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OBJECTIVES: The specific objectives of this project are to 1) Develop and rigorously test an analytical method for the measurement of dissimilatory nitrate reduction to ammonium (DNRA),a potentially important N cycle flux not previously measured in Maryland waters; 2) Conduct a pilot study in the Maryland Coastal Bays; 3) Communicate the importance of this process in the understanding and management of nitrogen in the Bays through various outreach activities; 4) Ultimately develop a mechanism for broader access to this capability. METHODOLOGY: DNRA flux measurements are carried out by adding isotopically labeled nitrate 15NO3 to a water or sediment core sample followed by the collection and measurement of ammonium (as 15NH4+), the product of DNRA. By measuring15N in NH4+, DNRA is distinguished from other assimilatory and dissimilatory pathways of nitrate reduction. This work involves the assembly, testing, and performance evaluation of an HPLC for detection of 15NH4+, whereby isotope ratios are determined from the shift in retention time of the eluting NH4+. This is the method of choice due to small sample size requirements and time of analysis. RATIONALE: DNRA is increasingly recognized as an important pathway in eutrophic environments. Compared to denitrification, for which the end product of nitrate reduction is N2 gas, that of DNRA is NH4, which remains in the system and continues to support algal production, often by harmful algae. DNRA, driven by new inputs of NO3-, often from groundwater, is favored when labile organic carbon levels are high. Decadal changes in NH4+ in the Coastal Bays suggest that this may be a significant process occurring in these bays, but no such measurements have heretofore been made. Knowledge of these rates will help to inform N management policies.
Glibert, PM; Hinkle, DC; Sturgis, B; Jesien, RV. 2014. Eutrophication of a Maryland/Virginia coastal lagoon: a tipping point, ecosystem changes, and potential causes. Estuaries and Coasts37(1):128 -146. doi:10.1007/s12237-013-9630-3. UM-SG-RS-2014-12.