Category: Biogeochemical Flows

Nitrogen and Phosphorus

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


  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 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.

Biogeochemical Flows: Nitrogen

Planetary Boundary

  • Global boundary: 62 Tg N/year
  • Source: *Steffen, W., K. Richardson, J. Rockström, S.E. Cornell, 2015. Planetary boundaries: Guiding human development on a changing planet. Science 347: 736, 1259855
  • Overall, the fixation of nitrogen through Haber–Bosch (120 Tg N yr−1) in 2010 was double the natural terrestrial sources of Nr (63 Tg N yr−1).
  • The overall magnitude of anthropogenic relative to natural sources of fixed nitrogen (210 Tg N yr−1 anthropogenic and 203 Tg N yr−1natural) is so large it has doubled the global cycling of nitrogen over the last century. 
  • The overall magnitude of anthropogenic relative to natural sources of fixed nitrogen (210 Tg N yr−1 anthropogenic and 203 Tg N yr−1 natural) is so large it has doubled the global cycling of nitrogen over the last century.
  • Source: Fowler, et al., The global nitrogen cycle in the twenty-first century, Philos Trans R Soc Lond B Biol Sci. 2013 Jul 5; 368(1621): 20130164.


The supply of Nr–reactive nitrogen (NH3, NH4, NO, NO2, HNO3, N20, HONO, PAN and other organic N compounds) is essential for all life forms, and increases in nitrogen supply have been exploited in agriculture to increase the yield of crops and provide food for the growing global human population. It has been estimated that almost half of the human population at the beginning of the twenty-first century depends on fertilizer N for their food. (Source: Fowler, et al.)

As nitrogen is a major nutrient, changes in its supply influence the productivity of ecosystems and change the competition between species and biological diversity. Nitrogen compounds as precursors of tropospheric ozone and atmospheric particulate material also degrade air quality. Their effects include increases in human mortality, effects on terrestrial ecosystems and contribute to the radiative forcing of global and regional climate. (Source: Lee, et al., see figure below)

Global N budget. Numbers represent global land N storage in TgN or annual N exchange fluxes in TgN yr−1 for contemporary (1991–2005 average) and preindustrial (1831–1860 average in parenthesis) times. See notes for this figure and the table below in:  Lee, et al., Prominence of the tropics in the recent rise of global nitrogen pollution.  Nature Communications (2019)10:1437  and Supplementary Table 1 (modified below) and Supplementary Note 1 ( 019-09468-4.)
Nitrogen Stores & Fluxes Published Estimates 1990’s Published Estimates 2000’s
Biological N Fixation 112; 139  
Agricultural 32 50-70
Preindustrial 58; 195  
Non-agricultural NA  
Natural 107; 128  
Atmospheric Deposition 59  
Haber-Bosch (Synthetic fetilizers) 100 120
Fluxes to the ocean 48 45
Fluxes to the atmosphere 189  
Denitrification N2 115 96
Other emissions 74 70
Fluxes to the land storage 60 27
Soils/litter storage 95,000  
Fluxes:  TgN/year    
Storage:  TgN    

Biogeochemical Flows: Phosphorus

Planetary Boundary

  • Global P Boundary: 11 Tg P/year*
  • Regional (watershed) P Boundary: 6.2 Tg P/year*
  • Current global rate of P fertilizer to croplands* (primary source of P to regional watersheds): 14.2 Tg P/year*
    • Total P flow through international agricultural trade increased from 0.4Tg to 3.0 Tg between 1961-2011**
    • The fraction of P taken up by crops that is subsequently exported increased from 9% to 20% between 1961 and 2011**
    • Global P flows through international trade of agricultural products have become an important feature of the global P cycle, accounting for 20% of the P in global crop production, 17% of the P globally used as mineral fertilizer, and 27% of the P that was traded as mineral fertilizers in 2011.**
  • (Sources: *Steffen, W., K. Richardson, J. Rockström, S.E. Cornell, 2015. Planetary boundaries: Guiding human development on a changing planet. Science 347: 736, 1259855, **Nesme, T.,  G.S. Metson, and E.M. Bennett. 2018. Global phosphorus flows through agricultural trade. Global Environ. 50:133–141.Change 50:133–141. doi:10.1016/j.gloenvcha.2018.04.004

*Phosphorus and Agricultural Trade

Global phosphorus fertilizer application to cropland


  • Critical element for all living organisms
  • Availability drives the productivity of many aquatic and terrestrial ecosystems worldwide
  • In agricultural systems, additional P can be supplied to soils as mineral fertilizer or manure to support crop growth and sustain high yields
  • Mineral P fertilizer production is dependent on the physical and economic availability of mined rock phosphate resources (non-renewable, diminishing, geopolitically concentrated)
    • The P cycle has been greatly transformed since the pre-Industrial era through increased agricultural mineral P fertilizer use
  • P losses to water bodies through runoff and erosion from fertilized agricultural soils and from the inadequate management of animal manure or human excreta has led to aquatic eutrophication
  • International trade of agricultural products (food, feed, fiber and fuel) are a key component of the global phosphorus cycle; agricultural flows of P are driven by trade of cereals, soybeans, and feed cakes
  • 28% of global P traded in human food, 44% in animal feed and 28% in crops for other uses in 2011