olelog

The Sea contains a much higher percentage of oxygen (34 %) than the atmosphere (ca. 21 %).
Most of the oxygen is of course bound to hydrogen in water molecules. A smaller part is bound in molecules of other substances (like calcium carbonate), and only part of the oxygen is freely available for respiration as oxygen gas dissolved in the water. Oxygen occurs as a by-product of photosynthesis in plants (plankton in the open sea). Oxygen is also dissolved at the interface between the sea surface and the atmosphere.
Most of the oxygen-rich ocean water and the animal life which uses oxygen for respiration is therefore found near the surface. As one descends into the depths, the amount of dissolved oxygen in the water drops rapidly, and so does animal life. In a zone occurring at depths of about 200 to 1,000 metres, depending on local circumstances, oxygen saturation in seawater in the ocean is at its lowest. This zone is called the Oxygen Minimum Zone (sometime referred to as the shadow zone). From the oxygen minimum layer downward the amount of dissolved oxygen increases initially, and another decrease occurs near the bottom.

Please note that oxygen profiles like the one shown here vary from location to location. Just now I would like to stress that the occurrence as such of a minimum oxygen zone is a natural phenomenon due to destruction of dissolved oxygen by respiration (something more or less like this CH₂O + O₂ = CO₂ + H₂O). I’ll come back to the degree of oxygen depletion later.

Surface ocean waters generally have oxygen concentrations close to equilibrium with the Earth's atmosphere. In general, colder waters hold more oxygen than warmer waters. This is obvious in the following image of annual mean sea surface dissolved oxygen for the World Ocean. Data from the World Ocean Atlas 2001.

Dissolved oxygen is measured in units as millilitres O₂ per litre (ml/l), millimoles O₂ per litre (mmol/l), milligrams O₂ per litre (mg/l) and moles O₂ m^-3. For example, in freshwater under atmospheric pressure at 20°C, O₂ saturation is 9.1 mg/l. (more about the unit mole at this Wikipedia page). 1 mol of oxygen (O₂) molecules weighs approximately 32 grams. For example an oxygen concentration in surface water of 0.27 mol O₂m^-3 approximates 6 millilitre per litre.

In the oxygen minimum layers the concentration of dissolved oxygen can approach zero, a condition called suboxic. Important mobile macroorganisms are stressed or die under hypoxic conditions; that is, when oxygen concentrations drop below ~60 to 120 mmol kg^-1 (3). Hypoxia occurs at different oxygen concentrations among various species of macroorganisms, so the threshold is not precise. Water lacking dissolved oxygen (0% saturation) is termed anoxic.

As I said above colder waters hold more oxygen than warmer waters. (Salinity is another important factor with differences between fresh water and sea water). The global ocean has warmed substantially over the past 50 years. So what happens during global warming. You would expect reduced oxygen levels and the oxygen minimum zones to expand. Reduced oxygen levels may have dramatic consequences for ecosystems and coastal economies.

According to the study Expanding Oxygen-Minimum Zones in the Tropical Oceans by Stramma et al. published in Science of 2 may 2008 the oxygen minimum zones of tropical oceans are expanding, restricting habitats for fish and other marine life, the researchers found that oxygen levels here have declined significantly over the past fifty years. The study was concentrated on the eastern tropical Atlantic and the equatorial Pacific simply because these areas have the best historical data, the situation may in fact be worse (or better?) in other parts of the world’s oceans. The expected impacts on subtropical and subpolar regions are larger than in the Tropics. Long-term oxygen changes have been observed and reported in the subpolar and subtropical regions.

And now back to the eastern tropical Atlantic and the equatorial Pacific during the past 50 years. The data reveal a vertical expansion of the intermediate-depth low-oxygen zones. The oxygen decrease in the 300- to 700-m layer is 0.09 to 0.34 micromoles per kilogram per year. In the tropical North Atlantic the vertical extent of the layer with oxygen concentrations of <90 mmol kg^-1 increased 85%, from a thickness of 370 m in 1960 to 690 m in 2006. The tropical ocean oxygen minimum zones in the central and eastern tropical Atlantic and equatorial Pacific Oceans appear to have expanded and intensified during the past 50 years.

Oceanic dissolved oxygen concentrations have varied widely in the geologic past. The anoxic ocean at the end of the Permian (251 million years ago) is associated with elevated atmospheric CO₂ and massive terrestrial and oceanic extinctions.

The trends have fundamental implications for marine ecosystems and thereby fisheries.


• http://www.sciencemag.org/cgi/content/abstract/320/5876/655
• http://news.nationalgeographic.com/news/2008/05/080501-dead-zones.html
• http://www.terradaily.com/reports/Oxygen_depletion_threatens_ocean_habitats_study_999.html
• http://www.scientificblogging.com/news_releases/the_decline_in_ocean_oxygen

In my post on Nitrogen and Bogs I wrote that “Biofuel may well be excellent to reduce emissions of the greenhouse gas carbon dioxide (CO₂), but it emits plenty of nitrogen oxides (NO_x), another greenhouse gas and pollutant”. This has now once again been confirmed by a new study led by Nobel prize-winning chemist Paul Crutzen (PJ Crutzen et al, Atmos. Chem. Phys. Discuss., 2007, 7, 11191).

Crutzen and colleagues have calculated that growing some of the most commonly used biofuel crops releases around twice the amount of the potent greenhouse gas nitrous oxide (N₂O) - also known as laughing gas - than previously thought, wiping out any benefits from not using fossil fuels and contributing to global warming.

For rapeseed biodiesel, which accounts for about 80 per cent of the biofuel production in Europe, the relative warming due to N₂O emissions is estimated at 1 to 1.7 times larger than the quasi-cooling effect due to saved fossil CO₂ emissions. For corn bioethanol, dominant in the US, the figure is 0.9 to 1.5. Only cane sugar bioethanol - with a relative warming of 0.5 to 0.9 - looks like a viable alternative to conventional fuels.

• http://www.rsc.org/chemistryworld/News/2007/September/21090701.asp
  

In a report on the impact of biofuels, the Organization for Economic Cooperation and Development (OECD) said biofuels may "offer a cure that is worse than the disease they seek to heal." Biofuels may in fact hurt the environment and push up food prices. When acidification, fertiliser use, biodiversity loss and toxicity of agricultural pesticides are taken into account, the overall environmental impacts of ethanol and biodiesel can very easily exceed those of petrol and mineral diesel.

• http://www.washingtonpost.com/wp-dyn/content/article/2007/09/11/AR2007091100942.html
  

Some of my posts on biofuels:

• Nitrogen and Bogs
  
• Biofuel
  
• Problematic Bioenergy