"Dead zones" are multiplying along the world's coasts. There are now more than 400 areas where the bottom waters have too little oxygen to support life. Scientists say these polluted regions pose the single greatest threat to coastal ecosystems, as Véronique LaCapra reports.

In this NASA SeaWiFS satellite image of the Black Sea, large algal blooms appear as light blue-colored swirls on the water's surface

When nitrogen from agricultural fertilizers and from the burning of fossil fuels gets into coastal waters, it stimulates the growth of algae. When these plants die, they sink to the ocean floor, and are consumed by bacteria and other organisms. The process uses up oxygen, which is normally replenished by water circulating down from the surface.

But marine scientist Robert Diaz says that doesn't happen if the surface and bottom waters can't mix. "It can be hotter water on top, cooler water on the bottom, or it can be fresher water on top, more salty water on the bottom." When coastal water becomes stratified in this way, respiration by bacteria and other bottom-dwelling organisms deplete the oxygen in the bottom layer of water, creating areas known as "dead zones."

A large "dead zone" sits just off the southern coast of the United States, in the Gulf of Mexico. The red and orange areas in this image represent water that is low in oxygen

Dead zones can be one-time events, or they can recur from day to day or year to year, lasting for just a few hours, or for an entire season. In extreme cases, once oxygen depletion sets in, dead zones can persist for years.

This loss of oxygen has major implications for marine life. Diaz – a professor at the Virginia Institute of Marine Science – says animals like fish, crabs and shrimp will try to escape oxygen-depleted bottom waters.

Less mobile animals like clams and worms cannot escape, and must do whatever they can to survive. Animals that normally stay in the sediment will come out into the water looking for oxygen. They'll stop feeding. And if oxygen levels drop even lower, says Diaz, "they'll try and basically turn off all their little biochemical or physiological systems and just sort of hunker down and wait until better oxygen times come."

And if those better times don't come? "Then what you what you see is mass mortality of all the organisms that are left behind."

In some cases, the drop in oxygen can be so rapid and widespread that even fish can't get away. Off the northwestern coast of the United States, for example, a dead zone is killing fish and other marine life in an area of over 3,000 square kilometers.

Other dead zones are even bigger. Diaz says that the world's largest is in the Baltic Sea. "At one time it was estimated at over 100,000 square kilometers, but it's reduced down now to somewhere between 70,000 and 80,000."

There are other large dead zones, as well, including one in the Gulf of Mexico and another in the East China Sea. Globally, Diaz estimates that these oxygen-depleted areas add up to more than a quarter million square kilometers – an area about the size of Britain or Laos.

Prior to the 1960s, scientists had identified fewer than 50 dead zones worldwide. Since then, that number has roughly doubled every decade. Diaz says that now, "we have well over 400 documented areas around the globe that have some form of low oxygen that is related to human activity."

Diaz expects that even more nutrients will enter coastal waters over the next 50 years, continuing the trend of increasing dead zones around the world. He believes this trend will not end until we can control the amount of nutrients getting into our rivers, estuaries, and seas.

Northeast of the Mediterrean, the Black Sea lies south of the countries of Ukraine and Russia

But if we can reduce the amount of nutrient pollution, Diaz says, we can bring the dead zones back to life. He says the best example is the Black Sea. In the 1970s and 80s, fertilizer runoff from agriculture reduced oxygen levels in the Black Sea over an area of 40,000 square kilometers.

"But with the collapse of the Soviet Union," explains Diaz, "subsidies were eliminated to a lot of the farmers in the area." The amount of nitrogen and phosphorus going into the Black Sea declined dramatically, with equally dramatic results. "Over a period of 3 years the Black Sea went from a 40,000 square kilometer dead zone, to zero."

Up until now, the vast majority of coastal dead zones have been found in the northern hemisphere, where most of the world's population – and aquatic research – is concentrated.

But Robert Diaz fears that tropical regions – whose coastal waters are naturally low in nutrients and oxygen – could be among the most sensitive to the effects of human development. Diaz says that in the tropics, it doesn't take much to upset the balance. "You just add a little bit of nutrients, and you can completely disrupt the way the system works, and create these dead zones." 

Diaz's analysis of global "dead zones" is published in the journal Science.