MEPS 210:25-39 (2001) - doi:10.3354/meps210025
Nitrogen cycling through a fringing marsh-aquifer ecotone
Craig R. Tobias1,*, Iris C. Anderson1, Elizabeth A. Canuel1, Stephen A. Macko2
ABSTRACT: Fringing wetlands are critical components of estuarine systems, and subject to water fluxes from both watersheds and estuaries. To assess the effect of groundwater discharge on marsh nitrogen cycling, we measured N-cycling in sediments from a fringing mesohaline marsh in Virginia which receives a seasonal groundwater input. Mineralization, nitrification, potential denitrification (DNF), and potential dissimilatory nitrate reduction to ammonium (DNRA) rates were estimated along with porewater concentrations of oxygen, sulfide, and conductivity during high (May 1997) and low (October 1997) groundwater discharge. All N-cycling processes were confined to the upper 1 to 1.5 m of marsh, where organic matter and ammonium were most abundant. Depth-integrated rates for mineralization, nitrification, DNRA, and DNF ranged between 1.0-11.2, 0.0-2.2, 0.9-6.1, and 1.8-17.6 mmol N m-2 h-1, respectively. During spring discharge (May), porewater conductivity, and dissolved sulfide decreased by approximately 50%, and a groundwater-driven O2 flux of 27 µmol m-2 h-1 into the marsh subsurface was estimated. Although mineralization, nitrification, and DNRA rates were up to 12 ×, 6x, and 7.5x greater in May, respectively, than during low discharge (October), DNF was 10x higher in October. The largest difference in seasonal rates was observed nearest the upland border, where groundwater discharge had the greatest effect on sediment geochemistry. We suggest that a synergy between an increased flux of electron acceptors, porewater mixing, and flushing of salt and sulfide was responsible for the elevated mineralization and nitrification rates in May. Natural-abundance δ15N measurements of the NH4+, NO3-, and N2 pools showed that nitrification is important in mediating N export by linking mineralization and denitrification in this marsh. However, despite accelerated mineralization and nitrification in May, there was not an equivalently large export of N via coupled nitrification-denitrification. The DNF:DNRA ratio in May (0.6) was 25-fold lower than that seen at low discharge, indicating that during spring discharge, a greater proportion of nitrified N was recycled internally rather than exported via denitrification.
KEY WORDS: Nitrogen · Marsh · Groundwater · Isotopes · Salinity
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