In eutrophic impoundments, this zone can be dominated by blue-green algae. The bottom zone contains organisms that live in and on the bottom sediments, including microbes, insects, and mussels. Algae bloom in Mounds Dam impoundment caused by eutrophication. Because nutrients and organic matter have relatively long residence times in impoundments, as well as abundant sunlight and higher temperatures, algae and plants may flourish. A eutrophic impoundment has undesirably high concentrations of phytoplankton and supports fewer aquatic invertebrates and fish because of the associated turbidity and low dissolved oxygen.
Eutrophic impoundments tend to only support rough fish, such as carp.
The still water of an impoundment allows plants to grow easily, without the need to be strongly rooted. Some aquatic plants found in impoundments may even be free floating. Impoundments support the types of invertebrates that do not require flowing water for feeding or respiration purposes. Invertebrates found in impoundments also tend to be able to tolerate warm water temperatures and relatively low dissolved oxygen. Mussels are among the most endangered aquatic organisms in North America, partly due to the fact that the greatest mussel diversity is found in flowing water habitats, not impoundments.