Ocean Fish Sing: Where Has All the Oxygen Gone?
by Bill Chameides | March 2nd, 2010
posted by Erica Rowell (Editor)
Can global warming make it harder for fish to breathe?
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?”filed under: animals, climate change, faculty, global warming, oceans
and: anoxia, dead zone, hypoxia, nitrogen, nutrient runoff, ocean minimum one, oxygen, phytoplankton, seafood