World
http://disc.sci.gsfc.nasa.gov/oceancolor...
Creeping Dead Zones
This is not the title of a sequel to a Stephen King novel. "Dead zones" in this context are areas where the bottom water (the water at the sea floor) is anoxic — meaning that it has very low (or completely zero) concentrations of dissolved oxygen. These dead zones are occurring in many areas along the coasts of major continents, and they are spreading over larger areas of the sea floor. Because very few organisms can tolerate the lack of oxygen in these areas, they can destroy the habitat in which numerous organisms make their home.
The cause of anoxic bottom waters is fairly simple: the organic matter produced by phytoplankton at the surface of the ocean (in the euphotic zone) sinks to the bottom (the benthic zone), where it is subject to breakdown by the action of bacteria, a process known as bacterial respiration. The problem is, while phytoplankton use carbon dioxide and produce oxygen during photosynthesis, bacteria use oxygen and give off carbon dioxide during respiration. The oxygen used by bacteria is the oxygen dissolved in the water, and that’s the same oxygen that all of the other oxygen-respiring animals on the bottom (crabs, clams, shrimp, and a host of mud-loving creatures) and swimming in the water (zooplankton, fish) require for life to continue.
The Gulf of Mexico Dead Zone
http://serc.carleton.edu/microbelife/top...
What Causes the Dead Zone?
The dead zone is caused by nutrient enrichment from the Mississippi River, particularly nitrogen and phosphorous. Watersheds within the Mississippi River Basin drain much of the United States, from Montana to Pennsylvania and extending southward along the Mississippi River. Most of the nitrogen input comes from major farming states in the Mississippi River Valley, including Minnesota, Iowa, Illinois, Wisconsin, Missouri, Tennessee, Arkansas, Mississippi, and Louisiana. Nitrogen and phosphorous enter the river through upstream runoff of fertilizers, soil erosion, animal wastes, and sewage. In a natural system, these nutrients aren't significant factors in algae growth because they are depleted in the soil by plants.
However, with anthropogenically increased nitrogen and phosphorus input, algae growth is no longer limited. Consequently, algal blooms develop, the food chain is altered, and dissolved oxygen in the area is depleted. The size of the dead zone fluctuates seasonally, as it is exacerbated by farming practices. It is also affected by weather events such as flooding (more info) and hurricanes.