Low Nitrogen Fixation Related to Shallow Nitracline Across the Eastern Indian Ocean

Abstract

Nitrogen fixation by diazotrophs provides a substantial source of bioavailable nitrogen in the ocean, supporting new production and net carbon uptake in the ocean (Gruber & Galloway, 2008;Karl et al., 1997). In the past two decades, sequencing of the nifH gene (which encodes the iron unit of nitrogenase) has revealed diverse diazotrophs, including cyanobacteria such as Trichodesmium and UCYN-A1, and non-cyanobacteria with broad geographic ranges from tropical and subtropical to polar regions (Zehr & Capone, 2020).The elucidation of the diversity of diazotrophs has shown that various factors control the activity and community structure of diazotrophs, including temperature, phosphorus, iron, nutrient supply ratios, and zooplankton grazing (

Diazotrophy in the Indian Ocean: Current understanding and future perspectives

“…More recent qPCR studies have either detected (Li et al 2021) or not detected (Sato et al 2022) UCYN‐A during the NEM, which may imply interannual variability in the composition of the diazotroph community for reasons yet to be understood. The southern edge of the BoB features anticyclonic eddies along 10° N during the IM period leading to strong water column stratification and oligotrophic conditions, favoring cyanobacterial diazotroph growth (Jyothibabu et al 2017; Wu et al 2021).…”

“…In austral summer, the Indian Ocean Dipole induces a positive sea surface anomaly in the western coast of Australia (Terray et al 2005), which together with iron‐rich dust deposition from the Australian landmass is thought to promote relatively high N 2 fixation rates during the austral IM (up to 77.89 nmol N L −1 d −1 ), despite the concomitant notable upward nitrate fluxes (Raes et al 2015; Sato et al 2022). The highest nitrate concentrations in this region are, however, found during the austral winter (up to 0.8 μ mol L −1 ).…”

“…This may be influenced by time constrained events such as seasonal cyclones and mesoscale features pulsing reactive nitrogen into the euphotic zone and inhibiting the development of diazotrophs (Singh and Ramesh 2015). Alternatively, the low iron to nitrogen (Fe : N) ratio of aerosol inputs has been suggested as a possible explanation for low N 2 fixation activity in the BoB (Sato et al 2022). A recent study (Saxena et al in review), argues that the negative effects of turbulence on diazotrophs may override the benefits of favorable nutrient conditions for diazotrophy in the BoB.…”

“…At ~ 0–10°S in the central EqIO, upwelling promotes important nitrate diffusive fluxes (46.9–397 μ mol m −2 d −1 ) with high N : P ratio values (Sato et al 2022). Together with the low dissolved iron concentrations of this region (Thi Dieu Vu and Sohrin 2013; Baer et al 2019), these biogeochemical conditions seem to inhibit diazotrophy in the central EqIO (Sato et al 2022).…”

“…At ~ 0–10°S in the central EqIO, upwelling promotes important nitrate diffusive fluxes (46.9–397 μ mol m −2 d −1 ) with high N : P ratio values (Sato et al 2022). Together with the low dissolved iron concentrations of this region (Thi Dieu Vu and Sohrin 2013; Baer et al 2019), these biogeochemical conditions seem to inhibit diazotrophy in the central EqIO (Sato et al 2022). In the eastern EqIO N 2 fixation rates are also low (0.03–0.20 and 0.10–1.16 nmol N L −1 d −1 during the IM and NEM, respectively; Sato et al 2022, Wu et al 2019), despite its deep thermocline and nutricline (Murtugudde et al 1999; Vinayachandran et al 2009; 2021).…”