The Gulf of  Mexico dead zone is the world’s second largest in the world

Source:  EPA Gulf of Mexico Dead Zone


(The Mississippi River/Gulf of Mexico Watershed Nutrient Task Force, a group working to reduce the Gulf dead zone through nutrient reductions within the Mississippi River watershed, has set a 5-year average measured size target of 1,900 square miles.)

2019 Forecast: Summer Hypoxic Zone Size, Northern Gulf of Mexico

NOAA and the United States Geological Survey (USGS) released their 2019 forecast for the summer hypoxic zone size in the Northern Gulf of Mexico on June 10, 2019. Scientists are expecting the 2019 area of low oxygen, commonly known as the ‘Dead Zone,’ to be approximately 7,829 square miles, or about the size of Massachusetts. This prediction is large primarily because of high spring rainfall and river discharge into Gulf.

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