Figure  1.  Sampling locations, bathymetry, and circulation in the Indian sector of the Southern Ocean, 62°−68°S and 30°−80°E. Sections C3, C4, C5, C6, and C7 are located in the Cosmonaut Sea, and Sections C8, C9, and P1 are located in the Cooperation Sea. All dots represent CTD observation and nutrient sampling sites. Yellow dots indicate nitrification rate measurements in situ and in the dark. The two yellow underlines indicate sites where substrate kinetic experiments for nitrification under light and dark conditions were performed. Oceanographic features highlighted include the Southern Antarctic Circumpolar Current Front (SACCF, 1.8℃ isotherm, brown line), the Southern Boundary of the upper CDW (SB, 1.5℃ isotherm, red line), and the Antarctic Slope Current (purple line) (Orsi et al., 1995). Dark blue circular arrows indicate the Antarctic Divergence zone (AD) between the SACCF and SB, with a large number of mesoscale eddies (Talley et al., 2011). The eastern boundary of the Weddell Gyre is marked by a green line with an arrow. The Prydz Bay Gyre is marked by a gray dashed circle (Heywood et al., 1999). The location of the Cape Darnley Polynyas (CDP) is marked off the Mawson Coast. The isobaths at 500 m, 1000 m, 3000 m, and 4000 m are represented by khaki, black dashed, turquoise, and blue lines, respectively.

Figure  2.  In-situ nitrification rates in different water masses above 500 m depth. a. Potential temperature (θ) versus salinity at sampling sites. The light grey curves represent the contours of potential density anomalies referenced to zero pressure (kg/m³). The dashed red lines along the x- and y-axes represent the surface freezing point (−1.89℃) and the salinity 34.65 contours. The water masses in the upper 500 m include Antarctic Surface Water (AASW), Winter Residual Water (WW), Thermocline Water (TCW), Circumpolar Deep Water (CDW), and Low-Salinity Shelf Water (LSSW). The characteristic range of potential temperature and salinity for each water mass is marked with black rectangles and lines. The detailed information and classification of water masses are given in the Text S1. b. The in situ nitrification rate [NR_{in situ}, nmol/(L·d), in terms of N] in each water mass, its average ± standard deviation (SD) is shown in the upper left corner.

Figure  3.  Profiles of apparent oxygen utilization (AOU), ${{\mathrm{NH}}_4^+} $, ${{\mathrm{NO}}_2^-} $, ${{\mathrm{NO}}_3^-} $, and nitrification rate (NR) in situ and under dark conditions in the upper 500 m water column. The yellow highlighted area indicates the depth at which the maximum value occurs. Black dashed lines in e and f represent 0.1% PAR depth. The legend below the plot shows the color and symbol representing each sampling site.

Figure  4.  Nitrification rate as a function of total ${{\mathrm{NH}}_4^+} $ concentration in substrate enrichment experiments at Sites C9-09 (a) and C9-05 (b). The samples for nitrification incubation at Site C9-09 were collected from 25 m (open square) and 200 m (solid square), and those at Site C9-05 were collected from 60 m (open square) and 200 m (solid square). The red and black dots and lines represent data obtained in situ [50%, 10%, and 0.1% photosynthetically active radiation (PAR)] and in the dark, respectively. The dashed and solid curves represent the fitting results of the M-M equation at p < 0.05 level for the data of 25 m/60 m and 200 m, respectively. Note that NR_{in situ} at 25 m from Site C9-09 does not fit the Michaelis-Menten (M-M) curve, but is linearly positively correlated (slope = 0.0051, R² = 0.98, p = 0.0002), reflecting the unsaturated state of the substrate.

Figure  5.  Light inhibition on nitrification. a. Spatial variation of site averages of nitrification rates (NR_avg, nmol/(L·d), in terms of N) under in situ (yellow bar) and dark conditions (grey bar). The brown, red, and purple lines show the Southern Antarctic Circumpolar Current (ACC) Front (SACCF, 1.8℃ isotherm), Southern Boundary of ACC (SB, 1.5℃ isotherm), and Antarctic Slope Current, respectively. b. Profiles of mean NR_{in situ} (yellow dots) and NR_dark (dark grey dots) at depth are shown with their respective error bars for all sites. The Light Inhibition calculated from the difference between mean NR_{in situ} and NR_dark is represented by horizon blue bars with light blue error bars. Note that a break is set at a depth of 122 m for better visualization. c. The relationship between NR_{in situ} and NR_dark under paired incubation experiments. The linear fit equation (red circles and dark red line) has a slope of 1.22, and the linear fit equation (blue triangles and dark blue line with 95% confidence band between double dashed blue lines) has a slope of 1.13 at low chlorophyll a (Chl-a) levels. The black solid line represents the 1:1 line.

