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