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Nutrient distributions and nitrogen-anomaly (N*) in the tropical North Pacific Ocean
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Aiqin Han1, T, Yu Wang1, T, Yunlong Huo1, Cai Lin1, Kaiwen Zhou1, Fangfang Kuang1, Hui Lin1, *
Acta Oceanologica Sinica | 2022, 41(11) : 23 - 33
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Acta Oceanologica Sinica | 2022, 41(11): 23-33
• Ecosystem and Environmental Baseline in COMRA’s Contract Area in the Clarion-Clipperton Zone •
Nutrient distributions and nitrogen-anomaly (N*) in the tropical North Pacific Ocean
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Aiqin Han1, T, Yu Wang1, T, Yunlong Huo1, Cai Lin1, Kaiwen Zhou1, Fangfang Kuang1, Hui Lin1, *
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  • 1 Third Institute of Oceanography, Ministry of Natural Resources, Xiamen 361005, China
Published: 2022-11-25 doi: 10.1007/s13131-021-1918-8
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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.

tropical North Pacific Ocean  /  nutrients  /  nitrogen anomaly  /  nitrogen fixation  /  aeolian deposition
Aiqin Han, Yu Wang, Yunlong Huo, Cai Lin, Kaiwen Zhou, Fangfang Kuang, Hui Lin. Nutrient distributions and nitrogen-anomaly (N*) in the tropical North Pacific Ocean[J]. Acta Oceanologica Sinica, 2022 , 41 (11) : 23 -33 . DOI: 10.1007/s13131-021-1918-8
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1 Introduction
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The tropical North Pacific is a critically important region affecting the global climate system, where water masses, heat and salt exchange actively, and is also under the influence of the western Pacific warm pool and the El Niño-Southern Oscillation (ENSO) (Hu et al., 2015; Liu et al., 2017). The region is characterized by low chlorophyll a concentration and low primary production relative to the other parts of the world ocean (Harrison et al., 1999; Ma et al., 2019), and this limitation in phytoplankton growth and production is attributed to its extremely low macronutrient levels (Kim et al., 2014; Matsumoto et al., 2016; Zhang et al., 2019b). Thus, careful examination of the nutrient distribution and nutrient concentration ratios is important to identify the limiting factors for this low production (Kim and Kim, 2013), yet nutrient data are still scarce in this oceanic region. Nutrient sources from nitrogen gas (N2) fixation could also be important to support oceanic productivity in the subtropical gyres (Kitajima et al., 2009; Böttjer et al., 2016). However, the spatial patterns of N2 fixation in this region are largely unknown due to the extreme lack of in situ observations.
Drawing from the classic sections of the Geochemical Ocean Sections Study (GEOSECS) (1972−1978) and World Ocean Circulation Experiment (WOCE) (1993−1994), Gruber and Sarmiento (1997) and Deutsch et al. (2001) adopted the quasi-conservative geochemical tracer N* (N*=N−16P+2.9) to investigate the distribution and rates of N2 fixation and estimate the world ocean’s nitrogen budget. Deutsch et al. (2001) also indicated that N2 fixation rates were the highest, leading to an increase in nitrogen concentration and N* values in the Pacific Ocean compared to the Atlantic and Indian Oceans.
In the present study, nutrients and their ratios were investigated in a vast area of the tropical North Pacific during July to November 2017. By combining these with the nutrient data from the WOCE program, the distributions of dissolved inorganic nitrogen and phosphorus (DIN and DIP, respectively) were examined in the region and the spatial distribution of N* was evaluated, as this could be vital to understanding N2 fixation dynamics. In addition, N* was related to the distribution of diazotrophs’ abundance as well as aerosolized nitrogen and iron deposition to further identify the controlling factors for N2 fixation in this region.
