Hypoxic zones can occur naturally. They can actually be created with the absence of pollutants. In estuaries, for example, the freshwater flows from a river into the sea. This water is less dense than salt water and creates stratification. The bodies of water are unable to mix well vertically, resulting in the restriction of oxygen being supplied to the surface and back to the bottom of the ocean. Although hypoxia does occur in regions through this process, the main concern of scientists are the areas created or enhanced by human activity. The primary cause is nutrient pollution created by humans. Like the process fertilizer goes through to reach the ocean, other nutrients like phosophorus and nitrogen can run off into rivers and coasts and aid in extreme algae growth, which then sinks and decomposes in the water. The decomposition process then consumes oxygen and depletes the supply available to healthy marine life. Other harmful human activity issues include sewage, vehicular and industrial emissions that result in depleted underwater oxygen levels.13
Another issue is the multitude of animals in the ocean being affected directly by hypoxia. Hypoxia can kill marine organisms that cannot escape low oxygen water, as well as decrease the health of ecosystems.3