Dead Zones

Once the consequences of eutrophication are felt in coastal waters, there is little that can be achieved to redress the situation by ecological engineers. The scale of coastal waters and their open boundaries make management difficult or impossible. Examples of this are the dead zones of hypoxic and anoxic waters in the Gulf of Mexico and the Black Sea. Only the surface waters of the Black Sea remain oxic and most of this deep basin is naturally anoxic. Two-thirds of the continental USA agricultural lands drain down the Mississippi into the Gulf of Mexico. The runoff of excess fertilizers increases nutrient inputs and also causes imbalances in the nutrient ratio. The plume of the Mississippi can be seen from satellite imagery but the effects of the nutrients are more widespread and can be detected through remote sensing by the high chlorophyll concentrations from the phytoplankton blooms. This excessive production unbalances the ecosystem. The microbial decomposition ofthe increased organic matter depletes the water of oxygen leading to hypoxia and even anoxia. The effect is particularly marked during the summer months when the waters are warm and saline therefore oxygen is less soluble. No effective ecological engineering measures are possible offshore to counter this effect that in some years covers a very large area of the Gulf of Mexico leading to the loss of valuable fisheries. However, the summer is also the hurricane season in the Gulf of Mexico and the turbulence and mixing caused by large and frequent storms can at times break up the 'Dead' zone.

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