2 Materials and methods
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2.1 Study area
The tropical North Pacific Ocean is characterized by complex interactions between the westward-flowing North Equatorial Current (NEC) along a latitude of ~13°−20°N, the eastward North Equatorial Countercurrent in the south, and the eastward North Pacific Subtropical Countercurrent in the north (Qiu et al., 2015; Liu et al., 2017). Throughout the year, the tropical North Pacific Ocean is oligotrophic, characterized by low nutrient concentration and low chlorophyll a concentration (Schlitzer, 2004; Cabrera et al., 2015), with DIN concentration ranging from <10.0 μmol/L to 2.5 μmol/L and DIP concentration <0.25 μmol/L in the upper 200 m at ~10°−30°N, while DIN concentration ranging from 2.0 μmol/L to 20.0 μmol/L and DIP concentration from 0.5 μmol/L to 2.0 μmol/L within 75−200 m depth at 5°−10°N. Chlorophyll a levels were <0.1−0.2 mg/m3, with the subsurface chlorophyll maximum (SCM) layer located at ~100−150 m (Kitajima et al., 2009; Shiozaki et al., 2009).
2.2 Sampling and measurements
Field sampling in the present study covered 40 stations along two transects (Fig. 1): one was located at ~20°N, 155°−165°E, including a series of seamounts, i.e., Demao and Batiza, Niulang, and McDonnell Guyots; the other was located at ~10°N, 180°E−152°W, and included the China mining claim contract area (China Ocean Mineral Resources R&D Association contract area) and related deep-sea mining Clarion-Clipperton Zone. The cruise was conducted from July 12 to November 18, 2017 on board of the R/V Xiangyanghong 03.
Water samples from the upper 1 000 m were taken with 10 L Niskin bottles mounted onto a rosette sampler assembly, equipped with a conductivity-temperature-depth (CTD) recorder (SBE911 plus, Sea-Bird Co., USA). Nutrient samples were analyzed on board using standard methods. Nitrate (${{\rm {NO}}_3^-} $) and nitrite (${{\rm {NO}}_2^-} $) were determined by the pink azo dye method, and DIP was determined with the molybdenum blue method (Strickland and Parsons, 1972). The detection limits for ${{\rm {NO}}_3^-} $, ${{\rm {NO}}_2^-} $, and DIP analysis were 0.15 μmol/L, 0.06 μmol/L and 0.06 μmol/L, respectively. And the analytical precisions for the nutrients field measurements were better than 2.9% for ${{\rm {NO}}_2^-} $, 4.2% for ${{\rm {NO}}_3^-} $, and 3.1% for DIP, respectively. Since most of the ammonium concentrations in the open ocean were extremely low, normally less than 90.0 nmol/L (Zhu et al., 2018), the contribution of ammonium to DIN could be neglected for N* estimation.
Water samples (2 L) for analysis of Trichodesmium spp. were collected with Niskin bottles at different depths via CTD, specifically at 5 m, 50 m, 75 m, 100 m, 150 m, 200 m and the SCM layer. Samples were preserved in 4% buffered formalin for further analysis. In the land-based laboratory, the trichomes of Trichodesmium species were identified and counted using a Sedgewick Palmer Maloney chamber under a Zeiss Z1 inverted microscope (Germany) based on their morphological characteristics (Hynes et al., 2012). The abundance of Trichodesmium spp. was expressed as filament counts (filaments/L) regardless of the presence of colonial forms.
Aerosol samples were collected for aeolian nitrogen and iron deposition analysis in a parallel cruise and a prior cruise conducted from April 8 to May 8, 2017 on the R/V Kexue. Two large flow atmospheric particulate matter samplers (Lao Ying 2035, China) were installed on the compass deck (~15 m above sea level) to collect aerosol particles with a wind direction control system. Sampling conditions were such that the wind speed was >0.5 m/s, the relative wind direction was centered on the bow, and samples were collected when the wind direction was within 30 degrees. The samples for aeolian nitrogen (${{\rm {NO}}_3^-} $+${{\rm {NH}}_4^+} $) and dissolved iron (FeDI) deposition analysis were collected using cellulose acetate (Whatman 41) and quartz filters (Pall), respectively, and the filters were frozen and stored at −20°C. Aeolian nitrogen (μg/m3) and FeDI (ng/m3) were measured by ion chromatography (ICS-1600, Thermo Fisher, USA) and by inductivity coupled plasma-mass spectrometry (ICP-MS, Agilent 7700X, USA) (Piazzalunga et al., 2013; Shi et al., 2019). The analytical uncertainty was <2%. The dry deposition fluxes were roughly estimated as F=Vd×Ca, where Vd is the dry deposition velocity and Ca is the atmospheric concentration of aerosol species. For simplicity, dry deposition velocities for individual species were assumed to be constant: 1.0 cm/s for ${{\rm {NO}}_3^-} $ (Luo et al., 2016), 0.1 cm/s for ${{\rm {NH}}_4^+} $ (Luo et al., 2016), and 1.0 cm/s for FeDI (Chen et al., 2008; Baker et al., 2013), respectively.
