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Numerical model run showing drogue path
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Fate of 'reactive nitrogen' derived from agricultural sources in coastal lagoons. U.S. Department of Agriculture Ecosystems Panel
Proposal Project Summary
PI: Karen McGlathery, Department of Environmental Science, University of Virginia,
Co-PIs: Iris C. Anderson and Carl Friedrichs, Post-Doc David Fugate, School of Marine Science, Virginia Institute of Marine Sciences, College of William and Mary
Coastal lagoons are typically shallow and well-mixed with limited freshwater input. Their high surface area to water volume ratio promotes sediment–water column interactions and associated biological transformations. Although coastal lagoons and bays constitute a major type of land margin ecosystem on most continents, the retention and transport of nutrients through these systems have received far less attention than in large estuaries. Hog Island Bay (HIB) is a coastal lagoon located between the mainland of Virginia’s Delmarva Peninsula and offshore barrier islands. It is part of the Virginia Coast Reserve, managed by the Nature Conservancy, and an LTER site (VCR-LTER). Like most other coastal lagoons it is shallow, well mixed, and receives no riverine input. Dominant autotrophs in the system are macroalgae and benthic microalgae. Nutrient inputs from its small watershed are thought to be primarily from the shallow aquifer, which is heavily impacted by agricultural activities. Because the system is well constrained by having a small watershed and a single inlet for exchange of water with the coastal ocean, it is an excellent site to explore the relative roles of biological and physical processes in regulating the flow of “reactive nitrogen” across the land-sea margin. In order to determine the potential for HIB to remove and/or retard the flow of reactive N and C to the coastal ocean, an interdisciplinary study is proposed to relate rates of biological processes to estimated watershed inputs of dissolved inorganic and organic species and to water residence time in the lagoon This proposal will examine the following hypotheses:
I. Hog Island Bay is a net autotrophic system in which excess organic matter is stored primarily in micro- and macroalgae, which effectively remove and retard “reactive” nitrogen during its transport from the watershed to the coastal ocean.
a. The lability of dissolved organic matter produced during decomposition will, depending on transport time, determine how effective the lagoon is in retarding or removing nitrogen.
b. Dissolved inorganic nitrogen regenerated during decomposition of micro- and macroalgae will be removed in the lagoon primarily by coupled nitrification-denitrification.
II. Because biological process rates mediated primarily by micro- and macroalgae and bacteria will exceed physical transport rates, immobilization of groundwater-derived and remineralized N within the lagoon will be more rapid than export by tidal flushing.
Groundwater inputs of N will be estimated based upon measurements of water flow and N concentrations in three transects of wells and lagoon-edge wells installed within the HIB watershed. These data will be used to calibrate a watershed model which will allow prediction of temporal variation in inputs of N to HIB. Within the lagoon measurements will be performed of both sediment and water column processes that produce ammonium (NH4+) (mineralization, N-fixation), immobilize NH4+ (bacterial, micro- and macroalgal uptake), and remove DIN (nitrification-denitrification) and DON (burial, bacterial, micro- and macroalgal uptake). By using coupled hydrodynamic and advection-dispersion models to relate biological processing rates to residence time, net retention of N within the coastal lagoon will be estimated.
Download January 2003 update on modeling results (PDF 180K)
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