CNET también está disponible en español.

Ir a español

Don't show this again

Sci-Tech

Troubled waters for ocean 'dead zones'

newsmaker Robert Diaz, a leading marine scientist working with the U.N., explains the increasing phenomenon of lifeless habitats.

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.

Diaz is also a contributor to the U.N.'s "Global Environment Outlook Year Book 2006."

Q: Much has been talked about "" 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.

The World Meteorological Organization/UNEP "Scientific Assessment of Ozone Depletion: 2006" (click for PDF) said that despite the Montreal Protocol, which curbed world use of ozone pollutants, the ozone hole is still not going to repair itself completely until 2065, and it will probably get worse before it gets better. Is this the situation with the world's dead zones? If the U.N. can get countries to agree to curb ocean pollution, will we see an immediate improvement, or is this another situation where Earth will need time to recover?
Diaz: It may be a little bit of both if you look at areas where, basically, nutrient management--either from runoff, sewage treatment improvements or erosion control in systems--has been implemented. For example, in the Mersey Estuary that runs by Liverpool or the Mondego Estuary in Portugal, these low oxygen zones have been drastically reduced and eliminated, and over a very short period of time. How long was that?
Diaz: It was called severely hypoxic by researchers in 1997 and here we are in 2006. And I think changes in hydrology and runoff control have eliminated the hypoxia--I think eliminated it by 2003, 2004. So within a few years, you can get the system to respond.

Then there are other places like the Chesapeake Bay where there has been pretty intense nutrient management in terms of what runs into the bay and there have been some measurable reductions, but the level of hypoxia hasn't really gone down. The Chesapeake is a system that may be prone to hypoxia, so it has a longer memory and will probably take several more decades before we see significant improvements.

But I think we would have other areas, some of the low-oxygen zones in Europe, that I think would improve very quickly with nutrient management.

Which zones?
Diaz: The central part of the Dead Sea is actually the largest anoxic, no-oxygen, zone on Earth and it's natural in origin. But to the north, along the Ukrainian area, the Volga River comes in there and that carries a lot of nutrients and other things into the shallow sea on the continental shelf, and creates an annual dead zone that is even larger than the one we have here in the U.S.

It's interesting because when agricultural industrial subsidies were eliminated for a period of time--when the Soviet empire was breaking up--there was a period following right after where nutrients and pollution going into the Volga were drastically reduced, and this produced an almost instantaneous decline in the area of hypoxia. So, even large systems can respond very quickly. Now that nutrients are coming back in, the hypoxia has increased again.

So how are dead zones affected by wind, wave movements and ocean current? And what about more drastic things, especially in the case of the Gulf of Mexico, like hurricanes?
Diaz: Yes, winds and storms are positive in terms of mixing water and breaking up hypoxia. So in a sense, the rough weather is one way of getting rid of the low-oxygen areas.

If you look at the Gulf of Mexico, dead zones...If the storms come earlier, then the hypoxia breaks up earlier; if they come later, it breaks up later.

What about NASA scientists saying that Earth is the warmest it has been in a million years, especially in parts of the Pacific Ocean? How is that affecting dead zones?
Diaz: Well now, that's a damn good question, as far as the sea surface temperatures are really what control our weather and climate from year to year. If we get shifts in our weather patterns, which will mean shifts in rain patterns, then I think what you would find is areas that tend to get wetter with more rainfall will probably tend to get more hypoxia. Areas that get drier will probably tend to have less hypoxia.

Why is that?
Diaz: Well because, in the Gulf of Mexico that's a very good example there, where the size of the dead zone is really highly correlated with the runoff from the Mississippi River, high (runoff) flow years produce more hypoxia than low-flow years.

The same thing happens in the Chesapeake Bay where the size of the low-oxygen area is highly correlated with spring runoff. So as the globe warms up, and climate...and weather patterns change, weather systems will probably increase in hypoxia because in a dry year you don't get as much as stratification; you know, that isolation factor.

I am working with a group of people from University of Delaware in a small tributary up in Delaware (Pepper Creek) looking at the effects of low oxygen...Right now, we are in the middle of trying to determine if (the fish) are actually taking advantage of stressed invertebrates.

