William B. Hilgartner

PALEOECOLOGY OF SUBMERGED MACROPHYTES IN THE UPPER CHESAPEAKE BAY

Fossil seed distributions of submerged aquatic vegetation (SAV) from dated sediment cores in tributaries of the upper Chesapeake Bay show prehistoric changes in species composition and abundance and reflect the response of SAV species to human disturbance since European settlement. The interval of time spanned by the cores includes several centuries prior to, and three centuries following, European settlement. Species diversity is greatest in the low-salinity northern and upper tributaries, while areas of higher salinity and extensive salt marshes are characterized by low diversity or absence of SAV. Mapped distributions of seed abundances show the migration from upstream to downstream in some tributaries of the brackish species Potamogeton perfoliatus, Zannichellia palustris, and Ruppia maritima following deforestation. The largest increase in SAV, represented by the highest abundance of fossilized seeds, occurred during the 1700s after Europeans first cleared the land for farms, and the largest and most widespread decline took place in the 1960s and 1970s after most of the watershed had been at one time or another cleared and heavily fertilized for agriculture. Distributions of SAV are highly variable both temporally and spatially, reflecting the dynamic nature of estuarine habitats. Despite high environmental variability, local and regional extinctions occurred only in the most recent decades, indicating a threshold response to land use changes and nutrient loading which had begun at least two centuries earlier and intensified in the mid- to late 19th century.

Biogeomorphology of an upper Chesapeake Bay river-mouth tidal freshwater marsh.

Field mapping and monitoring of vegetation, sedimentation patterns, substrate characteristics, and geomorphology in the Bush River tributary to upper Chesapeake Bay has been conducted since 1991 to ascertain the process-morphology dynamics in a tidal freshwater marsh. Nine plant associations from 5 distinct marsh habitats were identified by clustering species abundance measurements from 115 quadrats throughout an 84-hectare area. High spatial variability in physical habitat conditions such as summer-average sediment deposition, summer-average organic content, and surface-sediment grain size distributions were explainable using combinations of independent variables, including elevation, plant distributions, and distances to the tidal inlet and an adjacent stream. Sedimentation and vegetation were both observed to show a predictable response to disturbance by animal activity.