Characterization of nitrogen isotope fractionation during nitrification based on a coastal time series

Abstract

Isotopic enrichment factors are key to using stable isotope signatures in biogeochemical studies. However, these are typically determined in laboratory experiments and their applicability to environmental conditions is difficult to test. Here, we analyzed nitrogen stable isotope changes associated with nitrification in a coastal basin using weekly time-series measurements of δ 15 N in particulate nitrogen, ammonium, nitrite, and nitrate. Two year-long time series were selected as contrasting natural experiments in the ammonium-rich, aphotic bottom water of Bedford Basin, Nova Scotia, Canada. In 2014, ammonia oxidation (AO) was associated with Thaumarchaeota and nitrite concentrations remained low (< 0.5 μmol kg À1 ). In contrast, transient nitrite accumulation (~8 μmol kg À1 ) and a more rapid δ 15 N NH4 increase in the fall of 2017 were likely caused by ammoniaoxidizing bacteria, associated with higher AO rates and, possibly, stronger nitrogen-isotope enrichment ( 15 ε AO ). Estimates of 15 ε AO (21.8 AE 2.2‰, 24.1 AE 1.1‰) were derived empirically using Rayleigh models applied to field data from restricted periods during which the bottom waters approximated a closed system and influence on 15 ε AO from other processes was demonstrably insignificant. Using a numerical reactive-transport model, we

Fjords Are Potential Hotspots of Refractory Dissolved Organic Matter Production: Insights From 1 Year of Weekly Time‐Series Observations of Fluorescent Dissolved Organic Matter in the Bedford Basin, a Northwestern Atlantic Fjord

“…Haas et al. (2022) suggested that suspended POM may contribute to the observed N‐isotopic differences with steady δ 15 N PON values observed in near‐surface waters with a simultaneous δ 15 N PON increase in the bottom water. Alternative reasons for this observation may be that sinking POM has different carbon/nitrogen compositions.…”

“…Specifically, chl a increased from January to March and remained relatively high until July and then exhibited a slight decrease, while POC and PON increased from January to July and then decreased. Haas et al (2022) suggested that suspended POM may contribute to the observed N-isotopic differences with steady δ 15 N PON values observed in near-surface waters with a simultaneous δ 15 N PON increase in the bottom water. Alternative reasons for this observation may be that sinking POM has different carbon/nitrogen compositions.…”

Tracking Agulhas leakage in the South Atlantic using modern planktic foraminifera nitrogen isotopes

“…We attribute this lack of isotopic difference to the comparatively short integration time of much of the suspended PON pool (that we assume comprised mainly phytoplankton) compared to zooplankton in general and foraminifera in particular (Eppley et al, 1983;Altabet and McCarthy, 1985;Fasham et al, 1990;Capone et al, 2008). Indeed, δ 15 NPON can be rapidly altered (within hours to days; Savoye et al, 2003;Treibergs et al 2014) by several processes, including a switch in the dominant N form consumed by phytoplankton (i.e., from nitrate to ammonium) (Liu et al, 2007;Treibergs et al 2014) and mixing with surrounding water masses (Mino et al, 2020;Haas et al, 2022). The isotopes of suspended PON are further discussed in section 4.3.4 below.…”

Sources and sinks of bottom water oxygen in a seasonally hypoxic fjord

“…The Sackville River delivers freshwater into the head of the basin and at its seaward end the basin is connected to the outer Halifax Harbour and the Scotian Shelf via a 20 m deep sill. The freshwater input (5.3 m 3 s -1 ) is small compared to the mean tidal volume (2.5 × 10 7 m 3 ) and the total basin volume (5.1 × 10 8 m 3 ) (Petrie and Yeats, 1990;Haas et al, 2022). The presence of the sill restricts the estuarine circulation to the upper 20 m and the bottom water can become isolated from the surface layer due to stratification, leading to development of seasonal hypoxia.…”

“…This includes weekly CTD casts for hydrographic (T, S) and several chemical properties in both the surface (1, 5, 10m) and bottom (60m) waters. Bedford Basin has been the focus of numerous studies examining geological (Miller et al, 1982;Buckley and Winters, 1992;Fader and Miller, 2008), chemical (Shi and Wallace, 2018), climatological (Li and Harrison, 2008), biogeochemical (Conover, 1975;Hargrave, 1978;Taguchi and Hargrave, 1978;la Roche, 1983;Azetsu-Scott and Johnson, 1994;Punshon and Moore, 2004;Li et al, 2010;Burt et al, 2013;Georges et al, 2014;Haas et al, 2021;Haas et al, 2022;Robicheau et al, 2022) and environmental (Rao et al, 1993;Pan and Subba Rao, 1997;Scott, 2005;AMEC, 2011;Lacasse et al, 2013;Vercaemer and Sephton, 2014) aspects of the basin. Several of these studies have focussed on processes influenced by the transient hypoxic characteristics of the basin.…”