Chesapeake Bay

Category: Marine and Oceanic Biomes.

Geographic Location: North America.

Summary: The Chesapeake Bay is the largest estuary in the United States, of great importance to wildlife and fish, and the subject of numerous programs to improve and reverse past environmental degradation.

The Chesapeake Bay is the largest of over one hundred estuaries occurring in the conterminous United States and the third-largest in the world. Bordering the Atlantic Ocean, it extends from its mouth near Norfolk, Virginia, north to Havre de Grace, Maryland. The bay is 180 miles (290 kilometers) long and 3 to 35 miles (5 to 56 kilometers) wide. The main bay averages less than 30 feet (9 meters) deep, and the entire bay, including tributaries, averages about 20 feet (6 meters). Several deep holes indicating the remnants of the ancient Susquehanna River exceed 171 feet (52 meters). Substrate is comprised primarily of gray silt and clay in the bay's center, with fine and medium sands near the shore.

More than 11,000 miles (17,000 kilometers) of shoreline, including mainland and islands, are associated with the bay. The bay's shorelines are a mixture of developed and conserved lands. The Baltimore, Maryland, metropolitan area, with 2.7 million people, is the most populous city directly on the bay's shore. Two major roadways cross the bay: the 4-mile (7-kilometer) Chesapeake Bay Bridge near Annapolis to Kent Island, Maryland; and the 17-mile (28-kilometer) Chesapeake Bay Bridge Tunnel from Virginia Beach to Cape Charles, Virginia. Substantial revenues are generated from trade and commercial fishing in the Chesapeake Bay waterways. In addition, tourism and recreation contribute billions of dollars to the regional economy.

Hydrology

The Chesapeake Bay started to form its present shape over 3,000 years ago, following repeated glacial melting that began about 18,000 years ago in the ancient Susquehanna River watershed. Evidence suggests that a bolide (meteor or comet) struck the region 35 million years ago, creating a 55-mile-wide (89-kilometer-wide) crater. These conditions eventually led to the formation and placement of the Chesapeake Bay and its watershed, an area that stretches across 64,000 square miles (165,000 square kilometers) of land.

The bay is fed by 19 large rivers and over 150 smaller rivers, which in turn are fed by more than 100,000 streams in the watershed. The Susquehanna River provides the largest—nearly half—of freshwater inflow. Rivers on the western shore generally are larger than those on the eastern shore. The Chesapeake Bay watershed encompasses portions of Delaware, Maryland, New York, Pennsylvania, Virginia, and West Virginia. This land area is primarily forested (57 percent) although much reduced from historic levels due to agriculture and development. In 2020, an estimated 18.4 million people lived in the watershed.

Tides rise and fall twice daily, with tidal range varying from less than 1 foot to 3 feet (0.3 meters to 0.9 meters) as of 2022. Salinity varies based on location, season, and depth, averaging 15 parts per thousand and varying from 0 to 36 parts per thousand. Salinity is generally greater near the bottom, due to the lighter freshwater emptying into the bay from the streams and stratifying above the heavier ocean-derived saltwater. The mouth of the Chesapeake Bay is saltier and generally deceases farther north due to the greater influence of streams. The bay occurs in the humid subtropical climate zone, which exhibits hot, humid summers and mild to cool winters. Ice often builds up during the winter along the Delmarva Peninsula, located on the eastern shore, due to the prevailing northwest winds. During extreme cold periods, ice may even accumulate on the western shore.

Biodiversity

The Chesapeake Bay estuary supports more than 3,600 species of plants and animals. Typical habitat features of the bay include sand beaches, intertidal flats, shallow subtidal zones, wetlands, oyster reefs, and subtidal deepwater. Sand beaches are most common in the southern part of the bay but overall are less abundant than on the Atlantic Coast proper.

Beach “fleas” (genus Emerita), ghost crabs (Ocypode quadrata), and horseshoe crabs (Limulus polyphemus) are common inhabitants of sandy beaches. Horseshoe crabs, with their characteristic domed bodies and spiked tails, are primitive crustaceans dating back 360 million years. These crabs lay their eggs in the sand, particularly in the southern and middle portions of the bay, during high spring tides. Although not as abundant as in the Delaware Bay, horseshoe crabs are still extremely common here. Each crab lays 10,000 to 100,000 eggs, attracting shorebirds that consume many of the eggs. Humans harvest the crabs for use in the biomedical industry and as bait for commercial fishing.

