The benthos is an aggregation of organisms living on or at the bottom of a body of water. The name benthos is derived from the Greek, meaning "depths of the sea." The benthic community is composed of a wide range of plants, animals and bacteria from all levels of the food web.

Benthic organisms can be divided into three distinct communities:

• Infauna: Plants, animals and bacteria of any size that live in the sediment.
• Epifauna: Plants, animals and bacteria that are attached to the hard bottom or substrate (for example, to rocks or debris); are capable of movement; or that live on the sediment surface.
• Demersal: Bottom-feeding or bottom-dwelling fish that feed on the benthic infauna and epifauna.

Click to see a sample benthic community.

Benthic organisms link the primary producers, such as phytoplankton, with the higher trophic levels, such as finfish, by consuming phytoplankton and then being consumed by larger organisms. They also play a major role in breaking down organic material. Benthic algae and submerged aquatic vegetation (SAV) provide ideal habitat for juvenile fish. Benthic invertebrates are among the most important components of estuarine ecosystems and may represent the largest standing stock of organic carbon in the Chesapeake Bay. Many benthic organisms, such as hard clams, softshell clams and bottom-dwelling fish, are the basis of the Bay’s commercial fisheries. Other bottom-dwelling organisms, such as polychaete worms and crustaceans, contribute significantly to the diets of economically important fish.

Infaunal benthic communities often are considered to be "just worms." In reality, however, these groups that inhabit the sediment include animals from all trophic levels–the primary producers, such as diatoms, and primary consumers, such as mollusks and worms; secondary consumers, such as worms and crustaceans; and "decomposers," such as bacteria and flagellates.

Benthic invertebrate communities are used as prime indicators of environmental conditions within the Chesapeake Bay because:

• they have limited mobility and thus are unable to avoid adverse conditions;
• they live in sediments where they are exposed to environmental stressors, such as chemical contaminants and low dissolved oxygen levels;
• their life spans are long enough to reflect the effects of environmental stressors; and
• their communities are taxonomically diverse enough to respond to multiple types of stress.

A good picture of the current conditions in the Bay may be derived by looking at benthic indicators such as species diversity, abundance and biomass.

The Bay’s epifauna are the most familiar of all the benthic organisms. They include the plants and animals one sees while wading in tidal pools or among pilings or rocks. These communities include SAV and seaweeds; oysters, mussels and barnacles; and snails, starfish and crabs. They also include animals that span a wide evolutionary range, from primitive sponges to early vertebrates (for example, tunicates, such as sea squirts). These varied organisms share an important characteristic: they live either attached to the hard substrate or move on the sediment surface.

The demersal community includes some of the most economically valuable fish in the Chesapeake Bay. In order to adapt to life on the bottom, benthic fish have developed some of the most diverse physical characteristics found in any fish community. Soft-bottom fish include the flounders, puffers, searobins and cownose rays. Hard-bottom fish include those found near reefs, such as the oyster toadfish and the goby, which, when stationary, resemble rocks. Other demersal species include the Atlantic blue crab and bottom-feeding juvenile and adult finfish such as spot, croaker, striped bass and white perch. Most of these fish rely on the polychaete worms and crustaceans for food.