Growing more food on land may be depleting the oceans of fish.
The United Nations Environmental Program (UNEP) issued a warning on Oct. 19 that the number of "dead zones" in the world's oceans and seas--areas devoid of oxygen and marine life--has increased from 149 in 2004 to 200 in 2006.
The findings were disclosed as part of the U.N.'s Global Programme of Action summit in Beijing in October, where countries met to discuss ways of strengthening environmental laws.
Robert Diaz, a professor at the Virginia Institute of Marine Science at the College of William and Mary, was one of the experts the U.N. relied on for its "State of the Marine Environment" report.
Q: Much has been talked about "
the increase of dead zones" in the Earth's waters. What qualifies as a dead zone? Diaz: A dead zone would be an area where you would expect to find low oxygen, no fisheries resources, and possibly some dead vertebrates or crabs and clams. Somewhere around 3 down to 2 milligrams of oxygen per liter of water constitutes the scientific definition of a dead zone...and where most people think that the effects of the low oxygen are really severe, and start to change what's going on in the bottom or near the bottom.
What are the causes of these dead zones?
Diaz: The very first one is somehow the surface water has to be isolated from the bottom water through some physical process that oceanographers call stratification. It can be temperature, with the hotter water floating on top of cold water, or salinity, with fresher water sitting on saltier water. When you get these conditions, although you can't really see it with your eye, these stratification layers actually block the movement of oxygen molecules from the surface to the bottom or vice versa.
The second is some source of organic matter that settles into the bottom layer and starts to consume the oxygen as bacteria decomposes the organic matter.
So, you have to have an isolation effect, restratification and then you need an organic matter source that drives the oxygen low.
The great lake of Lake Erie is the second largest dead zone we have here in the U.S., actually. It is only second to the Gulf of Mexico zone, and it's created very similarly although it's in fresh water.
Explain how pollutants have an effect on the creation of these dead zones. Diaz: What happens in our coastal and estuarine systems is that nutrients get into the systems, and the primary nutrients are nitrates and phosphates.
When we have too much of them getting in, then the phytoplankton respond by growing more than they typically do. (Just as a dead plant on land is decomposed by bacteria and fungus), phytoplankton sinks to the bottom where the same process happens. The matter decomposes, uses oxygen in decomposition, no new oxygen gets in, the bottom odors become hypoxic, and that's the start of a dead zone.
So, it's not really pollution in the sense of toxics that are creating these. You could say anything that you put into our rivers and estuaries would be a pollutant if it's put in in too large an amount.
The U.N. report said that the food supply is affected because hypoxia (a lack of available oxygen) and the introduction of toxins into the waters means fewer fish, shellfish and ocean vegetation. But are these dead zones also contributing to the amount of fish carrying toxins? Diaz: I would think not, because the fish avoid these zones...There is a lot of very good data from the Gulf of Mexico and from Europe that clearly show when the oxygen gets down, that is 3 to 2 mg level, the fish just disappear.
Though the low oxygen does create some problems for mobilizing some elements that are in the sediments, so...things that were sequestered in the sediments are now leaving the sediments when the oxygen gets very low. In marine areas, phosphate comes out of the sediments at low oxygen, and then at very low levels, we get hydrogen sulfide coming out of the sediments. And that's not very good for animals.
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