Nitrate elimination and regeneration as evidenced by dissolved inorganic nitrogen isotopes in Saanich Inlet, a seasonally anoxic fjord

In this study, we used natural abundance isotope measurements of dissolved inorganic nitrogen (N) species to evaluate the effect of different oxygenation regimes on N transformation and elimination in Saanich Inlet, a seasonally anoxic fjord in British Columbia, Canada. We analyzed dissolved nutrient concentrations and the N (and O) isotope composition of nitrate (NO3-) and ammonium (NH4+) at different depths throughout the water column near the mouth of the inlet between April 2008 and April 2009. A gradual increase in both the NO3- delta N-15 and delta O-18, associated with a decrease in NO3- concentration and an increase in biological excess N-2, was observed after bottom water renewal events in August-October 2008, indicating NO3- consumption by denitrifying bacteria in an expanding suboxic water column. An increase in the delta N-15 of NH4+, with depth toward the suboxic/hypoxic transition, indicated net consumption of NH4+, most likely by micro-aerobic or anaerobic NH4+ oxidation and dissimilatory consumption by microorganisms. Deviations from a 1:1 correlation between the NW delta N-15 and delta O-18 (Delta(15,18)) that appears characteristic for both assimilatory and dissimilatory NW consumption in the ocean, were observed in surface waters and close to the hypoxic/suboxic transition. Lowered Delta(15, 18) values can most plausibly be explained by aerobic nitrification of newly remineralized NH4+ and/or low delta N-15-NO3- inputs from atmospheric precipitation in the surface mixed layer, and NO3- regeneration through NH4+ oxidation and/or the reoxidation of nitrite (NO2-) in deeper waters. Closed and open system model-derived N isotope effects for NO3- consumption calculated from time-series samples collected near the sediments in anoxic bottom waters were significantly lower (as low as similar to 11 parts per thousand) than the biological N isotope effects of similar to 20-30 parts per thousand for water column denitrification reported in other studies. We argue that the reduced N isotope effect is mainly due to the combined effects of water column and sediment denitrification, the latter occurring with a highly suppressed N isotope fractionation at the ecosystem level. We estimated that similar to 40-60% of the denitrification occurs within the sediments of the inlet. (C) 2013 Elsevier B.V. All rights reserved.