Benthos are the organisms that live at the bottom of the Chesapeake Bay and its streams and rivers. The word benthos comes from a Greek term meaning “depths of the sea.” Benthic communities are complex and include a wide range of animals, plants and bacteria from all levels of the food web. Clams, worms, oysters, shrimp-like crustaceans and mussels are all examples of benthic organisms.
There are two groups of benthic organisms, based on their habitat: epifauna and infauna. Epifauna live attached to a surface and infauna live within bottom sediments.
Epifauna live attached to hard surfaces such as rocks, shells and pilings or directly on the surface of the Bay’s bottom. Epifauna include oysters, sponges, sea squirts, sea stars and barnacles. An oyster reef is an example of an epifaunal benthic community.
Infauna burrow into bottom sediments. Worms, clams and other infauna form their own communities that are connected to the water by tubes and tunnels. A healthy infaunal community contains many different species.
Benthos play several important roles in the food web and serve as an excellent indicator of environmental conditions in the Bay and its streams and rivers.
Benthos link primary producers—phytoplankton—with higher levels in the food web.
Additionally, the bacteria, decomposers and detritus-feeders that live at the bottom of the Bay break down waste products and dead plants and animals.
Scientists study benthic organisms because they provide a good snapshot of environmental conditions in the Bay and its streams and rivers. Most benthic creatures cannot move very far—if at all—so they can’t avoid pollution or unhealthy water conditions.
Benthic communities are exposed to many stressors, including low oxygen levels, excess sediment and chemical contaminants.
Working with the Maryland Department of Natural Resources and Virginia Department of Environmental Quality, the Chesapeake Bay Program has monitored the health of benthic organisms in the tidal Chesapeake Bay since 1984. Each year, researchers with the Chesapeake Bay Benthic Monitoring Program collect hundreds of samples and compare species abundance, biomass, diversity and other attributes to conditions that would be expected in a healthy environment. The sample results are then scored on a one-to-five scale called the Benthic Index of Biotic Integrity, or B-IBI.
Over the past decade, the health of bottom habitat in the tidal Bay has remained poor. Researchers did observe minor improvements in 2015, however, with 62 percent of the Bay’s tidal bottom meeting restoration goals (compared to 59 percent in 2014). In other words, while 38 percent of the tidal Bay’s bottom habitat is marginal, degraded or severely degraded—home to more pollution-tolerant species, fewer species overall, fewer large organisms deep in the sediment and a lower total mass of organisms—almost two-thirds of this habitat is home to a healthy community of benthic organisms. Furthermore, the extent of degraded and severely degraded conditions was the lowest it has been since 1996. Experts attribute this improvement in bottom habitat to improvements in dissolved oxygen. Improvements in bottom water quality are thought to be the result of low spring river flow, which meant lower polluted runoff flowing into the Bay.