Which organisms are primarily found in the benthic zone of a river?

The organisms primarily found in the benthic zone of a river are benthic invertebrates. This zone is the lowest layer of a river, located on the bottom, where the substrate is composed of sediment, rocks, and detritus. Benthic invertebrates, such as worms, mollusks, and various types of insect larvae, thrive in this habitat. They play a critical role in the ecosystem by breaking down organic matter and serving as a food source for other animals, such as fish.

In contrast, floating plants are typically found in the surface layer of the water, where they can access sunlight for photosynthesis. Fish species inhabit various zones of a river, but not all fish are strictly associated with the benthic zone; many prefer the water column. Zooplankton are small, drifting organisms that live in the water column, primarily feeding on phytoplankton and are not typically found at the bottom of the river. Thus, the presence of benthic invertebrates is what uniquely characterizes the benthic zone.