• Slow moving bodies of water and ponds can become stagnant.wetland image by Colin Buckland from Fotolia.com
  
  Stagnant water is water that is not moving and has low levels of dissolved oxygen. It can be found in a container as small as a teacup or as large as a lake. Stagnant water is unhealthy for human recreation and for most animal life although some frogs and insects thrive in it. There are certain characteristics, or features, found in nearly all bodies of water that are considered stagnant.
  
  

Mosquitoe Larvae Present

• Mosquito larvae are nearly always present in stagnant water exposed to the open air during the summer. They are small worm-like organisms, visible to the naked eye. They are often called "wrigglers" because of the way they wriggle when moving from the top to the bottom of the body of water to escape predators. Mosquitoes are responsible for spreading a host of diseases, including malaria, encephalitis, dengue fever and heart worms in dogs.
  
  

Low Levels of Dissolved Oxygen

• Stagnant water has low levels of dissolved oxygen because of the lack of agitation that would normally increase the gas exchange needed to bring oxygen into the water. If there are fish in the water consuming the available oxygen, then the oxygen levels can be low enough to kill the fish, who need oxygen to survive. Fish affected by depleted oxygen levels in stagnant water are often seen at the surface of the water, gasping for air.
  
  

Bad Odor

• Stagnant water often has a bad odor resembling rotting eggs or rotting vegetation. This is caused by the death of organisms that need oxygen to break down organic matter. Without the organisms present to consume organic matter, such as fish waste or dead plants, the organic matter accumulates, rots and results in a bad odor.
  
  

Presence of Algae

• Low oxygen levels help increase the level of algae on the surface, bottom and sides of anything holding the stagnant water, because some types of algae thrive in a low-oxygen environment. Algae can be removed by hand or discouraged by aerating the water through agitation.