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تنوع مکانی و زمانی در ویژگی های رسوب معلق در لایه سطحی خلیج چساپیک فوقانی
Spatial and Temporal Variability in Suspended Sediment Characteristics in the Surface Layer of the Upper Chesapeake Bay
Periodic high discharge events flush suspended sediments from the Susquehanna River and Conowingo Dam reservoir into the upper Chesapeake Bay, which extends from the mouth of the Susquehanna River to the Bay Bridge near Annapolis, MD. Sediment characteristics in the surface layer of the upper Bay and changes in these characteristics with varying river discharge and distance downstream are not well known. In order to develop an integrated understanding of surface layer sediment dynamics, several in-situ data sets were examined at the Bay head and downstream along the Bay’s center channel, providing data on the spatial and temporal variability of suspended particle characteristics including concentration, settling speed, bulk density, and size. It was found that particles are entirely disaggregated at the Dam, later aggregating to a limited extent down Bay, and that downstream characteristics are more weakly linked to Susquehanna flow at lower flows and longer distances.
سرنوشت مواد مغذی در دو سیستم آب شیرین ساحلی
The Fate of Nutrients in Two Coastal Freshwater Systems
Human activities including fertilizer application and fossil fuel burning have increased nutrient concentrations in coastal waters. Nutrient inputs can be difficult to constrain at the coastal interface where multiple waters mix, including river water, groundwater, and lake or ocean water. At coastal interfaces, rivers distribute their nutrient loads across delta wetlands, where processes like anaerobic respiration and plant uptake may reduce nutrient concentrations. Beneath the coast, groundwater also carries nutrients offshore, where biogeochemical reactions alter the nutrient chemistry and discharge rates are difficult to measure. I aim to improve the assessment of nutrient loads to coastal waters in these challenging environments through two case studies. First, I estimate groundwater discharge, a previously unaccounted source of nutrients, to the United States Great Lakes coast using high-resolution geospatial analysis. By integrating land use data, I also identify areas of the coast that are vulnerable to high nutrient loads from groundwater. My analysis shows that almost one-third of Lake Erie’s United States coastline is vulnerable to contamination from groundwater nutrient sources. By collecting field measurements at a vulnerable beach site, I show that the nitrogen load from groundwater exceeds 1 gram/day/meter of coastline, which constitutes a small but non-negligible source to Lake Erie. In the second case study, I use benthic chambers to measure nitrogen removal rates in a coastal wetland in Wax Lake Delta, Louisiana. Results suggest that summertime nitrate removal kinetics are highly correlated with a widely available remotely-sensed vegetation index (NDVI). Heavily vegetated, submerged levees at intermediate elevations in the delta are thus predicted to be the most reactive habitats. Though less reactive, larger channels primarily on the eastern half of the delta may contribute most to nitrate removal, as they receive the greatest mass fluxes of nitrate. Numerical simulations of reactive nitrate transport in Wax Lake Delta and six synthetic deltas suggest that nitrate removal may be intrinsically limited in river-dominated deltas to a small fraction of the incoming nitrate load. Removal increases with delta topset gradient, and smaller, high-sitting deltas remove more nitrate than larger, low-lying deltas. From a management standpoint, nitrate removal efficiency can be improved by designing river diversions to build steeper deltas. Steeper deltas are created by accessing coarser sediments in river diversion projects. However, manmade deltas alone cannot remove most nitrate discharging to the sea. Policy that addresses the nitrate load upstream is necessary to further reduce coastal nitrate loading.