Sediment-water column nutrient exchanges in southern Chesapeake Bay nearshore environments

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Virginia Water Resources Research Center, Virginia Polytechnic Institute and State University

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.