Dead Zones are referred to as being hypoxic zones. They are
essentially areas in which the level of oxygen in the water is too low
to support any kind of marine life. These hypoxic events happen annually and are a summertime phenomena.
An unusual chain of events initiates this oxygen deficiency.
Coastal ecosystems rely on nutrient runoff from the surrounding
landmasses, and/or deep-water upwellings of nutrients to fuel the
coastal marine food web. Any disruption of this delicate balance
impacts right throughout the ecosystem.
It has been discovered
that there is such a disruption occurring around many coastal areas,
and that disruption can begin many miles from the oceans it impacts as
it causes these Dead Zones.
Runoff of leached fertilisers,
particularly those high in nitrogen and phosphorus, from neighbouring
landmasses build up in concentration in areas of the marine
environment. This usually, though not always happens near waterways
that feed into the sea.
As the levels build phytoplankton and
algae find these concentrations of nutrients, warmer water temperatures
and increased light levels ideal, and a bloom results. After the
initial build-up of these micro-organisms occurs they reach their peak
population density and a mass die-off happens. After this happens the
organisms sink to the seabed. There they are digested by other
micro-organisms. This process of digestion removes vast amounts of
oxygen from the surrounding water.
The oxygen needs, in marine
ecosystems, are met by surface gaseous exchange and as a by-product of
the natural life processes of the same phytoplankton that caused the
bloom, have now died off.
Average seawater has a dissolved
oxygen level of around 10 ppm (parts per million) of oxygen. However,
researchers have found that the oxygen levels in the " Dead Zone" were
around 0.5 ppm.
Under normal conditions the dissolved oxygen is normally mixed right throughout the layers of the ocean down to the seabed.
However, in this case this is not happening and like a toxic cloud the
area of oxygen depletion clings to the seabed, condemning every
creature in the area to suffocation.
Most fish begin to develop
respiratory stress when the oxygen levels reach 5 ppm. Large fish, such
as sharks vacated areas in Dead Zones once the oxygen level reached 3
ppm. However it was found many fish species held out until the oxygen
level reached 2 ppm before, in most cases, vacating to the periphery of
the Dead Zone areas where the oxygen levels were higher. This provided
a bonus for many commercial fishermen as large catches were easily
obtained in these peripheral areas. This also brought fish into areas
of heavier predation increasing the stress on the fish resident to that
area.
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The greatest casualties however are among
the sedentary, territorial and slower moving species. Crabs, shellfish,
starfish and shrimp are unable to flee the area and are suffocated.
These zones of death remained well hidden, as the water above them
closer to the surface is still able to continue its process of gaseous
exchange, in fact often showing a large amount of fish species not
normally so abundant as they avoid the low oxygen area below them.
It is generally the vast array, and amount, of marine species washed
ashore dead that indicate an ecological disaster such as this has
occurred.
In some areas these zones occur seasonally, while some are now permanent features in areas such as parts of the Baltic Sea.
These Dead Zone events impact on the Commercial Fishing Industry of the
affected areas, as a lot of the species impacted are of commercial
value.
In the Gulf of Mexico, crab and shrimp industries have
noticed a decline in catch quotas. Reduction in quotas has an effect on
local industry and economy, but also impacts on the national economy as
well.
These Dead Zones first started to be more noticeable in
the 1970s, greater notice began to be taken of the problem in 2000. It
was discovered that the number of these zones had doubled with each
passing decade, with 2002 being the worst period for the appearance of
this phenomena on record. This has probably coincided with the
increased use of artificial fertilisers for pastoral and agricultural
production.
There are no quick and easy cures for these Dead
Zones. Reduction in the amount of artificial fertilisers used in areas
where they can leach into rivers and waterways, and hence into the sea,
has been shown to help, as shown by initiatives implemented by the
Danish government.
Re-establishment also of the devastated areas ecosystem and ecological balance is a long, slow but achievable solution as well.
The effects of these ecological disasters impact on the ecosystem as a
whole, therefore it is the whole ecosystem that has to be assessed in
order to find out where the problems have developed, both those that
are evident and those not so evident, in order to begin fixing the
damage and restoring an ecological balance again.
Without action vast areas of seabed will become a barren testimony to man's rape of the marine environment.
This only proves what we do on the land can also have a major effect on what happens out at sea.
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