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Prehistoric habitat stability and post-settlement habitat change in a Chesapeake Bay freshwater tidal wetland, USA

Engineering Programs for Professionals, The Johns Hopkins University, Baltimore MD 21218, and Science Department, Friends School, Baltimore MD 21210, USA. Mailing address (Home Office): 222A Donnybrook Lane, Towson MD 21286, USA hilgartner{at}jhu.edu

Grace S. Brush

Department of Geography and Environmental Engineering, University, Baltimore MD 21218, USA

Analyses of fossil seeds and pollen grains preserved in seven sediment cores and borings were combined with historical land-use records to reconstruct an 1800-yr history of Otter Point Creek (OPC), a freshwater tidal deltaic wetland in the upper Chesapeake Bay. The objectives of the study were to document the role of natural and anthropogenic disturbance on habitat development at different sites within the wetland. Fossil seeds revealed a 1500-yr period (AD 230 to 1700) of subtidal habitat stability characterized by the aquatic macrophytes Zannichellia palustris, Najas gracillima, N guadalupensis, Elodea canadensis and Vallisneria americana. Natural disturbance had little or no impact on the estuarine habitat during this time. By the early 1700s, forest clearance for agriculture and lumber during European settlement led to higher erosion rates in the watershed. This led to rapid sediment efflux and broad habitat changes along a hydrologic gradient within the estuary. The aquatic macrophytes disappeared when estuarine sedimentation rates increased from a pre-1700 mean of 0.05 cm/yr to 0.60 cm/yr after 1730-1750. As the estuary continued to fill behind a prograding delta from AD 1750 to 1950, sub-tidal species were replaced sequentially by increasingly less flood-tolerant dominants of low marsh (Zizania aquatica), middle marsh (Typha angustifolia or Leersia oryzoides), high marsh (Acorus calamus), shrub marsh (Typha latifolia and Salix nigra) and riparian forest (Acer negundo, Fraxinus pennsylvanica and Betula nigra). Habitat change progressed in pulses followed by periods of habitat stasis. The most rapid and extensive period of change occurred between 1840 and 1880, synchronous with the period of highest sedimentation rates, mid-century storms and intensive land use in the watershed. The rate and pattern of community change differed among sites where position relative to the watershed, local disturbance, and channel shifts at or adjacent to the site influenced different community pathways. A conceptual model is proposed to describe the spatial and temporal development of eastern US freshwater tidal wetland habitats, reflecting the importance of the impact of colonial deforestation.

Key Words: Chesapeake Bay • deforestation • freshwater tidal wetland • human impact • plant macrofossils • sedimentation • wetland habitat development

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