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Hypoxic zones can occur naturally, for example, coastal upwelling zones along the western side of the continents are highly productive, thus respiration of fluxes of organic matter can produce severe hypoxia. There are several persistent oxygen minimum zones created by natural conditions, such as the one in the Pacific Ocean. The most important difference between natural and human induced hypoxia, is that in natural hypoxic zones the marine life has adapted to the low oxygen conditions, whereas in human created dead zones most of the marine life of the area cannot survive.
The Gulf of Mexico Hypoxic Zone
In the Gulf of Mexico, every summer, a hypoxic zone appears as a consequence of the massive amounts of nitrogen and phosphorous coming from the Mississippi River. This zone has an average size of 15,000 km2 and has been measured from 1985 to the present. It is the second world largest hypoxic zone. This zone presents a strong seasonal dependence, it is created every spring, when the Gulf receives a massive amount of nitrate flux, and factors such as rainfall, warmer temperatures, and sunlight favor algal growth. Moreover, the waters are calmer, which prevents vertical mixing of the ocean. This lack of vertical mixing means that colder, oxygen depleted waters cannot interact with the fresher, warmer waters of the surface, thus maintaining the hypoxic zone. Then, the hypoxic zone is breaks early September when hurricanes and tropical storms mix the ocean.