Marine “Dead Zones” Will Increase, Scientists Predict
So-called “dead zones”–patches of ocean lacking aerobic (oxygen breathing) life–will most likely increase due to a rise in carbon dioxide (CO2) concentrations.
Because of CO2 build up in the atmosphere, its level of concentration in the oceans also increases. There, some of the CO2 converts to a simple acid called carbonic acid. (H2CO3).
What is becoming a much-studied phenomenon in recent years, these dead zones of depleted oxygen (02) - typically found at depths between 300 and 600 meters–are the result of several factors working separately and in combination: lower sea surface 02 levels, less heat exchanging (”ventilation”) with mid-level ocean depths due to over-all warming, and “euthrophication events” (an over-growth of a species due to excess nutrients).
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The lack of adequate ventilation also allows ocean acidification to penetrate more rapidly to lower depths than normal. This acidification of ocean waters, along with rising temperatures and decreasing O2 levels puts serious strains on much of the life that would exist there.
At such depths, little light penetrates, and so, oxygen-producing phytoplanktons do not exist in sufficient amount to absorb CO2 and respire O2 for animals to breathe. Thus fish and other creatures rely on dissolved O2 reaching these depths from oceanic ventilation and mixing between surface and mid-ocean depths. Additionally, as a general rule, the colder the water temperature, the more dissolved oxygen can be contained in any given volume. As ocean temperatures rise, water warms, and this means less dissolved oxygen. As usual in nature, there is seldom one factor alone which can be cited as a cause of any given effect. It is most often a combination of factors, and calculating these factors in order to gain an accurate model or map of things, gets tricky and complex.
Bacteria set the practical limit for all aerobic respiration in the Earth’s oceans. The ability of an aerobic microbe species to survive adequately in water is gauged in terms of the minimum concentration of dissolved O2. When the saturation of dissolved O2 (in parts per million) gets too low, it becomes inefficient for aerobic microbes to consume O2. But microbes are highly adaptive and can switch their metabolic functioning over to manganese (Mn !V) or nitrous oxide (NO3) or iodine oxide IO3).
But larger, oxygen-breathing animals (invertebrates and fish) are not so adaptive. The ‘dead zones ” for these animals are defined as areas in which respiration is severely limited and energy expenditures (to get food, swim to higher O2 areas, etc.) are are highly constrained. There is also evidence that hypoxic (low O2) conditions inhibit normal reproductive activity in most fish species. But calculating and estimating these limits is very challenging because there are no universally agreed upon figures (or formulas) to use when trying to map shifting marine conditions (such as ventilation, acidification, and eutrophication).
Marine scientist understand the chemistry of what’s happening, but they lack a way to quantify this knowledge, and thus make better predictions.
Reporting in the June 12 edition of Science, researchers Peter G. Brewer and Edward T. Peltzer of the Monterey Bay Aquarium Research Institute have developed a “respiration index” (RI) which they believe will give researchers a better picture of how these factors combine, and thus how dead zones are formed. The RI allows researchers to calculate fairly accurately the effect of marine species of shifting chemical relations–shifts (as in reduction/oxidation reactions) that result from increasing CO2 concentrations.
Still, the formulas being used are relatively simple (typically controlling for only one variable at a time, like the fossil-fuel signal CO2) and thus the scientists believe that their formula greatly under-estimates the problem. So, although well-oxygenated ocean regions will be relatively unaffected by rising CO2 (the report’s authors surmise), existing ocean dead zones (whether hypoxic or anoxic - minimal/zero O2 levels) will most likely expand their reaches. With rising CO2 and decreasing O2 saturation (from warming waters), the researchers warn, “the combined effect will be severe.”
A 2008 survey by researchers Diaz and Rosenberg couned 405 ocean dead zones world-wide..
photo credit: (video frame capture) Uwe Kils, GFDL
