The history of diatom community structure, eutrophication and anoxia in the Chesapeake Bay as documented in the stratigraphic record

摘要

Stratigraphic records preserved in the sediments of the mesohaline Chesapeake Bay were used to reconstruct a 2,000 year 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 cores collected from a transect across the Chesapeake Bay from the Choptank River to Plum Point, Maryland. Surface salinities in this area average 8-15%.;Four cores were collected in areas which currently experience different patterns of hypoxic and anoxic bottom waters. The sediments within the cores were dated using radiocarbon and pollen techniques. Sedimentation rates were determined using pollen methods, and a chronology for each core was compiled. 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 of the Chesapeake Bay, some for the first time. The distributional patterns of diatom species in terms of relative abundance of species, community diversity, centric/pennate ratios, distance between samples, and cluster analysis of diatom communities were related to changing water quality and land use patterns of the watershed over time. The taxonomy, autecology and observed valve morphology of the 50 most abundant diatom species are discussed.;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. Diatom community diversity exhibits a continuing decline while centric/pennate ratios rise dramatically in most recent sediments. Diatom communities from samples of all four cores dated within similar time periods of land use are clustered together.