Dead Zones - where marine life is not supported

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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