Geoscience Reference
In-Depth Information
mid-twentieth century the amount of reactive nitrogen at the Earth's
surface has roughly doubled. This means that one species—the
human species—is putting as much nitrogen into the Earth system as
the rest of biology put together.
Not all of these fertilizers go where they are intended, into the
crops. Part is washed from the fields into streams and rivers, and then
into the sea. There they stimulate plankton blooms that, on dying, fall
to the sea bottom. In those regions where fertilizer inwash is high and
the sea's circulation is naturally sluggish, the decaying plankton can
use up all the oxygen. The result is suffocation for all the animals liv-
ing on, in, or just above the sea floor. There are now about 400 such
dead zones in coastal areas around the world, covering in total about
a quarter of a million square kilometres, of which the most notorious
are in the Gulf of Mexico, the Baltic Sea, and Chesapeake Bay on the
eastern US seaboard. 108 It is generally an annual phenomenon—
mostly, winter storms stir in enough oxygen to allow recolonization
of the sea floor by a new generation of animals—animals that are
fated only to survive until the next summer; the ecology of the dead
zones is now restricted to those organisms that can tolerate such
short and interrupted life cycles.
It is hard to think of these new marine dead zones as anything
other than horrific. But the bottom of the sea is rarely in people's
thoughts, and this significant change to the world's marine ecosys-
tems is mostly out of sight and out of mind. Nevertheless, as ever
more fertilizers will be needed to grow the crops to feed an expanding
global population, the dead zones seem set to spread ever more
widely. These localized zones can be added to the more widespread
oxygen depletion of the warmer, more stratified seas of the near
future. For the myriad organisms for whom the oceans are home, it
will simply be harder to breathe.
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