Monday, March 17, 2008

Hypoxic zones around the world

The World Resources Institute and scientist Bob Diaz of Virginia Marine Institute have compiled a new map of the world's coastal hypoxia zones like the famous Gulf of Mexico "Dead Zone" we discuss in the recent PNAS paper on nitrogen pollution and corn production for ethanol. The new map includes 169 documented hypoxic areas, 233 are areas of concern and 13 areas in recovery.

Bottom-water hypoxia can develop when high input of nutrients like nitrogen promote the excessive algae growth. When algae eventually dies and sinks to the bottom, it decomposes, and that process depletes oxygen from the water.

It is worth noting hypoxia will not arise anywhere simply because nutrients are added. To get things started, you still need to feed the algae, and nutrient pollution does the trick. But certain
coastal areas are more naturally prone to hypoxia.

If, for example, the water column is highly "stratified", by that I mean less dense water lying above more dense water there is little mixing between the surface and the bottom waters (think of making a simple oil and vinegar salad dressing). It is then difficult for oxygen from the air to diffuse to the bottom and replace the oxygen consumed by decomposition.

The outlet of big rivers like the Mississippi can be ideal for hypoxia development because the fresh and therefore lighter water introduced by the river creates a stratified water column. That explains some of the year-to-year dynamics of the hypoxic zones like the Gulf Dead Zone. First, hypoxia development can be much worse in a wet or flood year because of the addition of more nutrients and the increased stratification. Second, if a hurricane blows through the Gulf, it encourages mixing just like you do by shaking that bottle of salad dressing, and can break-up the Dead Zone.

The new map supposedly includes only human-driven cases of hypoxia. Which raise the question, what is the cause of the zone between very sparsely populated Somerset and Cornwallis Islands in Nunavut, in the Canadian Arctic? If you have an answer, let me know.

3 comments:

fragment said...

"We hypothesize that the pools form annually, as the sea ice expels dense brine, which sinks and collects in previously formed ice gouge depressions on the shallow slopes of Resolute Bay. Benthic respiration would be sufficient to drive the stratified water in the pools to anoxia in the absence of currents and turnover, resulting in microbial production of highly toxic sulfides."

Kvitek, Conlan, Iampietro 1998. If they're right, it's not anthropogenic. Maybe there's been more work on it in the last decade...

Anonymous said...

Professor, can we extrude tubing and use a laser to drill small holes. Then place miles of this tubing at lower levels and force air into tubing. Air bubbles will emit 24/7 over the tubing's length and add Oxygen and disturb the water at the needed depth...

Simon Donner said...

If I understand you, the idea is to wire up the continental shelf of the northern Gulf and other areas that experience hypoxia like giant aquariums. Technically, I suppose it is possible (?) but given the scale of many hypoxic zones, it would be a monumental engineering effort.