2.3 Data compilation
To expand our understanding of the nutrient distribution and N*, as well as related large-scale nitrogen fixation (N2 fixation) characteristics of the tropical North Pacific, hydrological and nutrient data from the WOCE Hydrographic Program (https://www.ewoce.org and https://cchdo.ucsd.edu/search?q=WOCE) were combined. WOCE data were scanned for outliers using the automatic and interactive methods of the Ocean Data View software (*_hy1.csvfiles from WOCE Hydrographic Program Office web-site as of May 27, 2002). The scanned WOCE data were downloaded from the Ocean Data View file, covering a spatial range ~5°−27°N, ~145°E −149°W (Table 1). Station locations are shown in Fig. 1, and data were obtained from the upper 1 000 m.
3 Results and discussion
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3.1 Hydrography of the tropical North Pacific Ocean
Sectional distributions of potential temperature (θ) and salinity and the relationship between them in the tropical North Pacific are shown in Figs 2 and 3. The surface tropical North Pacific Ocean was occupied by North Pacific Tropical Surface Water (NPTSW) with high temperatures (~28.5°C) and a wide salinity range (~33.5−35.3) probably due to the influence of other water masses over the large coverage of the study area (Fig. 3). The latter may be mainly affected by the westward NEC with a strong speed of ~200 mm/s (Hu et al., 2015; Liu et al., 2017). Within the water column, θ generally decreased to ~5.0°C at 1 000 m depth in both latitudinal (along 155°E) and longitudinal (along 10°N) transects. The difference between two transects was that the 15°C isotherm showed a southward rise from ~400 m at 28°N to ~150 m at 10°N, which is consistent with the distribution of isopycnal of 25.0 kg/m3 (Figs 2a, b, i and j). While the isotherm remained at ~150 m from ~150°−170°E to ~100 m and rose gradually to ~100−150 m at ~150°W (Fig. 2c). In contrast to θ, the salinity distribution was much more complex. Along 155°E, the maximum salinity (~35.3) appeared at the surface around 25°N and dropped abruptly to 200 m depth at 15°N, then rose slightly to 100−150 m at salinity of ~34.7, meanwhile, the density (σ0) was within 23.0−24.0 kg/m3 (Figs 2e, i), implying the water mixing between NPTSW and the North Pacific Central Water (NPCW) (Fig. 3). Along 10°N, this salinity maximum remained at 100−150 m at ~150°−170°E and rose to ~100 m at 160°W, outcropping at 150°W (Fig. 2g). This water mass with the maximum salinity was the NPCW (Pickard and Emery, 1990) (Fig. 3).
Under the maximum salinity layer, a water mass with low salinity of ~34.5 and high density of ~25.3−26.0 kg/m3 appeared at 100−150 m at 150°W and inclined downward to ~250−300 m at 150°E (Figs 2g, h, k and l). This water mass, referred to as the California Current System (CCS), appears at low latitudes but not at high latitudes (north of ~13°N) (Tsuchiya, 1968). With increasing water depth, another water mass (salinity, ~34.8; σ0, ~26.5−26.8 kg/m3) was observed from 150°W (at ~200−500 m) to 150°E (at ~400 m), named the South Pacific Central Water (SPCW) (Sverdrup et al., 1942; Zhang et al., 2019a), which again only appeared at low latitudes (Fig. 2g, h, k and l). This water mass may be connected with the eastern tropical and South Pacific water mass (θ , ~8.0°C; salinity, ~34.75) with an oxygen minimum value <25.0 μmol/kg (Schlitzer, 2004).
At high latitudes (20°−25°N), North Pacific Intermediate Water (NPIW) with an extremely low salinity of ~34.2 and density of ~26.8 kg/m3 was located at 500−800 m (Figs 2f, j). At water depths of ~1 000 m, Antarctic Intermediate Water (AAIW) was identified from its temperature of 5.0°C, salinity of 34.3−34.5, and density of ~27.4 kg/m3 (Liu et al., 2017; Zhang et al., 2019a).
