Figure 1.
Location of stations superimposed upon seawater temperature at 75-m-depth.
(A) the 2010 cruise (R/V Atlantis), and (B) the 2011 cruise (R/V Melville). Station numbering are identical to that of 2 companion papers [28], [42]).
Figure 2.
Horizontal and vertical distributions of hydrological and biogeochemical parameters during the 2010 cruise (R/V Atlantis).
(A, B) temperature, (C, D) dissolved oxygen, (E, F) chlorophyll fluorescence in the northern transect (left panels) and southern transect (right panels).
Figure 3.
Horizontal and vertical distributions of hydrological and biogeochemical parameters during the 2011 cruise (R/V Melville) – Northern transect (10°S).
(A) temperature, (B) dissolved oxygen, (C) chlorophyll a fluorescence, (D) NO3− concentrations, (E) PO43− concentrations, (F) Mean N2 fixation rates (n = 3).
Figure 4.
Horizontal and vertical distributions of hydrological and biogeochemical parameters during the 2011 cruise (R/V Melville) – Southern transect (20°S).
(A) temperature, (B) dissolved oxygen, (C) chlorophyll a fluorescence, (D) NO3− concentrations, (E) PO43− concentrations, (F) Mean N2 fixation rates (n = 3).
Figure 5.
Vertical profiles (0–2000 m) of N2 fixation rates (nmol L−1 d−1) during the 2010 cruise (R/V Atlantis).
Open circles: individual N2 fixation measurements at each depth. Black line: dissolved oxygen concentrations (µmol Kg−1) divided by 500 to fit on the same scale.
Table 1.
Areal N2 fixation rates (µmol N m−2 d−1) calculated from measurements performed in the euphotic and aphotic zones during the 2010 and 2011 cruises.
Figure 6.
Vertical profiles (0–2000 m) of N2 fixation rates (nmol L−1 d−1) during the 2011 cruise (R/V Melville).
Open circles: individual N2 fixation measurements at each depth. Black line: dissolved oxygen concentrations (µmol Kg−1) divided by 500 to fit on the same scale.
Figure 7.
Effect of glucose additions on mean N2 fixation rates (n = 3) during the Atlantis cruise (2010).
Data from bioassay experiments performed in the OMZ at Stations 5, 7 and 11. The error bars represent the standard deviation of triplicate incubations. Treatment means were compared using the 2-tailed non parametric Mann-Whitney mean comparison test (n = 3, α = 0.05, unpaired samples). Means that are significantly different (p<0.05) from the control are labeled with an asterisk.
Figure 8.
Effects of nutrient additions on mean N2 fixation rates (n = 3) during the Melville cruise (2011) (C: Control, AA: Amino acids, CH: Carbohydrates).
Data from bioassay experiments performed in the OMZ at (A) Stations 7, (B) Station 9, (C) Station 11, (D) Station 5, (E) Station 1, and (F) Station 6 during the Melville cruise (2011). The error bars represent the standard deviation of triplicate incubations. Treatment means were compared using the 2-tailed non parametric Mann-Whitney mean comparison test (n = 3, α = 0.05, unpaired samples). Means that were significantly different (p<0.05) from the control are labeled with an asterisk.
Figure 9.
Results from the molecular analyses of the 2010 glucose amendment experiments.
Neighbor joining tree of partial nifH nucleotide sequences. Nodes are labeled with nifH cluster designations according to the convention established [74]. The number of sequences recovered from stations and treatments for each phylotype are indicated in boxes to the right of the tree.
Table 2.
Examples of published studies showing the range of oceanic N2 fixation areal rates measured in some contrasting oceanic environments.
Table 3.
Comparison between estimated fixed N losses via denitrification and anammox and fixed N gains via N2 fixation (estimated from the 0–2000 m depth integrated rates measured in this study) in the ETSP.