Nutrient distribution and nitrogen and oxygen isotopic composition of nitrate in water masses of the subtropical South Indian Ocean

Abstract

<p><strong>Abstract.</strong> Vast subtropical gyres are important areas for the exchange of carbon between atmosphere and ocean in spite of low nutrient concentrations, and supposedly for the influx of reactive nitrogen to the ocean by dinitrogen fixation. To identify sources and transformation processes in the nitrogen cycle of the southern Indian Ocean subtropical gyre, we investigated concentrations of water column nutrients and stable isotope composition of nitrate of samples from two expeditions in 2016 (MSM 59) and 2017 (SO 259) in the subtropical gyre between ~ 30 °S and the equator. Low nitrate and phosphate concentrations mark the thick mixed layer of the oligotrophic gyre with values of < 5.9 µM NO<sub>3</sub><sup>−</sup> and < 0.5 µM PO<sub>4</sub><sup>3−</sup> (< 310 m; σ < 26.4 kg/m³). Increased nutrient concentrations towards the equator represent the northern end of the gyre, characterized by typical strong horizontal gradients of the outcropping nutriclines. Measurements of stable isotopes of nitrate (δ<sup>15</sup>N and δ<sup>18</sup>O) indicate isotopic maxima of δ<sup>15</sup>N (> 7 ‰) and δ<sup>18</sup>O (> 4 ‰) centred at 400–500 m, representing the preformed nitrate exported from the Southern Ocean with mode water and induced by partial N-assimilation there. Additionally, a residue of nitrate affected by denitrification in the Arabian Sea is imported into the sub-thermocline of the gyre, indicated by a strong N deficit (N* < −1 µM) within the northern study area, accompanied by elevated isotopic ratios of nitrate (δ<sup>15</sup>N > 7 ‰; δ<sup>18</sup>O > 3 ‰). The subtropical South Indian Ocean is thus supplied by nitrate from lateral influx of water masses that have similar isotopic character, but antagonistic origin (preformed versus regenerated). A significant contribution of N<sub>2</sub>-fixation within the Indian Ocean subtropical gyre (17° S–25° S) is promoted by low nitrate to phosphate ratios in the surface layer, where approximately one-third of the nitrate in the upper ocean is derived by newly fixed N.</p>

Lithogenic Particle Flux to the Subantarctic Southern Ocean: A Multi‐Tracer Estimate Using Sediment Trap Samples

“…The calculation of FLith, MB by mass balance is thus made under the assumption that the remaining fraction of the TMF minus biogenic flux components can be attributed to lithogenic material as shown in Equation 6. This method has been used by a number of sediment trap studies to estimate lithogenic particle fluxes (Wefer and Fischer, 1993;Ratmeyer et al, 1999b;Francois et al, 2002;Helmke et al, 2005;Nowald et al, 2015;Wynn-Edwards et al, 2020;Harms et al, 2021).…”

“…Higher correlation between FLith, MB and POC and opal compared to that between FLithAv and POC and opal together with similarities between the variability of FLith, MB and TMF fluxes (Figure 5) indicate that FLith, MB is more associated with export production compared to FLithAv. The use of mass balance to estimate lithogenic flux (Wefer and Fischer, 1993;Ratmeyer et al, 1999a;Ratmeyer et al, 1999b;Francois et al, 2002; In submission of S7Z -Master of Marine and Antarctic Science Research Component -42 -Helmke et al, 2005;Fischer and Karakaş, 2009;Nowald et al, 2015;Harms et al, 2021) might therefore overestimate the correlation between lithogenic particles and POC export at 1000m depth. As a consequence, FLith, MB estimates may exaggerate the role of lithogenic particles in the ballast process of POC (Klaas and Archer, 2002).…”

“…Our derivation of FLith, MB (from Wynn-Edwards et al (2020) data) uses POM = 2.199´POC (Klaas and Archer, 2002). Francois et al (2002) and Harms et al (2021) employ the (Anderson, 1995) empirical marine organic matter formula C106H175O42N16P, yielding POM = 1.87´POC (from mean algal elemental composition). The POM:POC ratio may vary regionally due to local phytoplankton taxonomic composition and to nutrient depletion (Suess and Ungerer, 1981;Sharoni and Halevy, 2020).…”

The Agulhas Current Transports Signals of Local and Remote Indian Ocean Nitrogen Cycling

“…First, diazotrophs have been observed during the late-summer bloom that occurs irregularly to the southeast of Madagascar (Poulton et al, 2009), as well as within and south of the Mozambique Channel (Huggett & Kyewalyanga, 2017;Karlusich et al, 2021), with surface N 2 fixation rates of 2-18 nM.d −1 measured southeast of Madagascar during the most recent (2020) late-summer bloom (Metzl et al, 2022). Second, one third of the thermocline nitrate at the north-eastern edge of the greater Agulhas region (∼23°S, 70°E; Figure 1) has previously been estimated to be newly fixed based on subsurface nutrient N:P ratios (Harms et al, 2019). Here, the δ 15 N of thermocline nitrate, which was measured to average 5.2 ± 1.1‰, and the mean AOU and nitrate concentrations of 27.9 ± 10.5 μM and 1.6 ± 1.3 μM, respectively, are remarkably similar to those of STTW in our data sets (Table 1) (Harms et al, 2019), strongly suggesting that newly fixed nitrate is retained in the greater Agulhas region.…”

“…Second, one third of the thermocline nitrate at the north-eastern edge of the greater Agulhas region (∼23°S, 70°E; Figure 1) has previously been estimated to be newly fixed based on subsurface nutrient N:P ratios (Harms et al, 2019). Here, the δ 15 N of thermocline nitrate, which was measured to average 5.2 ± 1.1‰, and the mean AOU and nitrate concentrations of 27.9 ± 10.5 μM and 1.6 ± 1.3 μM, respectively, are remarkably similar to those of STTW in our data sets (Table 1) (Harms et al, 2019), strongly suggesting that newly fixed nitrate is retained in the greater Agulhas region. Third, the mean N* (= [NO 3 − ] − 16 × [PO 4…”