3.2 Distributions of DIN, DIP and nitrite
Sectional distributions of DIN and DIP concentrations in the upper 1 000 m, and nitrite concentrations in the upper 200 m are shown in Fig. 4. Along 155°E, nutrients in the NPCW were extremely low, with DIN concentration less than 1.0 μmol/L and DIP concentration less than 0.1 μmol/L in the upper 100 m, and DIN and DIP concentrations of ~5.0 μmol/L and ~0.2 μmol/L, respectively, at 200 m depth (Figs 4a, e). With submergence of the NPCW, the DIN and DIP isolines of ~10.0 μmol/L and ~1.0 μmol/L, respectively, were located at ~400 m. Following the southward flow of the NPCW, the DIN and DIP isolines rose synchronously to ~150 m at low latitudes (~10°N). Thus, nutrients at low latitudes (~5°−10°N) showed a pronounced increase within the upper 200 m water column, as DIN and DIP concentrations increased from <1.0 μmol/L and <0.1 μmol/L at the surface (NPTSW) to ~23.0 μmol/L and ~1.8 μmol/L at 200 m (CCS). At high latitudes (~20°−25°N) nutrients increased gradually with increasing water depth, while at low latitudes (~10°N) nutrient concentrations increased markedly until they stabilized at 1 000 m where DIN and DIP concentrations attained ~38.0−45.0 μmol/L and ~2.8−3.3 μmol/L, respectively (Figs 4b, f).
Zonal nutrient distributions showed that they were dominated by a shift in water masses. Along 10°N, the NPTSW upper 100 m layer was again oligotrophic, with the same nutrient levels as the meridional NPTSW. Within the NPCW water, nutrient DIN and DIP contour lines ranging from 5.0 μmol/L to 20.0 μmol/L and from 0.2 μmol/L to 1.5 μmol/L, respectively, lay between 125 m and 200 m in the western region (150°E), while the contour lines became noticeably shallower and thinner between 75 m and 100 m in the eastern region (150°W). Between 100 m and 200 m, nutrient concentrations in the SPCW water mass at 150°W increased noticeably and were as high as ~35.0 μmol/L and ~2.5 μmol/L for DIN and DIP, respectively (Figs 4c, g). In the western CCS water mass, similarly to the nutrient levels observed in the latitudinal CCS water mass, DIN concentrations were within 20.0−30.0 μmol/L and DIP concentrations between 1.5 μmol/L and 2.0 μmol/L. Since the AAIW occupied depths of ~1 000 m in the tropical North Pacific Ocean (Fig. 3), nutrient levels at longitudinal mid-deep water depth (~1 000 m) were comparable to those of latitudinal intermediate water as mentioned above (Figs 4d, h).
With respect to nitrite distributions, the maximum nitrite layer (MNL) became deeper, from ~100 m to ~200 m from 25°N to 20°N, corresponding to the NPCW submergence, with a concentration of ~0.15 μmol/L. Thereafter, the MNL became shallower at low latitudes (5°−10°N) with a higher nitrite concentration of 0.4 μmol/L at depths from ~75 m to ~125 m, consistent with the nutricline depth (Fig. 4i). Shallower MNL at 5°−10°N might be uplifted by southward flow of the NPCW. Zonally, nitrite concentrations along 10°N were comparable to those found along 155°E, with a value of ~0.3 μmol/L. The depth of the MNL was located at ~125 m and fluctuated between 75 m and 125 m. In contrast to the western region, the highest nitrite concentrations (0.6−0.8 μmol/L) occurred in the eastern tropical region (160°−150°W) at depths of 75−125 m (Fig. 4j), which indicated stronger nitrification in the eastern tropical Pacific Ocean compared to the western region.
3.3 Nutrient dynamics as indicated by the relationship between DIN and DIP and the N-anomaly (N*)
Nutrient dynamics, especially N2 fixation, may be reflected by the variation in DIN/DIP concentration ratios and the N-anomaly, N* (Gruber and Sarmiento, 1997; Deutsch et al., 2001). Wong et al. (2007) also indicated that variation in N* could be influenced by the remineralization of organic matter. The N* index is defined as N*=N−16P+2.9 (Gruber and Sarmiento, 1997; Deutsch et al., 2001), where N and P are the concentrations of DIN (>1.0 μmol/L) and DIP (>0.1 μmol/L), respectively (Wong et al., 2007). The value of 2.9 is the global average deficit in nitrate resulting from denitrification (Wong et al., 2007).
