The dead zone…

by Jim Caryl

rb2IMAGINE that you’re walking along a path in the middle of a prairie, minding your own business. As you’re walking, you find yourself getting out of breath; you find you’re unable to exert yourself as you can’t seem to breath fast enough. Pretty soon you start becoming slovenly, slow moving, dulled; there’s something wrong with the air, there’s not enough oxygen; you’re in the ‘dead zone‘. You try to move away from where you are, but you don’t know where the bad air started. Before long, it’s too late.

A similar experience may happen to deep ocean fish. They need oxygen too, they just manage to get it from the water, but there’s a problem. A report in the April 17th edition of Science, by Peter Brewer and Edward Peltzer (Monterey Bay Aquarium Research Institute, Moss Landing) describes how ‘Ocean “dead zones” [defined as regions where normal respiration is greatly limited and the expenditure of effort is physiologically constrained], devoid of aerobic life, are likely to grow as carbon dioxide concentrations rise.’ In order to understand what their report is about, we need a little background.

We’re all familiar with the phenomenon of rising CO2 levels in the atmosphere, largely attributed to the actions of humans (if you’re not familiar, then get yourself over here and see ‘how it all ends’). It has long been known that the oceans are the largest ‘sink’ for CO2. This happens in two ways: the first, and by far the greater process, is simply via the gas dissolving in the seawater; the second is a biological sink, where the CO2 is used by single-celled plants called algae, becoming fixed in biological matter; these, and the other micro-organisms that then consume them, and any waste matter, all sink toward the bottom of the ocean, holding the carbon in place.

When CO2 dissolves in water, it is stabilised by forming several new chemical species, which effectively reduces the concentration of the CO2 molecule itself in the water; once the CO2 is ‘stabilised’ in this way, more CO2 can dissolve into the water. Liken it, if you will, to putting milk on your cereal. You add milk and it floods the cereal, but the cereal then absorbs all the milk, until the milk is no longer visible. The milk is ‘stabilised’, and now, there’s room for a little more milk. Of course, it is an imperfect analogy as eventually the cereal becomes saturated and can’t take in any more milk; it is thought that the ocean has a theoretical limit to the amount of ‘carbon’ it can hold, but we’re a long way off that, so is not an issue.

The new chemical species that CO2 forms when it dissolves in water are weakly acidic, and this process results in the acidification of the water. Now, in the ocean, as you can guess, there is a very complicated interaction between numerous cycles that continually restore balance; these processes are very slow moving. For CO2 to be stabilised in the ocean, other minerals such as calcium are required; this calcium comes from the shells and coral of organisms in the upper levels of the ocean, and it takes time (understatement!) for the slightly acid water to leach the calcium from this biological source of calcium and for this to be distributed throughout the ocean. There is a concern that this process cannot occur fast enough to help deal with the rising CO2 levels, but on the flip side, increasing acidification can kill off such organisms damaging the delicate balance of nutrient exchange in the ocean.

Another issue is increased warming of ocean surfaces. As the ocean surfaces warm, less oxygen is dissolved. All gases are more soluble in cold water; this is why the cold water you pour into a glass starts to form bubbles as it warms up; it’s the dissolved gases becoming less soluble and therefore escaping. Cold surface waters are rich in oxygen, and being cold they are denser, so sink, carrying replenishing oxygen into the ocean interior. This initiates a convection current that literally ventilates the oceans. However, warm water acts as a lid, preventing the mixing of surface water with the deeper water, so any oxygen produced as a result of our surface-dwelling algae is not transferred to the ocean interior, it is instead lost to the atmosphere. This is a natural process that varies between winter and summer, thus there are always periods when the interior of the ocean is low in oxygen; but what if the oceans are warmer for longer in a globally warmed planet?

So what has this all got to do with our ‘dead zones‘? Here is where we return to our Science paper. Typically the ocean’s ability to support life has been assessed based upon the concentration of dissolved oxygen, known as pO2. The dissolved CO2 concentration has largely been dismissed upon the ‘unspoken assumption that pCO2 levels are low’. However, the authors suggest that with increasing atmospheric CO2, the oceans chemical relations may be reset. Their concerns are based upon the fact that the balance of oxygen and CO2 in the blood of higher sea animals is delicately balanced (as it is in us too), where a naturally high concentration of oxygen displaces, and subsequently exchanges with, CO2. However, if the ratio of Oxygen:CO2 changes, a higher CO2 concentration can start to displace oxygen, resulting in asphyxia.

[As an aside, a radio programme in the UK – Radio 2 Drive-time with Chris Evans – last week attempted to determine how long you could survive in a car full of air, should you end up under water. They calculated the volume of air in the car, and the volume of air you exchange as you breath per hour, and made their conclusion upon this, but forgot two factors. Firstly, the proportion of oxygen in the air is 21%, but we typically only use 4-6% when we inhale. So we can re-use the air to a certain extent; however, secondly, they forgot that CO2 levels will increase, and as they do, it will start to displace the oxygen in our lungs. We will asphyxiate, despite there still being theoretically sufficient oxygen for use to breathe.]

The authors, Brewer & Peltzer, define a respiration index (RI), essentially a ratio between pO2/pCO2 that can be used to assess the physiological limits of deep-sea animals:

An RI value of ≤ 0 is a formal dead zone for aerobic (oxygen breathing) life, however, even at RI = 0.0 to 0.4, aerobic respiration is not observed.

An RI value of 0.4 to 0.7 is the practical limit for aerobic bacteria.

An RI value of 1 or slightly less, is sufficent for some, but not all, marine life.

An RI value >1 is necessary for marine life.

They hope that their proposed RI calculations will provide a more precise and quantitative way for oceanographers to identify, track and/or predict ocean ‘dead zones’, and monitor how rising oceanic CO2 levels affect this ratio. Their example calculation, comparing the extent of the dead zone between current modern levels with 2x and 3x greater surface CO2 levels, indicated an alarming increase of dead zone area, and this they point out is a understatement, as rising ocean temperatures further decrease oxygen saturation.

Brewer, P., & Peltzer, E. (2009). OCEANS: Limits to Marine Life Science, 324 (5925), 347-348 DOI: 10.1126/science.1170756

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