210Pb based particle residence time within the mixed layer are on the order of centuries. Mixing in the biologically dominated side of the river is generally shallow (<40 cm), with low 210Pb biodiffusion rates (0.43-3.35 cm 2 y-1). The physical mixing results in an impoverishment benthic community which is composed primarily of small, opportunistic species. X-radiographs reveal that the mixed layer consists of centimeter to decimeter scale units of finely to coarsely laminated strata bounded by hiatal surfaces, indicative of physical mixing. Within the physically dominated side of the river, the mixed layer is characterized by 210Pb profiles with one or more segments ( ∼ 25-100 cm thick) of nearly uniform excess activity. A strong cross-estuary gradient is observed with one side, including channel, flank and shoal, dominated by frequent deep erosion and re-deposition (physical mixing), while physical mixing is reduced on the other side resulting in a greater preservation of biological mixing. Throughout much of the energetic microtidal York River, the seabed is characterized by deep physical mixing (25-200 cm). A 35 month time series using kasten cores reveals that along with ∼ 25 cm differences in mixing depths due to the fortnightly time formation and destruction of furrows, a ∼ 100 cm depth scale signal of mixing exists annual to interannual time frame which is unrelated to the formation of erosional furrows. Varying furrow morphologies were observed depending on tidal flow, ranging from: (1) no bedforms during the higher flow conditions such as spring tide to (2) large patches of meandering furrows as the mean flow decreases to (3) large, variably spaced (5-7 m) linear furrows during the lowest mean current conditions of neap tide. Within the upper York River, transient, longitudinal erosional furrows regularly form within a broad flat secondary channel. 210Pb profiles reveal low sediment accretion rates within the mainstem sites (80 vs 6-30 cm3 y-1). X-radiographs indicate complete biological reworking of sediments. Within the lower Chesapeake Bay two sites were selected as biologically dominated, both are located within the bay stem plains and are characterized by muddy sand and an abundance of large, deep-dwelling organisms. By using radioisotope profiles from sediment cores, comparisons are made in terms of depth and rate of sediment mixing, deposition and accretion. ![]() ![]() To investigate the relationship between biological and physical mixing in forming strata, the lower mainstem of Chesapeake Bay has been contrasted with the York River Subestuary.
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