Nutrient distributions and nitrogen-anomaly (N*) in the tropical North Pacific Ocean
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Abstract: Based upon cruise observations broadly covering the tropical North Pacific during July−November 2017, together with data obtained from the World Ocean Circulation Experiment Hydrographic Program, this study examined the distribution of dissolved inorganic nitrogen (DIN, nitrate (
${{\rm {NO}}_3^-} $ )+nitrite (${{\rm {NO}}_2^-} $ )), dissolved inorganic phosphorus (DIP) and related N* (nitrogen-anomaly, N*=N−16P+2.9, where N and P are the concentrations of DIN (>1.0 μmol/L) and DIP (>0.1 μmol/L)), used as an index of N2 fixation, in the upper 1 000 m of the water column. Nutrient concentrations displayed distinct spatial variability in the upper ocean but became relatively constant at a depth of 1 000 m: they were high at low latitudes and in the eastern region, with an obvious nutricline at ~150 m (DIN, ~32.0 μmol/L; DIP, ~2.4 μmol/L) and then generally increased with depth; they decreased markedly (DIN, ~1.2 μmol/L; DIP, ~0.1 μmol/L; at ~150 m) at high latitudes and in the western region, where a nutricline was not apparent. The N* index showed significant meridional and zonal variation, with the most negative values located at low latitudes and in the eastern region (~10°N, ~150°−170°E), while becoming positive towards the northwest (the north of ~18°N, ~160°E westward). A N* concentration larger than 2.0 μmol/L which often used as an indicator of N2 fixation, was observed between 155°E and 165°E; N* values were 2.0 μmol/L to 6.0 μmol/L at ~15°−28°N, i.e., much higher than those in the southern sector (0−2.0 μmol/L at ~5°−10°N). Zonally, N* decreased gradually from west (−2.0 μmol/L to 4.0 μmol/L, ~145°−165°E) to east (−2.0 μmol/L to −8.0 μmol/L, ~155°W) along ~10°N, which was consistent with the distribution of Trichodesmium abundance and N2 fixation rates. Furthermore, since such region was also supplied with aeolian deposition, high N* was probably not only induced by N2 fixation but also influenced by iron and/or nitrogen deposition.-
Key words:
- tropical North Pacific Ocean /
- nutrients /
- nitrogen anomaly /
- nitrogen fixation /
- aeolian deposition
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Figure 1. Sampling locations in the tropical North Pacific Ocean based on the World Ocean Circulation Experiment (WOCE) program and our in situ cruise conducted from July 12 to November 18, 2017 (a, b). Stations for aerosol sampling conducted from April 8 to May 8, 2017 (black stars) and from July 19 to August 24, 2017 (black plus sign). Field sampling in the in situ cruise 2017 covered 40 stations (red dots with black circles) including a series of seamounts, i.e., Demao and Batiza (NA), Niulang (NLG), and McDonnell (MP4) Guyots, the China mining claim contract area (China Ocean Mineral Resources R&D Association contract area, COMRA) and related deep-sea mining Clarion-Clipperton Zone (CCZ). ALOHA: A Long-term Oligotrophic Habitat Assessment.
Figure 2. Observed depth profiles of potential temperature (θ) (a−d), salinity (e−h), and density (σ0) (i−l) of transects along ~155°E and ~10°N in the tropical North Pacific Ocean, including the dataset from the WOCE program and our in situ cruise in 2017.
Figure 3. Relationship between potential temperature(θ) and salinity in the tropical North Pacific Ocean showing observed water masses with depth indicated by colors, including data from the WOCE program (14 cruises listed in Table 1) and from our in situ cruise during August−November, 2017 (field data indicated by black circles). Isopycnal was indicated by grey lines. NPTSW, North Pacific Tropical Surface Water; NPCW, North Pacific Central Water; CCS-LL, California Current System waters observed at low latitudes (~13°N southward); SPCW-LL, South Pacific Central Water observed at low latitudes (~13°N southward); NPIW, North Pacific Intermediate Water; AAIW, Antarctic Intermediate Water.
Figure 4. Meridional and zonal distributions of dissolved inorganic nitrogen (DIN, a−d) and phosphorus (DIP, e−h) concentrations, respectively, in the upper 1 000 m. Meridional and zonal distributions of nitrite (
${{\rm {NO}}_2^-} $ ) concentrations (i, j) in the upper 200 m along the 10°N and 155°E transects in the tropical North Pacific Ocean. Data were based on both WOCE dataset and our in situ cruise observations in 2017.
Figure 5. Relationship between dissolved inorganic nitrogen and phosphorus, DIN and DIP, in the tropical North Pacific Ocean based on the WOCE dataset and in situ cruise data in 2017. Red dashed lines indicate the nutrient discrepancies in latitudinal variation, as DIN concentration ~25.0 μmol/L and DIP concentration ~1.7 μmol/L; 14:1 indicates the slope of the fitted linear regression.
Figure 6. Vertical water column depth distribution of DIN/DIP concentration ratios and nitrogen anomaly (N*) in the tropical North Pacific Ocean. Black circles represent in situ data from the 2017 cruise, the remainder correspond to the WOCE program.
Figure 7. Meridional (along ~155°E and ~165°E) (a) and zonal (along ~10°N and ~20°N) (b) water column depth sections of the nitrogen anomaly (N*, μmol/L) in the upper 200 m of the tropical North Pacific Ocean based on WOCE data and in situ data from the 2017 cruise. Depth-integrated Trichodesmium spp. abundance in the upper 200 m is represented by green dots; N2 fixation activity of Trichodesmium spp. along 155°E measured according to Kitajima et al. (2009) is represented by dark green stars. Latitudinal and longitudinal fluxes of aeolian nitrogen and FeDI deposition were obtained in April−May 2017 and July−August 2017, respectively.
Table 1. Cruise information from the WOCE Hydrographic Program
Cruise Cruise date P03W_31TTTPS24_2 May 10, May 14−15, 1985 P04C_32MW893_2 March 9−21, 1989 P04E_32MW893_3 April 9−15, 1989 P04W_32MW893_1 February 28−March 1, February 23−24,
February 19−March 4, 1989P10_3250TN026_1 October 21−23, 1993 P13_3220CGC92_2 September 30−October 12, October 10, 1992 P14N_325023_1 August 3−4, 1993 P15N_18DD9403_2 October 24−25, 1994 P16C_31WTTUNES_3 September 22−23, 1991 PR04_49RY9006_1 June 20−26, June 24−25, 1990 PR04_49RY9106_1 June 11−18, June 16−17, 1991 PR04_49RY9206_1 June 16, June 19, 1992 PR04_49RY9306_1 June 15−21, June 20, 1993 PR15_35COSURTPAC_6 June 26−27, 1986 
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