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.
What types of benthic organisms live in the Chesapeake Bay?
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.
A typical healthy benthic community
Why are benthic organisms important?
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.
An important link in the food web
Benthos link primary producers—phytoplankton—with higher levels in the food web.
- Filter feeders such as clams and oysters consume plankton and organic particles.
- Many benthic creatures, particularly clams and worms, serve as food for larger, economically important species such as blue crabs, striped bass, spot, croaker and white perch.
Additionally, the bacteria, decomposers and detritus-feeders that live at the bottom of the Bay break down waste products and dead plants and animals.
An environmental snapshot
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.
- In summer, high temperatures and nutrient pollution often lead to low-oxygen areas at the bottom of the Bay and its rivers.
- Excess sediment suspended in the water can block sunlight from reaching bay grasses growing at the bottom. When sediment finally settles, it can bury oyster bars and other benthic species.
- Many chemical contaminants bind to bottom sediments, remaining there for years. Benthic species become contaminated when they feed and live in these toxic sediments.
Working with the Maryland Department of Natural Resources and the 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.