Introduction

Pollen preserved in lake and peat sediments has been used to reconstruct changes in vegetation induced by climatic events (Davis, Spear, & Shane, 1980; Webb, Cushing, & Wright, 1983; Davis & Jacobson, 1985) and by anthropogenic alterations to the landscape (Brugam, 1978; Burden et al., 1986). Only recently has the record from estuarine sediments been similarly exploited (Brush, 1986; McGlone, 1988). In this chapter, we describe the general pathways of pollen in an estuary, and show from laboratory experiments and field observations how the transport and deposition of pollen in the estuary results in distributions in the sediments that record regional vegetation, land use and the effect of land use on rates and patterns of estuarine sedimentation. Our area of study is the Chesapeake Bay estuary extending between latitudes 37° and 39° 30' N in the mid-Atlantic region of the USA (Fig. 3.1).

Estuarine circulation

First, we examine the fluid motion, sediment motion, and salinity characteristics of Chesapeake Bay. Chesapeake Bay and many of its tributaries are partially mixed estuaries (Dyer, 1973), characterized by vertical, lateral and longitudinal gradients in salinity, but with no abrupt change from fresh to salt water as is found, for example, in the salt wedge environments of many fjords. On the other hand, Chesapeake Bay estuary is neither completely mixed nor homogeneous, as are estuaries characterized by large tidal flows and small river inflows. An example of an idealized, partially mixed estuary is shown in Figure 3.2A. When river water enters the estuary and meets ocean water, partial mixing occurs and causes lines of equal salinity to deform.