When you are referring to stressed invertebrates, what type of marine life is that?
Diaz: Oh, small worms, small clams, little small amphipods. The fish are eating off of the small invertebrates because the invertebrates for the most part cannot move. They have to sit tight and try and survive. And one of the common survival strategies is to basically come out of the sediment and lay on the surface of the sediment, and hope enough oxygen washes by you to keep you alive until oxygen returns. And then you can burrow back down into the sediment.

This is what happened to the Norway lobster, which were in a big fishery on the west coast of Sweden and Denmark. The fishermen were catching lots of these lobsters, and unusually high catches, but what they didn't understand was that they were all oxygen-stressed.

Does that make them dangerous to eat?
Diaz: No, not at all. But what it does is it eliminates their habitat. In one year, the lobster fishermen trawled up a record catch. The next year, they didn't get any because they had all died. And right now...Kattegat (a 2,000- to 3,000-square kilometer area) doesn't support the lobster fishery.

So, they had a record catch one year, and then the next year they had none?
Diaz: Yeah. So that's one of the better examples of the direct effect of low oxygen on a fishery species. It stresses them and changes their behavior. They are easy to catch, but then oxygen gets a little lower and they die. And they just don't recuperate, and the area just doesn't support the fishery anymore.

You and your team created a map of the world's dead zones. How do you find them?
Diaz: I have some students and we regularly look through the literature that's published to find accounts of oxygen or fish kills-- anything that would indicate that there is some environmental anomaly. When we find one, we track it down to look at it in more detail to see if it was an oxygen problem. If it is, then we list it. Well, we are investigating over 200 areas. I think we have about 175 confirmed to date that are actually low-oxygen zones that are associated with human activity.

Are you saying that this is being added by fertilizer runoff, or are there other types of things?
Diaz: There are really three major areas that are probably contributing: people through sewage, and people through needing food, and people through burning fossil fuels. So the answer comes down to too many people.

In almost all cases the two most obvious sources are sewage from people and runoff from farm land, agricultural land. The other source, which is less clear (to the public), is atmospheric deposition, which can be very significant. There is a lot of nitrate in fossil-fuel burning. I think something on the order of 25 percent of the nitrogen, that enters the Chesapeake Bay, comes in through atmospheric deposition: power plants, cars, you know, anything that burns fossil fuel.

So, I think it's a little difficult to sort of point out that it's agriculture that's really the problem. Agriculture is just responding to the needs of people, the real problem is just too many people..., agriculture concentrated in the middle part of the U.S. It's just a bad combination for the environment.

What do you offer as a suggestion?
Diaz: I knew you were going to ask that. I don't know. I honestly don't know what the answer is, other than smarter agriculture practices, better erosion control--if you can keep things from getting into the watersheds, then the problem will be solved--better recycling, better treatment of waste.

Is there anything else you think people need to know about this?
Diaz: Well, my impression is that these low-oxygen zones, these dead zones, really are a major environmental problem that we need to deal with quickly because the consequences to our fisheries resources are really pretty (messed up). If you would look, a lot of these zones have eliminated fishing and from different areas, primarily in Europe.

So far in the U.S., (there are) small areas where fish aren't collected anymore, but in Europe there are some really large areas, and the Baltic Sea for example...

Let me stop you. Is that due to U.S. regulation of runoff and pollution, or is that just due to the location of the North American continent and the situation of surrounding waters?
Diaz: Well, the Clean Water Act that was passed in the '70s was instrumental really in starting the cleaning up and putting attention on water quality...The nature of our coastline and circulations have also helped. We have not had the mass mortalities that have been reported elsewhere in the world.

Of course, now I think that the population is so large that we have to really start thinking hard about doing more to control water quality or improve water quality.

Now, you were going on to explain something about the Baltic Sea?
Diaz: That's probably the largest hypoxic area attributed to humans on Earth. It depends on the year, but it's about 800,000 square kilometers and there you have an area that is permanently--well I shouldn't say permanent--persistently hypoxic, so the oxygen is low all year-round.

It's low all year-round, so you have no bottom fish in it, and the thickness of this layer is increasing.

Basically, what happens is the cod lay their eggs in the water column. They sink down, and they sort of settle in at this stratified level because they are sort of floating on higher salinity water. And the hypoxia reaches up, and eliminates lots of the eggs and kills them.

So, this is a pretty serious consequence for the fishing industry there. You don't need too many years of that happening, year after year, before the stocks are eliminated.