Perilous Times
Ocean Dead Zones On The Rise
by Staff Writers
Washington DC (SPX) Jul 20, 2010
The size and number of marine dead zones-areas where the deep water is
so low in dissolved oxygen that sea creatures can't survive-have grown
explosively in the past half-century. Red circles on this map show the
location and size of many of our planet's dead zones. Black dots show
where dead zones have been observed, but their size is unknown.
It's no coincidence that dead zones occur downriver of places where
human population density is high (darkest brown). Some of the
fertilizer we apply to crops is washed into streams and rivers.
Fertilizer-laden runoff triggers explosive planktonic algae growth in
coastal areas.
The algae die and rain down into deep waters, where their remains are
like fertilizer for microbes. The microbes decompose the organic
matter, using up the oxygen. Mass killing of fish and other sea life
often results.
Satellites can observe changes in the way the ocean surface reflects
and absorbs sunlight when the water holds a lot of particles of organic
matter. Darker blues in this image show higher concentrations of
particulate organic matter, an indication of the overly fertile waters
that can culminate in dead zones.
Naturally occurring low-oxygen zones are regular features in some parts
of the ocean. These coastal upwelling areas, which include the Bay of
Bengal and the Atlantic west of southern Africa, are not the same as
dead zones because their bottom-dwelling marine life is adapted to the
recurring low-oxygen conditions. However, these zones may grow larger
with the additional nutrient inputs from agricultural runoff.