Intertidal flats composed of mud and sand are valuable areas for a variety of invertebrates, including fiddler crabs (Uca spp.), marsh crabs (Sesarma reticulatum), and snails. Shallow subtidal zones are extremely productive areas, with an abundance of submerged aquatic vegetation (SAV) and a suite of fish, invertebrates, and waterbirds. SAV is a common and integral component of Chesapeake Bay; the 26 species occurring in the bay contribute to clean, productive water. Areas of higher salinity are dominated by widgeon grass (Ruppia maritima) and eelgrass (Zostera marina), which harbor a less diverse assemblage of species than freshwater areas of the bay. SAV is important for filtering pollutants and nutrients, including nitrogen and phosphorous, from the water and for providing food for fish, crabs, and waterfowl. If pollution levels become too high, SAV declines. In 2020, the Chesapeake Bay had a recorded 62,160 acres of SAV, down 42 percent from 108,078 acres in 2018. The Environmental Protection Agency (EPA) had a goal of increasing SAV to 130,00 acres by 2025.

Chesapeake Bay is well known for native American or Eastern oyster (Crassostrea virginica) beds. These dense oyster reefs attract diverse assemblages of micro- and macro-organisms that live in and among the oyster beds. Eastern oysters are ecologically important inhabitants of the bay and are central to the region's commercial fishery. Their abundance has declined since the mid-1800s, but at their peak, they filtered the entire volume of water in the bay every three to four days, removing algae and particulates from the water.

Oyster beds also provide habitat for numerous other species. In 2021, the oysters were only 1 5 percent of their historic densities. The decline is attributed to increased efficiency of harvest techniques, degradation of water quality and habitat, and increased prevalence of diseases and parasites. Historic harvest records are rare, but in 1980, 3.1 million bushels (109,000 cubic meters) were harvested, compared with only 370,000 bushels (13,000 cubic meters) in 2001. As of 2021, man-made oyster beds in Chesapeake Bay were helping to restore the Eastern oyster population. The National Oceanic and Atmospheric Administration (NOAA) set a goal of bringing back oysters in 10 Chesapeake Bay tributaries by 2025.

The forested, shrub, and emergent wetlands, including both tidal and nontidal areas, span the gradient from freshwater to saltwater, providing the plant material that breaks down into detritus, forming one of the primary foundations of the bay's complex food web. Indeed, consumption of detritus by organisms is about 10 times greater than herbivorous grazing in the bay. The eastern shores have larger expanses of marshes, providing habitat for migratory waterfowl during the spring and fall migrations, as well as during the winter. About 40 percent of the waterfowl in the Atlantic Flyway overwinter on Chesapeake Bay. Avian types range from ducks, terns, and gulls to apex predators including osprey (Pandion haliaetus) and bald eagles (Halieaeetus leucocephalus).

Large fish, including bluefish (Pomatomus saltatrix) and red drum (Sciaenops ocellatus), are among the most impressive inhabitants of the subtidal deepwater zones, but phytoplankton and zooplankton are dominant, integral contributors to the bay's productivity. Several anadromous fish species, including striped bass (Morone saxatilis), American shad (Alosa sapidissima), and American eel (Anguilla rostrata), live in the deepwater zones and enter the streams feeding Chesapeake Bay to spawn.

Human Impact and Conservation

Chesapeake Bay has a long human history; paleo-Indians first inhabited the Chesapeake Bay watershed about 13,000 years ago, following the retreat of the glaciers. Oyster shells were used extensively by these early inhabitants as tools and jewelry, but the extent to which they were consumed for food is unclear. Certainly, mastodons (Mammut spp.), mammoths (Mammuthus spp.), and other large mammals were pursued for their food value. As the climate warmed in the region, the paleo-Indians were replaced by the archaic Indians about 9,000 years ago; they also primarily subsisted on hunting but apparently had a stronger connection to the aquatic resources, including fish and shellfish, and the migratory birds provided by the Chesapeake Bay. Woodland Indians moved into the Chesapeake Bay region some 3,000 years ago, remaining until after European settlers arrived; evidence of many villages and stronger relation to bay resources survives.

