Dead ones generally occur near areas where heavy agricultural and industrial activity spill nutrients into the water. The excess nutrients, especially nitrogen and phosphorous, cause the rapid growth of microscopic algae called phytoplanktons. This phenomenon is called the nutrients, grow, die and sink to the bottom, where they are decomposed by bacteria. The bacteria inhale the dissolved oxygen in the water as they decompose the phytoplankton. This leads to the depletion of oxygen available to other marine life.
If the phytoplanktons are cyanobacteria, the problem is further intensified they are not consumed by zooplankton and fish. Hence they accumulate in water, leading to the expansion of dead zones. The bacterial degradation of their biomass consumes more oxygen in the water.
While nutrients run-off has been known for decades, researchers say that climate change is making the lack of oxygen worse. As more carbon dioxide is released enhancing the greenhouse effect much of the heat is absorbed by the oceans. In turn, this warmer water can hold less oxygen.
Stratification
The other worrying factor is the possible hindrance to replenishment of oxygen. This happens when the water is warmer than usual. Oxygen in the air and the surface usually gets dissolved in the water below by tides and winds. This helps restore the gas consumed by bacteria. When the temperature of the water is high, less dense water flowing into the ocean from rivers sits on top of salty dense water. This process is called stratification. A barrier is formed between the separated masses of water, hampering the replenishment process.
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