Figure  6.  The relationship between on-site nitrification rate and environmental factors. a. Relationship between nitrification rate in darkness (NR_dark) and apparent oxygen consumption (AOU). The NR_dark in the upper 100 m water column increases exponentially with the increase of AOU (dark red circle). No correlation was observed between the two at the depth of 200−500 m due to the low available substrate (black square). b. Relationship between depth-integrated nitrification rate (DINR) and primary productivity (PP). Results in situ and the dark are shown in red and black, respectively.

Figure  7.  Light is the primary limiting factor for nitrification compared to ammonium in the epipelagic zone. a. Relationship between nitrification rate (NR) and total ${{\mathrm{NH}}_4^+} $ concentration under different photosynthetically active radiation (PAR). Data obtained at different light intensities are represented by different colors. b. The change of photosensitivity index (PI) value with tracer addition at Sites C9-09 and C9-05. The data at 25 m and 200 m at Site C9-09 are represented by solid pink squares and hollow squares, respectively. The data at 60 m and 200 m at Site C9-05 are represented by solid yellow circles and hollow circles, respectively. The pink arrow marks the most significant decrease in PI values under dark and 50% PAR conditions.

Figure  8.  Nitrification substrate affinity (α) and its variation in our study sites and the global ocean. a. Comparison of α values under light (white bars) and dark (gray bars) conditions at Sites C9-09 and C9-05. Compared with light, the folds of α increase in the dark in the same layer marked with blue arrows and numbers. A negative correlation was observed between α and photosynthetically active radiation (PAR) (slope = −0.12, R² = 0.66, p = 0.0145). b. The relationships between α_light and α_dark. Data from our study sites, the Western North Pacific Ocean (WNP) and the East China Sea (ECS) (Xu et al., 2019) are represented by red and dark green open squares, and dark green open triangle, respectively. Linear fit lines and their 95% confidence are plotted, with red and dark green lines for our study sites and WNP, respectively. c. Profiles of α in the global ocean. The α-value data include those from our study sites, the South China Sea (SCS) (Wan et al., 2018), the Hood Canal in Puget Sound (Horak et al., 2013), the WNP (Zhang et al., 2020), the Indian sector south of Africa (Mdutyana et al., 2022b), the eastern Equatorial South Pacific (ETSP) (Peng et al., 2016), the Sargasso Sea (Newell et al., 2013), the WNP and the ECS (Xu et al., 2019). Data for summer, autumn, winter, and spring are represented in red, yellow, blue, and green, respectively. Filled and open symbols represent data under dark and light conditions, respectively. d. Relationship between α_dark and ${{\mathrm{NH}}_4^+}_{\mathrm{amb}} $ in summer, winter, and the Southern Hemisphere oceans, shown in red, blue, and black, respectively. Data from winter were sparse and discrete, resulting in poor fitted correlation (p > 0.1). e. Variation of α with latitude in the global ocean. The absolute values of the slopes (|slope|) and p-values of the fitted equations for the Southern and Northern Hemisphere oceans are shown.

Figure  9.  Relationship between nitrification rate (NR) and upwelling intensity indicators. a. Spatial variation of nitrate anomaly at 100 m. b. Spatial variation of temperature anomaly at 100 m. c. Spatial variation of salinity anomaly at 100 m. d. Spatial variation of NR_{in situ} at 100 m. e. Spatial variation of NR_dark at 100 m. f. Spatial variation of ΔNR (NR_dark minus NR_{in situ}) at 100 m. g. Relationship between NR and ${{\mathrm{NO}}_3^-} $ concentration. Data under in situ and dark conditions are represented by red and black open circles, respectively. h. Relationship between NR and nitrate anomaly at 100 m. Data under in situ and dark conditions are represented by red and black open triangles, respectively.

Figure  10.  Depth-averaged profiles of nitrification rate in situ light (NR_{in situ}) and in darkness (NR_dark) and photoinhibition alleviation between upwelling and non-upwelling sites. NR_{in situ} for upwelling and non-upwelling sites are indicated by solid pink triangles and solid blue circles, respectively. NR_dark for upwelling and non-upwelling sites are indicated by open triangles and circles, respectively. The percent alleviation from photoinhibition calculated from the difference between the percent inhibition of upwelling and non-upwelling sites is represented by green horizontal bars with dark green error bars. Note that a break is set at a depth of 122 m for better visualization.

Figure  11.  Conceptual diagram for the mechanism of photoinhibition alleviation and nitrification promotion in the summer Cosmonaut and Cooperation seas of the Indian Sector in the Southern Ocean.