MEPS 237:41-50 (2002)  -  doi:10.3354/meps237041

ABSTRACT: Rates of nitrate (NO₃^-) reduction to nitrogen gas (N₂) and ammonium (NH₄⁺) were measured in August and December 1999 on intact cores (Laguna Madre and Baffin Bay, Texas) using flowing seawater enriched with ¹⁵NO₃^-. The combination of membrane inlet mass spectrometry (MIMS) and high performance liquid chromatography (HPLC) allowed accurate and simple estimation of these 2 dissimilatory pathways of NO₃^- reduction. NO₃^- enrichment (~100 µM ¹⁵NO₃^-) did not stimulate denitrification (mean ± SE = 55 ± 16 and 69 ± 15 [Aug 99], -11 ± 16 and 11 ± 18 [Dec 99] µmolN m^-2 h^-1 before and after ¹⁵NO₃^- addition, respectively; n = 8). However, ¹⁵NH₄⁺ production rates increased after the ¹⁵NO₃^- addition (69 ± 14 [Aug 99], 50 ± 9 [Dec 99] µmolN m^-2 h^-1), comprised about 1/3 of total NH₄⁺ flux, and were comparable to denitrification rates. A larger portion of added ¹⁵NO₃^- was converted to ¹⁵NH₄⁺ (15 to 75%) than to N₂ (²⁹⁺³⁰N₂; 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.

KEY WORDS: Dissimilatory nitrate reduction to ammonium · DNRA · Denitrification · Brown tide · Nitrogen conservation · Laguna Madre/Baffin Bay