Tobias, C.R., I.C. Anderson, E.A. Canuel and S.A. Macko. 2001. Nitrogen cycling through a fringing marsh-aquifer ecotone. Marine Ecology Progress Series 210:25-39. Keywords=nitrogen, marsh, groundwater, isotopes, salinity End_keywords 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 1-1.5 meters 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 mmoles N m-2 hr-1, respectively. During Spring discharge (May), porewater conductivity, and dissolved sulfide decreased by approximately 50 %, and a groundwater driven O2 flux of 27 µmoles m-2 hr-1 into the marsh subsurface was estimated. Although mineralization, nitrification, and DNRA rates were up to 12x, 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 d15N 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. End_abstract Notes= submitted by iris@vims.edu, Fri Apr 13 15:44:44 EDT 2001 End_notes