The relationship between DIN and DIP is shown in Fig. 5. A linear relationship of DIN=14.21(±0.03)×DIP−1.31(±0.06) (coefficient of determination, r2=0.992, n=4 162) , where DIN and DIP are the concentrations of DIN (μmol/L) and DIP (μmol/L), provided an excellent fit once all nutrient data were included. The slope of 14.2 was slightly lower than the classic Redfield ratio of 16:1 (Redfield et al., 1963), which provided a robust indication of nitrogen limitation. The DIN/DIP ratios varied with a clear latitudinal pattern that corresponded to the spatial variation of hydrological and nutrient distributions. As shown in Fig. 5, DIN/DIP ratios at high latitudes (north of ~20°N), where DIN and DIP concentrations were <25.0 μmol/L and <1.7 μmol/L respectively, systematically exceeded the ratios at southern latitudes with comparable nutrient levels. When DIN and DIP exceeded 25.0 μmol/L and 1.7 μmol/L, respectively, the meridional variation of DIN/DIP ratios clearly disappeared.
Examination of the molar vertical DIN/DIP ratios throughout the water column indicated that they ranged between 0.2 and 28 in the upper 200 m, and became stable and constant at ~14.2 at depths of 200−1 000 m (Figs 6a, c). Some high ratios of ~15:1−28:1 appeared at ~200 m at high latitudes; these were much higher than the fitted ratio of 14:1 (Fig. 5) and also apparently greater than the values (ranging from ~2 to 14) at southern latitudes (~5°−10°N) at ≤200 m, as well as some of those determined at northern latitudes at ≤200 m (Fig. 6a). Furthermore, DIN/DIP ratios at southern latitudes were between 2 and 14 and were concentrated in a relatively shallow layer of 75−120 m, while the ratios at northern latitudes were found at 150−200 m, corresponding to the nutricline distribution. Longitudinally, most DIN/DIP values in the eastern regions ranged between 2 and 14 in the shallower layer (≤100 m), and remained constant (14:1) below 100 m. Further westward, at longitudes up to 150°E, the layer of DIN/DIP ratios between 5 and 14 became deeper, and was found at ~150 m, again attributable to the shallower nutricline (~100 m) in the eastern sector compared to that in the western sector (~150 m) (Fig. 6c). Below 200 m (to 1 000 m), both the latitudinal and longitudinal variation in DIN/DIP ratios became undetectable, maintaining a constant value of ~14.2 (Figs 6a, c).
Scatter plots of meridional and zonal N* water column distributions are shown in Figs 6b and d. In the upper 500 m, N* varied widely, with values ranging from −9.0 μmol/L to 5.0 μmol/L, while N* exhibited noticeably negative values (−8.0−0 μmol/L) below 500 m. Moreover, in the upper layer (≤200 m), including the MNL where nitrification was most intense, N* varied conspicuously, with values ranging from −8.0 μmol/L to 6.0 μmol/L. In addition to the vertical N* variation, Fig. 6b shows the latitudinal and longitudinal difference in N*. Meridionally, N* values at high latitudes (>18°N) were significantly higher than those at low latitudes (<15°N) in the upper 600 m, and the meridional spatial variation became undetectable below 600 m. Furthermore, N* was much higher in the upper 500 m at northern latitudes, corresponding to the nutrient isohaline at ~600 m, with DIN and DIP concentrations of ~25.0 μmol/L and ~1.8 μmol/L, respectively, in the NPCW water mass (Figs 4b, f). Zonally, N* increased gradually from east (~150°W) to west (~150°E), with values ranging from −9.0 μmol/L to 0 μmol/L. The most negative N* value, located in the eastern Pacific Ocean at 500−600 m water depth, corresponded to the oxygen minimum zone characterized by a high denitrification rate (Gruber and Sarmiento, 1997). It is noteworthy that there was no obvious spatial difference in N* between 155°E and 165°E at northern latitudes (>18°N) throughout the water column, indicating that the maximum N* occurred in the Northwest Pacific.
