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Rates of nitrate (NO3 - ) reduction to nitrogen gas (N2) and ammonium (NH4 + ) were mea- sured in August and December 1999 on intact cores (Laguna Madre and Baffin Bay, Texas) using flowing seawater enriched with 15 NO3 - . The combination of membrane inlet mass spectrometry (MIMS) and high performance liquid chromatography (HPLC) allowed accurate and simple estima- tion of these 2 dissimilatory pathways of NO3 - reduction. NO3 - enrichment (~100 µM 15 NO3 - ) did not stimulate denitrification (mean ± SE = 55 ± 16 and 69 ± 15 (Aug 99), -11 ± 16 and 11 ± 18 (Dec 99) µmol N m -2 h -1 before and after 15 NO3 - addition, respectively; n = 8). However, 15 NH4 + production rates increased after the 15 NO3 - addition (69 ± 14 (Aug 99), 50 ± 9 (Dec 99) µmol N m -2 h -1 ), comprised about 1/3 of total NH4 + flux, and were comparable to denitrification rates. A larger portion of added 15 NO 3 - was converted to 15 NH 4 + (15 to 75%) than to N 2 ( 29+ 30 N 2; 5 to 29%) on both sampling dates. High dissimilatory NO3 - reduction to NH4 + (DNRA) and low denitrification suggest that sulfide may influence the processes. High sulfide concentrations inhibit nitrification and denitrification but may enhance DNRA by providing an electron donor. Inhibited denitrification and enhanced DNRA may preserve available nitrogen in Laguna Madre/Baffin Bay, which has limited water exchange with other bodies of water.

marine ecology. progress series

Dissimilatory nitrate reduction to ammonium (DNRA) as a nitrogen link, versus denitrification as a sink in a shallow estuary (Laguna Madre/Baffin Bay, Texas)