Browsing by Author "Reay, William G."

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    Sediment-water column nutrient exchanges in southern Chesapeake Bay nearshore environments
    Reay, William G.; Gallagher, Daniel L.; Simmons, George M. Jr. (Virginia Water Resources Research Center, Virginia Polytechnic Institute and State University, 1993-08)
    Using field and modeling methodology, the effects of benthic microcommunities and physical transport mechanisms on sediment nutrient flux were investigated for two nearshore sediment types on Virginia's Eastern Shore. Subtidal sandy mineral sediments were conducive to water transport and influenced by groundwater discharge to a greater degree than less-permeable organic silt-clay sediments. Sediment ammonium and dissolved inorganic phosphorus (DIP) fluxes were strongly dependent on benthic aerobic respiration rates for silt-clay sediments as compared to sandy substrates. In-situ studies showed a significant decrease in ammonium and DIP fluxes as a result of nutrient uptake by benthic microalgal communities. The importance of advective solute transport mechanisms within nearshore sediments was demonstrated by a one-dimensional, steady-state model for a conservative substance. By incorporating dispersive and advective transport mechanisms, model predictability for chloride sediment flux was significantly improved as compared to a diffusion-based model. Model results indicate that velocity-associated transport of solutes, driven by elevated upland hydraulic heads, is significant and can dominate over diffusive flux in sandy sediment. In contrast, silt-clay sediment solute fluxes were dominated by dispersive processes. The importance of sedimentary nutrient flux in a shallow coastal embayment was demonstrated by short water column dissolved inorganic nitrogen (DIN) and DIP turnover times ·with respect to sediment nutrient fluxes, by the significance of ammonium and DIP sediment nutrient fluxes with respect to phytoplankton nutrient assimilation demands, and by the relative importance of benthic respiration with respect to water column primary productivity. To evaluate water quality impacts of groundwater discharge on a larger scale, a surface, groundwater, and in-situ groundwater discharge water quality survey was conducted in a shallow estuarine embayment. Sediment inorganic nitrogen fluxes were elevated adjacent to agricultural fields without benefit of a vegetative buffer. Nitrogen contributions from direct groundwater discharge and groundwater-derived creek contributions appear to be of significant importance in terms of water quality and phytoplankton nitrogen assimilation demands. In summary, this study highlights the importance of nearshore sediments with regard to estuarine nutrient water quality and ecological processes. Neglecting groundwater discharge as a nutrient source and transport mechanism and potential biotic effects occurring at the sediment-water interface may lead to misinterpretation of data and error in water quality strategies.
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    Sediment-Water Column Oxygen and Nutrient Fluxes in Nearshore Environments of the Lower Delmarva Peninsula, USA
    Reay, William G.; Gallagher, Daniel L.; Simmons, George M. Jr. (Inter-Research, 1995)
    Sediment-water column exchanges of oxygen, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) were measured in situ over an annual cycle for sandy and silt-clay sediment types in a shallow Chesapeake Bay (USA) inlet. Benthic oxygen and inorganic nutrient fluxes differed between sediment types. Based on metabolic rate estimates and photosynthetic pigment concentrations, nearshore sandy sediments were more productive than silt-clay sediments. Overall benthic community respiration rates were 872 mu mol m(-2) h(-1) for sandy sediments and 2220 mu mol m(-2) h(-1) for silt-clay sediments. Elevated ammonium and DIP sediment fluxes were associated with silt-clay sediments. Sandy and silt-clay sediment ammonium fluxes ranged from -44 to 358 and -30 to 615 mu mol m(-2) h(-1) respectively, with DIP fluxes ranging from -5.3 to 42.0 and -3.3 to 35.7 mu mol m(-2) h(-1). Negative nutrient flux values denote sediment uptake. Sediment ammonium and DIP fluxes were dependent on benthic aerobic respiration rates for silt-clay sediments. In contrast, sandy sediment ammonium fluxes were less dependent and DIP fluxes showed no relationship to benthic aerobic respiration rates. Ammonium and DIP flux rates were significantly reduced in transparent chambers as compared to opaque chambers indicating the importance of the benthic microalgal community. On an annual basis, sandy sediments could supply 11% of the phosphorus and 6% of the phytoplankton nitrogen requirements based on gross productivity estimates, whereas silt-clay sediments could supply 11 and 14%, respectively. Positive correlations between sandy and silt-clay sediment DIN fluxes and phytoplankton DIN assimilatory demands emphasize the importance and interdependence of sediment heterotrophic and water column autotrophic processes. Short water column DIN and DIP turnover times, on the order of hours, were characteristic of summer conditions when water column nutrient concentrations were low and silt-clay sediment nutrient fluxes high. Conversely, nutrient turnover times on the order of days were characteristic of winter conditions when water column nutrient concentrations were high and sediment nutrient fluxes low.