A lotic ecosystem, such as a river or stream, is a freshwater habitat where there is a constant current in one direction based on gravity, which has many effects on the organisms living there. It is distinguished from lentic ecosystems, which are freshwater habitats that lack such currents, such as ponds.
The constant movement of water in a lotic environment means that maintaining a position while swimming requires significant expenditure of energy. Many organisms that live in these environments thus spend a great deal of their time clinging to the riverbed or other solid components of the water's channel. They also tend to be more streamlined than organisms in lentic environments, both to reduce the drag of running water and to gain easier access to small crevices and other shelters. Some parts of lotic environments tend to stir up a large amount of sediment, obscuring light and making it difficult for algae or other photosynthetic organisms to thrive.
Lotic environments also have advantages for the organisms that live within them. The constant movement of the water means that it tends to mix in plenty of oxygen and allows for the easy removal of waste. However, rivers and streams tend to be in a constant state of change, so an area that is hospitable for an organism at one time may not be at another.