A MILLENNIAL RECORD OF OYSTER COLONIES IN THE CENTRAL POTOMAC ESTUARY ALONG THE NORTHERN NECK OF VIRGINIA

The Potomac River is a tributary to the Chesapeake Bay that was once a thriving habitat of the American Oyster, Crassostrea virginica. Historical records indicate an abundance of oyster beds through the 17th and 18th centuries, however, current surveys indicate an absence of major beds in the central estuary. Increased sedimentation rates due to agricultural activity, sea level rise, nutrient pollution, disease, and overfishing have all contributed to the recent decline of oyster populations. Our objective is to extract short sediment cores and subject them to radiometric geochronology. The data gleaned from this will be combined with geophysical CHIRP surveys to assess the spatial-temporal distributions of oyster beds spanning the past several centuries.

The Potomac River in Westmoreland County, Virginia is a tidally influenced, oligo- to mesohaline, partially mixed estuary. Short cores (0.5-1.2 m) extracted off the coast of Westmoreland State Park yield brown, silty clay transitioning to gray clay with stratified Crassostrea virginica shell beds below ~72 cm. Radiometric dates of skeletal carbonate (oysters) yield an uncorrected AMS ¹⁴C age of 795±15 yr. Assuming a carbonate reservoir effect of 400 years, the age of the oyster beds are likely early to late 17th Century. Cesium-137 peaks in nearby cores (20-21 cm) corroborate this on the basis of calculated sedimentation rates. Multiple CHIRP lines acquired across a ~1 km² grid containing the locations of the coring sites indicate two strong "mound shaped" reflectors at approximately 1 m and 5-8 m below the sediment-water interface. The reflectors collectively form a gently sloping, upstream stacking pattern along the axis of the Potomac. The primary mound in the position of the core has an extent of 1-3 m depth below the sediment surface and a 150 x 100 m geospatial extent.

These data suggest that multiple mound shaped oyster beds were actively growing prior to land clearance of the region. The mound structure’s stratigraphic stacking patterns are attributed to late Holocene 1-2 mm/yr sea level rise, largely due to Glacial Isostatic Adjustment. Decline of the oyster beds was likely caused by increased sedimentation during the mid-19th century that coincided with changes in salinity and temperature stratification at the end of the Little Ice Age.