What is a Dead Zone?

About Dead Zone:

• The term “dead zone” or “hypoxia” refers to low-oxygen areas in the world’s lakes and oceans.
• Because most organisms need oxygen to live, few organisms can survive in hypoxic conditions. That is why these areas are called dead zones.
• Hypoxic zones can occur naturally, but human activities can also lead to the creation of new dead zones or the enhancement of existing ones.
• How are Dead Zones formed?
  • A dead zone occurs as a result of eutrophication, which happens when a body of water is inundated with too many nutrients, such as phosphorus and nitrogen.
  • At normal levels, an organism called cyanobacteria – or blue-green algae – feeds on these nutrients.
  • With too many nutrients, it can cause an overgrowth of algae in a short period of time, also called algae blooms.
  • Dead zones form when the algae die, sink to the bottom, and are decomposed by bacteria—a process that strips dissolved oxygen from the surrounding water.
  • Dense algal blooms also block sunlight, which prevents underwater grasses from growing. In turn, the animals that depend on these grasses for food and shelter suffer, as well.
  • Human activities mainly cause these excess nutrients to be washed into the ocean, which is why dead zones are often located near inhabited coastlines. 
• Once a dead zone forms, other factors can influence its size and duration.
  • For example, wind can mix oxygen from the surface into deeper water and help break up dead zones.
  • Hot temperatures can make dead zones worse by warming a layer of surface water that locks colder, denser water below where oxygen from the surface can’t mix in.
  • Heavy rainfall increases the amount of pollution washed into waterways.
• Shallow waters are less likely to stratify than deep waters, and so are less likely to develop hypoxic conditions. This is because shallow waters tend to be well-mixed by winds and tides.
• Additionally, waters that are shallow and clear enough to allow light to reach the bottom can support primary producers such as phytoplankton, algae and seagrasses that release oxygen during photosynthesis.