THEGREENGROK

Ocean Fish Sing: Where Has All the Oxygen Gone?


by Bill Chameides | March 2nd, 2010
posted by Erica Rowell (Editor)

Permalink | 3 comments

Can global warming make it harder for fish to breathe?

An upcoming article documents a decline in oceanic oxygen. And, believe it or not, there is a likely link between it and global warming.

Temperature Effect on Oxygen Relates to Solubility of Gases

As you increase the temperature of water, the solubility of gases in that water deceases. Seawater is, well, water, and its temperature has been, on average, increasing over recent decades as a result of global warming. Oxygen is a gas. It follows, therefore, at least in theory, that global warming is causing a decline in the amount of oxygen dissolved in the ocean.

Ocean Circulation Slows as Waters Warm

As global warming increases the amount of heat entering the ocean surface, the rate of vertical mixing between surface waters and deep ocean waters slows. This drag in the mixing rate can also deplete oxygen in areas of the ocean that lie between the surface layer and the deep ocean — a region, as we will see below, that is most sensitive to a decreased oxygen supply.

So What? So Get Ready for the Possibility of More Dead Zones

For most of the ocean’s water, less oxygen resulting from global warming should not be a problem. The ocean’s concentrations of oxygen are high enough already and the expected decline in oxygen is too small (estimated to be between four and seven percent by the end of the century) to affect significantly the abilities of fish and other ocean respirers to breathe.

But there’s one underwater region where a small decrease in oxygen can make a huge difference: the oxygen minimum zone (OMZ), which lies some 200 to 1,000 meters below the surface. The OMZ is produced by the rain of dead phytoplankton sinking from the photic zone, the upper layer of water that permits sunlight in, thus allowing photosynthesis to occur.

As the phytoplankton sink, they decay, consuming oxygen in the process. In the OMZ the rate of decay is at a maximum, with phytoplankton consuming relatively large amounts of oxygen — thus the term oxygen minimum zone or OMZ.

Most of the time the severity and extent of the OMZ are relatively mild, and the impact on life in the zone is minimal. But every once in a while the oxygen depletion can become so severe that large areas can become uninhabitable for many forms of life.

The concern in the case of climate change is that the expansion of the OMZ into typically more oxygenated zones could bring about a major ecological disruption over large regions of the ocean.

(If you need an example of how much damage large swaths of oxygen-depleted areas can do, read about the dead zones caused by nutrient runoff.)

Paper: Oxygen Levels Are Changing

So, is there any evidence that oxygen concentrations are changing? Yes, says Lothar Stramma of the Leibniz Institute of Marine Sciences, Germany, and colleagues in a new paper to be published in the journal Deep Sea Research I.

Comparing datasets of oxygen concentrations in the world’s ocean gathered between 1960–1974 and 1990–2008, the authors found evidence of oxygen decline in the OMZ over huge swaths of the tropical Atlantic, Pacific, and Indian Oceans. They also found oxygen levels in more than 4.5 million square kilometers of the OMZ (an area that would be about six percent of the Atlantic Ocean) had already fallen to levels sufficient to cause anoxic stress for some organisms. 

The authors did not find universal oxygen declines. Upticks in oxygen levels were found in regions of the subtropics (for example in a swath of the Atlantic Ocean extending from Florida to North Africa). These areas of increased oxygen are consistent with predicted changes in ocean circulation.

What are the implications of this decrease in oxygen for the world’s oceans? The authors offer some speculation, including these two possibilities: the abundance of jellyfish may become even more abundant while the restriction of tuna et al. fish to the ocean surface layer might make them even more susceptible to overfishing. There is also some evidence to suggest that increased ocean hypoxia could lead to more production of nitrous oxide, a powerful greenhouse gas.

Speculations are one thing, but we will only know for sure when we do the actual experiment. Some might argue that this is one of those experiments we’d be better off not doing. But given current political currents, the odds are high that we will continue with it.

Probably just my imagination, but do I hear a gurgling refrain of “when will they ever learn?”

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3 Comments

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  1. Ken Towe
    Mar 6, 2010

    Substituting “CO2” for “oxygen”: As you increase the temperature of water, the solubility of gases in that water deceases. Seawater is, well, water, and its temperature has been, on average, increasing over recent decades as a result of global warming. CO2 is a gas. It follows, therefore, at least in theory, that global warming is causing a decline in the amount of CO2 dissolved in the ocean. “As the phytoplankton sink, they decay, consuming oxygen in the process. In the OMZ the rate of decay is at a maximum, with phytoplankton consuming relatively large amounts of oxygen” Remember your Oct 28 post on the Arctic phytoplankton? Phytoplankton do not consume oxygen, they release it. It is the “marine fauna”, animal plankton, nekton who consume the oxygen, not the phytoplankton. The “marine fauna” are heterotrophs, the ones who use oxygen to recycle the photosynthetic carbon back to CO2. It is their high population at those shallow depths and their respiration that creates the OMZ. In our quixotic(?) zeal to manage the planet (while increasing our population) we cannot have it both ways… “seeding” iron to tie-up CO2 into organic carbon? Yet then worry about organic carbon lowering oxygen during its recycling. The only effective way to “hide” CO2 biochemically is to increase LONG-TERM organic carbon burial. This will increase net oxygen, not decrease it.

    • Bill Chameides
      Mar 11, 2010

      Ken, When phytoplankton are doing photosynthesis, they produce oxygen; when they are not, they are most definitely consuming oxygen. And so when phytoplankton fall below the photic zone, the zone where there is enough sunlight, they are oxygen-consumers. This is not to say that phytoplankton are not a net supplier of oxygen overall, but in the zone where they decay below the photic zone they are net sinks of oxygen.

      • Ken Towe
        Mar 11, 2010

        Aren’t you confusing gross and net primary productivity? Phytoplankton are aerobes and respire, esp. in the dark, using some percentage of the gross oxygen they have produced. But it is their net productivity that counts, not the gross. Then, when they sink below the photic zone they die…photosynthesis stops taking place. At that point (OMZ) they become a total oxygen sink and a source of CO2. Very little of the photosynthetic primary productivity ever reaches the sea floor. Phytoplankton at the base of the food chain support the marine fauna…consuming oxygen just like you and I do when we metabolize our spinach. This is why iron seeding experiments cannot help much, long-term, to tie up CO2 unless it is primarily the coccoliths with CaCO3 scales that are the ones being fed the iron. The CO2 in this calcite may sink to the bottom after their Corg has been consumed. Unfortunately, most of the high-latitude phytoplankton being fed iron are diatoms with opaline silica frustules. Silica contains no CO2 and will provide little help, long-term in sequestering CO2.

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