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

Abstract

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− δ15N and δ18O, associated with a decrease in NO3− concentration and an increase in biological excess N2, 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 δ15N 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 NO3− δ15N and δ18O (Δ(15,18)) that appears characteristic for both assimilatory and dissimilatory NO3− consumption in the ocean, were observed in surface waters and close to the hypoxic/suboxic transition. Lowered Δ(15, 18) values can most plausibly be explained by aerobic nitrification of newly remineralized NH4+ and/or low δ15N–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 ~11‰) than the biological N isotope effects of ~20–30‰ 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 ~40–60% of the denitrification occurs within the sediments of the inlet.