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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. Continued On Next Page (The dead zone, Page 2) ... AUTHOR: Tony Lucas TAGS: Life events death Food government water action aliens BOOKMARK: Digg it | Add to Del.ICIO | Add to FARK ACTIONS: Comment Save Print Register free acount |
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