Fig 1.
N–cycle emphasising the production of N2O through denitrification (grey arrows) and nitrification (red arrows).
The production of N2 through anammox is presented with green arrows. Arrow width indicates suggested pathway importance in the south-eastern Indian Ocean. Marker genes for the different steps of the nitrogen cycle are in italics. The nitrous oxide reductase gene (nosZ) is highlighted with a dashed blue circle. Heterotrophic denitrification (nir/nor genes) is highlighted as a C source while the autotrophic anammox process (hzsA genes) is highlighted as a C sink. Note: dissimilatory reduction of NO2− to NH4+ and N2O is not presented on this figure. Figure adapted from Throbäck et al. [53]
Fig 2.
Spatial extent of oxygen concentrations (integrated from the surface to 200m depth).
Data (black squares) sourced from all available Argo floats with oxygen sensor in the region and cruise voyages conducted on the RV Southern Surveyor in 2010, 2011, 2012 and 2013. Low dissolved oxygen (<125 μmol.kg-1) stretches from the tropics to the subtropics. Black circles highlight sampling stations for functional N genes (nosZ and hzsA). Grey diamonds denote sampling station for heterotrophic microbial counts. Regional water masses Subtropical waters (STW) and Leeuwin Current waters (LC) are denoted by black arrows.
Fig 3.
(A) Nitrous oxide reductase (nosZ) gene copies (copies mL-1) plotted against in situ temperature data (°C); nosZ: r2 = 0.27, slope = 32381 copies.°C-1, coefficient standard error = 13259, p = 0.027. (B) Microbial counts (cells mL-1) versus in situ temperature data; red diamonds denote stations where nosZ genes were measured.
These microbial counts suggest a positive correlation with temperature (r2 = 0.39, p = 0.098). Grey diamonds present regional microbial abundance but do not suggest a temperature correlation (r2 = 0.016, p = 0.5); see Fig 2 for spatial coverage. Fig 3 C presents a latitudinal relationship of oxygen (r2 = 0.6, p<0.0001) and temperature (r2 = 0.8, p<0.0001). Data are capped from the surface to 50 m depth and sourced from all available Argo floats through the Australian Argo DAC as part of the IMOS portal and cruise voyages conducted on the Southern Surveyor in 2010, 2011, 2012 and 2013.
Fig 4.
Spatial extent of shallow (100-200m) low dissolved oxygen layers stretching from the tropics to the subtropics.
Data (blue dots) sourced from all available ARGO floats with oxygen in the region and cruise voyages conducted on the Southern Surveyor in 2010, 2011, 2012 and 2013. The red rectangle indicates the latitudinal area where a longitudinal slice through the water column has been analysed. Low dissolved oxygen layers (μmol kg-1) are highlighted by black circles.