Doctor of Philosophy (Ph.D.)
Virginia Institute of Marine Science
Iris C. anderson
Elizabeth A. Canuel
The influence of groundwater discharge on the hydrology and biogeochemical cycling of nitrogen in a fringing intertidal wetland was studied by characterizing groundwater discharge, determining N-cycling rates in cores, and examining nitrate reduction in situ using 15N enrichment and natural gradient tracer techniques. Groundwater discharge was estimated by three independent methods: Darcy's Law, a water/salt mass balance, and a subsurface tracer test. Seasonal patterns of discharge predicted by Darcy's Law and the mass balance were similar. Discharge maxima and minima occurred in April and September, respectively. The water/salt mass balance provided the more reasonable estimate of groundwater flux at high flows, and the Darcy technique was better at estimating low flow at our site. The high discharge seasonally purged porewater from the marsh to the estuary, and marsh processing of groundwater solute loads would occur only during this period. Mineralization, nitrification, potential denitrification (DNF), and potential dissimilatory nitrate reduction to ammonium (DNRA) rates were estimated in cores during periods of high and low groundwater discharge. All N-cycling processes occurred in sediments <1.5 meters deep. Natural abundance isotope measures, and core experiments indicated that coupled nitrification-denitrification was a sizeable sink for mineralized N. Mineralization, nitrification, and DNRA rates were 6--12x greater during Spring high discharge. DNF rates, were 10x higher during Fall low discharge. Despite accelerated mineralization and nitrification during high discharge, the DNF:DNRA ratio was <1, indicating that more of the N cycled through nitrification was retained as ammonium rather than exported as dinitrogen through coupled nitrification-denitrification. Nitrate reduction pathways in the marsh were studied in situ by creating a nitrate plume enriched in 15N. Isotopic enrichment of the ammonium, PON, dissolved nitrous oxide, and dissolved dinitrogen pools initially accounted for 14--36% of the observed nitrate loss. Adjustment of these estimates with potential losses through gas evasion, and ammonium turnover, accounted for nearly all of the N missing from the mass balance. The adjusted mass balance indicated that 68% of the nitrate load was denitrified, and 30% was assimilated and retained in the marsh.
© The Author
Tobias, Craig -1967, "Nitrate reduction at the groundwater - salt marsh interface" (1999). Dissertations, Theses, and Masters Projects. Paper 1539616877.