CHESAPEAKE BAY WATERSHED HISTORICAL LAND-USE - IMPACT ON WATER-QUALITY AND DIATOM COMMUNITIES

Stratigraphic records preserved in the sediments of the mesohaline Che sapeake Bay were used to reconstruct a 2000-yr history of sedimentatio n, eutrophication, anoxia, and diatom community structure over time. D iatoms, pollen, total and organic carbon (TOC), total and organic nitr ogen, total sulfur, acid-soluble iron, an estimate of the degree of py ritization of iron (DOP), and biogenic silica (BSi) were used as paleo ecological indicators in four cores collected from a transect across t he Chesapeake Bay from the Choptank River to Plum Point, Maryland. Thi s paper covers results for diatoms, pollen, and BSi. Sediments were da ted using radiocarbon and pollen techniques, and sedimentation rates w ere determined (0.2-5.8 mm/yr) using pollen methods. Geochemical indic ators were measured and diatom species identified at subsampled interv als within each core. More than 400 diatom species, primarily marine a nd estuarine taxa, were identified in the sediments, some for the firs t time. Analysis of the data indicates that sedimentation rates, eutro phication, 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 in creased. Changes in diatom community structure and geochemical indicat ors reflect major changes in land use patterns of the watershed and in creasing population. Diatom community diversity exhibits a continuing decline, while centric/pennate ratios rise dramatically in most recent sediments.