The Chesapeake Bay has been negatively affected and will continue to be so, both by direct and indirect human impact According to US Fish and Wildlife reports, there is a long list of threatened and endangered species of flora and fauna in the bay area. Climate change is predicted to lower salinity due to increased flows from major rivers such as the Susquehanna, which could potentially affect important bay species that are intolerant of low salinity conditions, such as oysters and crabs. Moreover, sea-level rise from melting glaciers, due to global warming trends, will submerge some salt marshes. Models predict greater future increases in mean temperatures, and lower dissolved oxygen levels, which may affect numerous organisms.

The bay's productivity is limited by degradation of water quality caused by excessive chemical, nutrient, and sediment input. Sediment accretion rates in the bay have increased one and a half to seven times over the pre-European settlement rate. Europeans started settling the region in the early seventeenth and eighteenth centuries. Between 1830 and 1890, 80 percent of the land was cleared of timber, and much of it was plowed and put into agriculture. Clearing of the land led to increased flow of freshwater into the bay, resulting in greater sediment deposition. With the advent of commercial fertilizers since the mid-1800s—most notably those containing nitrogen and phosphate compounds—fertilizer discharge has increased, as have sewage discharge and industrial waste. The 1940s brought about common agricultural practices implementing a variety of pesticides. Starting in the 1960s, efforts have been made to reduce the effects of contaminated runoff. Water quality continues to fluctuate, as do population numbers of various species in the region.

In 2009, the Chesapeake Bay was declared a national treasure, and President Barack Obama issued an executive order to promote and restore its health, heritage, and natural resources. The US Environmental Protection Agency (EPA) mandated goals in the form of Total Maximum Daily Load for the amount of pollutants that can enter the bay. These goals and related solutions to reduce sediment, nitrogen, and phosphorous runoff were implemented for each state in the watershed. In addition to federal and state agencies, numerous local and regional watershed groups, such as the Chesapeake Bay Foundation, work to improve water quality in the bay. In 2022, the Maryland Department of the Environment identified 134 watersheds that were impaired by one or more substances.

Bibliography

Baird, Daniel and Robert E. Ulanowicz. “The Seasonal Dynamics of the Chesapeake Bay Ecosystem.” Ecological Monographs 59, no. 4 (1989).

Chesapeake Bay Program, 2018, www.chesapeakebay.net/. Accessed 29 June 2018.

Committee on Nonnative Oysters in the Chesapeake Bay. Nonnative Oysters in the Chesapeake Bay. Washington, DC: National Academies Press, 2004.

Cooper, S. R. and G. S. Brush. “Long-Term History of Chesapeake Bay Anoxia.” Science 254, no. 5034 (1991).

"EPA Outlines Final Stage of Chesapeake Bay Restoration Effort." Storm Water Solutions, 28 June 2018, www.estormwater.com/epa-outlines-final-stage-chesapeake-bay-restoration-effort. Accessed 29 June 2018.

Felver, Rachel. "Acreage of Underwater Grasses in the Chesapeake Bay Decline for a Second Straight Year." Chesapeake Bay Program, 28 Jul. 2021, www.chesapeakebay.net/news/blog/acreage‗of‗underwater‗grasses‗in‗the‗chesapeake‗bay‗decline‗for‗second‗stra. Accessed 22 Jul. 2022.

Lippson, A. J. and R. L. Lippson. Life in the Chesapeake Bay. 2nd ed. Baltimore, MD: Johns Hopkins University Press, 1997.

National Oceanic and Atmospheric Administration. “Chesapeake Bay Office.” http://chesapeakebay.noaa.gov.

"StoryMap: Oyster Reef Restoration in the Chesapeake Bay." NOAA Fisheries, 1 Mar. 2021, www.fisheries.noaa.gov/resource/map/storymap-oyster-reef-restoration-chesapeake-bay. Accessed 22 Jul. 2022.

United States Geologic Survey. “The Chesapeake Bay Bolide: Modern Consequences of an Ancient Cataclysm.” http://woodshole.er.usgs.gov/epubs/bolide.