Chesapeake Bay Watershed Historical Land Use: Impact on Water Quality and Diatom Communities

Abstract

Stratigraphic records preserved in the sediments of the mesohaline Chesapeake Bay were used to reconstruct a 2000‐yr history of sedimentation, eutrophication, anoxia, and diatom community structure over time. Diatoms, pollen, total and organic carbon (TOC), total and organic nitrogen, total sulfur, acid‐soluble iron, an estimate of the degree of pyritization of iron (DOP), and biogenic silica (BSi) were used as paleoecological indicators in four cores collected from a transect across the Chesapeake Bay from the Choptank River to Plum Point, Maryland. This paper covers results for diatoms, pollen, and BSi. Sediments were dated using radiocarbon and pollen techniques, and sedimentation rates were determined (0.2‐5.8 mm/yr) using pollen methods. Geochemical indicators were measured and diatom species identified at subsampled intervals within each core. More than 400 diatom species, primarily marine and estuarine taxa, were identified in the sediments, some for the first time. Analysis of the data indicates that sedimentation rates, eutrophication, turbidity, and anoxia have increased in the Chesapeake Bay since the time of European settlement of the watershed. There is also evidence that freshwater input to the mesohaline Chesapeake Bay has increased. Changes in diatom community structure and geochemical indicators reflect major changes in land use patterns of the watershed and increasing population. Diatom community diversity exhibits a continuing decline, while centric/pennate ratios rise dramatically in most recent sediments.