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