Certain species favor the still water of impoundments over the flowing water in rivers and vice versa. In impounded and free-flowing systems, the base of the food chain is made up of phytoplankton. The types of phytoplankton found in both systems are relatively similar. Phytoplankton, by converting nutrients and inorganic compounds to complex organic compounds, provide the primary biological productivity of the water body. Fish, in turn, feed on these aquatic invertebrates, and larger fish prey on smaller fish.
Some fish feed on aquatic invertebrates floating in or on top of the water.
Other fish feed on aquatic invertebrates that live on the bed of the river or impoundment. The near-shore zone contains large beds of aquatic plants rooted to the bottom of the impoundment. Two factors that determine the extent of this zone are the area of shallow, fertile sediments that provide the nutrients for the plant beds and the greatest depth that light can penetrate the water. The plant beds also provide habitat and food for fish and waterfowl. The open-water zone is inhabited by phytoplankton and some fish species.