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

Abstract

Abstract. The Indian Ocean subtropical gyre (IOSG) is one of five extensive subtropical gyres in the world's ocean. In contrast to those of the Atlantic and Pacific oceans, the IOSG has been sparsely studied. We investigate the water mass distributions based on temperature, salinity and oxygen data, and the concentrations of water column nutrients and the stable isotope composition of nitrate, using water samples collected between ∼30∘ S and the Equator during two expeditions: MSM 59/2 in 2016 and SO 259 in 2017. Our results are the first from this oceanic region and provide new information on nitrogen sources and transformation processes. We identify the thick layer of nutrient-depleted surface waters of the oligotrophic IOSG with nitrate (NO3-) and phosphate (PO43-) concentrations of < 3 and < 0.3 µmol kg−1, respectively (< 300 m; σ < 26.4 kg−1 m−3). Increased nutrient concentrations towards the Equator represent the northern limb of the gyre, which is characterized by typical strong horizontal gradients of the outcropping nutriclines. The influx of the Subantarctic Mode Water (SAMW) from the Southern Ocean injects oxygen-saturated waters with preformed nutrients, indicated by the increased N and O isotope composition of nitrate (δ15N > 7 ‰; δ18O > 4 ‰) at 400–500 m (26.6–26.7 kg−1 m−3), into the subtropical thermocline. These values reflect partial N assimilation in the Southern Ocean. Moreover, in the northern study area, a residue of nitrate affected by denitrification in the Arabian Sea is imported into intermediate and deep water masses (> 27.0 kg−1 m−3) of the gyre, indicated by an N deficit (N* ∼-1 to −4 µmol kg−1) and by elevated isotopic ratios of nitrate (δ15N > 7 ‰; δ18O > 3 ‰). Remineralization of partially assimilated organic matter, produced in the subantarctic, leads to a decoupling of N and O isotopes in nitrate and results in a relatively low Δ(15–18) value of < 3 ‰ within the SAMW. In contrast, remineralization of 15N-enriched organic matter from the Arabian Sea indicates higher Δ(15–18) values of > 4 ‰ within the Red Sea–Persian Gulf Intermediate Water (RSPGIW). Thus, the subtropical southern Indian Ocean is supplied by preformed nitrate from the lateral influx of water masses from regions exhibiting distinctly different N-cycle processes documented in the dual isotope composition of nitrate. Additionally, a significant contribution of N2 fixation between 20.36 and 23.91∘ S is inferred from reduced δ15N–NO3- values towards surface waters (upward decrease of δ15N ∼2.4 ‰), N* values of > 2 µmol kg−1 and a relatively low Δ(15–18) value of < 3 ‰. A mass and isotope budget implies that at least 32 %–34 % of the nitrate in the upper ocean between 20.36 and 23.91∘ S is provided from newly fixed nitrogen, whereas N2 fixation appears to be limited by iron or temperature south of 26∘ S.

Marine Water Quality at Diego Garcia: A Preliminary Study of Pollution Levels in Coastal and Lagoon Waters

“…Surface concentrations of nitrates and phosphates at the nearpristine sites were very low (below the limits of detection), as described by previous authors (e.g., McCreary et al, 2009;Harms et al, 2019;Sheehan et al, 2019). Deeper water masses of different origins are likely to provide the main sources of new nutrients which sustain primary production in the upper mixed layer or in the vicinity of the thermocline (e.g., Lévy et al, 2007;Dilmahamod et al, 2016;George et al, 2018).…”

“…Deeper water masses of different origins are likely to provide the main sources of new nutrients which sustain primary production in the upper mixed layer or in the vicinity of the thermocline (e.g., Lévy et al, 2007;Dilmahamod et al, 2016;George et al, 2018). Insufficient data were obtained during this study to investigate sources and cycling of nutrients, which has been reviewed elsewhere (e.g., Rayner and Drew, 1984;Sardessai et al, 2010;Garcia et al, 2018;Green et al, 2019;Harms et al, 2019;Twining et al, 2019). Observations from the non-pristine study sites suggest that nutrient levels were higher than concentrations that might be expected from the broadscale oceanography, particularly in the lagoon.…”

“…Broadscale oceanographic processes are influenced by atmospheric processes, regional currents and bathymetry (including seamounts and the Seychelles-Chagos Thermocline Ridge; Hermes and Reason, 2008;Xue et al, 2014;Masner et al, 2017), with seasonal winds and Ekman transport driving upwelling of nutrient-rich waters and sustaining plankton productivity (Fasolo, 2013;George et al, 2013;Dilmahamod et al, 2016). Surface waters are often nutrient depleted (Rayner and Drew, 1984;McCreary et al, 2009;Harms et al, 2019) with low chlorophyll (chl) concentrations (< 0.2 µg chl l −1 ; Resplandy et al, 2009;George et al, 2018). Higher concentrations (>5 µM nitrate, > 1 µg chl l −1 ) are typically found at or below the thermocline which can vary in depth from about 20 to 120 m (George et al, 2013).…”

Nitrogen isotopic analysis of nitrate in aquatic environment using cadmium–hydroxylamine hydrochloride reduction

“…In addition to the chemical reduction method, the denitrifier method is also widely applied in the determination of δ 15 N‐NO 3 − values in water samples 37–39 . Specifically, the δ 15 N‐NO 3 − values of the laboratory KNO 3 standard were 0.67 ± 0.14‰ and 0.66 ± 0.20‰ using the Cd‐Cu column–NH 2 OH method and the denitrifier method (Figure 6).…”

Megafauna of the German exploration licence area for seafloor massive sulphides along the Central and South East Indian Ridge (Indian Ocean)

“…These ecosystem functions include utilisation rates of surface derived detritus, dietary composition or the locomotion mode. Megafauna are likely subject to drastic changes due to mining activities and are expected to recover slowly related with slow rates of growth and recruitment (Ramirez-Llodra et al 2011, Amon et al 2016, Gollner et al 2017. Many megafauna taxa are considered as indicators of physical disturbance responding to disturbance events with changes in densities, dominating taxa or the community composition (Bluhm andGebruk 1999, Amon et al 2017).…”