Biological Sciences

A process-oriented sedimentary facies model is developed for the York River estuary, a sub-estuary of the lower Chesapeake Bay. This facies model was based on 210Pb and grain-size profiles, as well as X-radiographs taken from kasten cores and box cores collected in a series of across-river transects. Throughout most of the energetic microtidal York River, the seabed is characterized by physical mixing to depths of 25–200 cm. A strong cross-estuary gradient in processes is observed with one side, including channel, flank and shoal, dominated by frequent deep erosion and redeposition (physical mixing), while physical mixing is reduced on the other side, resulting in a greater preservation of biological mixing signatures. Within the physically dominated side of the river, the mixed layer is characterized by ‘stair-stepped’ 210Pb profiles with one or more segments (∼25–200 cm thick) of nearly uniform excess activity. X-radiographs reveal that, although a record of limited biogenic sediment modification is preserved, sedimentary structures within the mixed layer are dominated by centimeter to decimeter scale units of finely to coarsely laminated strata bounded by hiatal surfaces. This demonstrates that mixing results primarily from erosion, resuspension and deposition. Reduced salinity limits the number of benthic species in the York River. Physical disturbance leads to an impoverishment of this community, which is composed primarily of small, opportunistic species with a paucity of larger macrofauna. As a result, mixing in the biologically dominated side of the river is generally on the order of a few centimeters, but may be as deep as 40 cm, and 210Pb geochronology yields low biodiffusion rates (0.43–3.35 cm2 yr−1). X-radiographs reveal the presence of some laminations which suggest that although the mixing is controlled by biological processes the mixing intensity is relatively low. Based on 210Pb geochronologies, residence time estimates for particles within the mixed layer are on the order of centuries. Residence time calculations based on the sediment mass in the physically mixed layer is equivalent to 70 yr of river sediment yield, consistent with century-scale residence times from core data. The frequency and intensity of seabed mixing appears to differ between the lower and upper river. The lower York River is wider and deeper, and is more susceptible to large storms and sea surges, which we suspect drives much of the recorded seabed mixing. Within the upper river, longer-term events (storms) may cause the deepest mixing, but much of this record is destroyed by shorter-term, high-frequency events which produce shallow to mid-depth (<50 cm) mixing, probably driven by spring/neap tides, co-phased tidal constituents, and river flooding.

... of manmade features Mainly channels roughness smooth (dredge cuts, anchor drags, etc.) (Elizabeth River) Z ... Tidal current velocities measured at 1 m above the bed attain speeds on the order of ... from linear theory to range from about 15 cm s' under fairweather conditions to ...

... Ronan Jr, TE 1977 Formation and palaeontological recognition of structures caused by marine annelids. ... Zucker, CL, Olsen, CR, Larsen, IL Cutshall, NH 1984 Inventories and sorptiondesorption trends of radiocesium and radiocobalt in James River estuary sediments. ...

... NH4+ and animal irrigation behavior Marilyn S. Mayerll*, Linda Schaffner 2, W. Michael ~ernp' ... Benthic macrofauna can significantly increase rates of nitrification (oxidation of NH4+ to NO3-) in sediments (Knstensen et al. 1991, Mayer 1992, Henriksen et al. 1993). ...

ABSTRACT Effects of macrobenthos on hydrophobic organic contaminant (HOC) transport during two seasons characterized by high (summer) and low (winter) bioturbation were studied in the laboratory. HOC burial and loss from microcosms were followed in the presence and absence of macrofauna after the introduction of particle-associated polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs) to the sediment−water interface. During summer, HOC burial depths and modeled diffusive losses (DL) increased with compound relative effective diffusivity ( ), consistent with enhanced fluid transport rates due to macrofauna. Although more than 50% of HOC loss was independent of macrofauna, macrofauna-mediated processes had significant effects on the long-term (56 d) removal of the hydrophobic compounds TCB and BaP. A possible mechanism for this effect is direct particle resuspension by macrofauna, which has been shown to enhance loss of materials from our experimental systems. During winter, macrofauna had little effect on HOC burial but may have enhanced HOC retention. Thus, effects of macrofauna-mediated processes depend on HOC physico-chemistry as well as the temporal phasing and duration of contaminant−biota interac tions. Bioturbation has important implications for contaminant fate because macrofauna significantly alter the distributions of HOCs near the sediment−water interface.

... Leptocheirus plumulosus, in Water-only and Sediment Exposures Susan B. Kane Driscoll,* Linda C. Schaffner and Rebecca M. Dickhut ... Samples were extracted with chloroform, methanoi, and water, using a previously described protocol (Bligh and Dyer, 1959; Kane Driscoll ...