As indicated earlier, the highly variable water column N* with both positive and negative values may be attributable to complex biogeochemical processes, i.e., nitrogen deposition (Kim et al., 2011, 2014; Kim and Kim, 2013), N2 fixation (Capone et al., 2005; Bhavya et al., 2019), preferential uptake of DIP over DIN by phytoplankton (Clark et al., 2002) and/or nitrification of nitrogen-rich organic matter with an N:P ratio exceeding the Redfield ratio of 16 (Redfield et al., 1963; Singh et al., 2017). These negative N* values could also be explained by the preferential release of DIP over DIN during particle remineralization (Hung et al., 2007).
Gruber and Sarmiento (1997) surveyed the N* distribution throughout the entire water column (0−6 000 m) in all major world oceans. They found that N* was less variable (ranging from −1.5 μmol/L to 1.0 μmol/L) in the Pacific region at 5°−25°N, ~170°E. Deutsch et al. (2001) further analyzed N* water column (0−6 000 m) data in the Pacific Ocean (along 180°E and 10°N) and suggested that N2 fixation occurred primarily in the western Pacific subtropical gyres. Consistent with these results, and as shown in Fig. 6, in the present study the most positive N* values in the upper 500 m were located at northwestern stations (20°N northward, 165°E westward). Moreover, Zhang et al. (2019b) reported that N2 fixation contributed an important N source which sustained 6.2%±3.7% of the primary production in the upper 200 m of the tropical North Pacific Ocean (13°−20°N, 120°−160°E). Recent studies indicated that the cyanobacterium Trichodesmium was one of the most important diazotrophs in the western tropical-subtropical Pacific Ocean (Chen et al., 2019). Kitajima et al. (2009) further demonstrated that microplanktonic diazotrophs were abundantly distributed in the North Pacific Ocean along a latitudinal gradient (with Trichodesmium spp. dominating at 26.5°N and Richelia intracelluaris around 8°N and 30°N). Thus, the present study focused on the N* data in the upper 200 m where a N2 fixation signal was observed.
3.4 Meridional and zonal N* characteristics in the upper 200 m water column
As shown in Fig. 7a, along longitudes 155°E and 165°E, the N* maximum decreased from ~2.0 μmol/L at 5°N to ~0 μmol/L at 10°N. Thereafter, N* attained more positive values until it reached a maximum of 2.0−6.0 μmol/L at northern latitudes (15°−28°N), coinciding with a salinity front of the NPCW. It was acknowledged that mixing of water masses with different endmember values of N* might bias in situ estimated N* (Kim et al., 2014). As shown in Fig. S1a, the potential temperature-salinity diagram for the upper 200 m water column along 155°E and 165°E illustrated a three end-member mixing scheme. In order to separate the temporal N* trend from the contribution of physical mixing, a three end-member mixing model (Han et al., 2012; Kim et al., 2014) was adopted. This analysis was also performed along density coordinates σ0 (~22.5−26.3 kg/m3) for the core density layers of the main water masses. The N* addition signals (positive ΔN*, i.e., N sources larger than N sinks) from the isopycnal analysis and θ-Salinity diagram was in good agreement with the latitudinal tendency for N* (Fig. S1b). Such consistency indicated that high N* along 155°E and 165°E was unlikely to be generated by physical mixing. This is also consistent with the previous study at northern Pacific Ocean which demonstrated that water mass mixing played a minor role for controlling the N* distribution in the upper water (Kim et al., 2014).
Given that N*>2.0 μmol/L have been proposed as indicators of N2 fixation (Gruber and Sarmiento, 1997; Deutsch et al., 2001), we hypothesized that N2 fixation, being one of the possible N sources, would occur with greater spatial variability and higher intensity at high latitudes than that at low latitudes.
The depth-integrated Trichodesmium abundance of 24.0 filaments/L at ~20°N, was ~4-fold higher than that at lower latitudes (~10°N) (Fig. 7a). It was in good agreement with the meridional N* distribution, suggesting that Trichodesmium spp. might dominate N2 fixation during our cruise period. Although direct measurements of N2 fixation rate were not obtained in the present study, the Trichodesmium abundance and N* signals obtained agreed well with the results of Kitajima et al. (2009). These authors found that the rate of N2 fixation activity at northern latitudes (~15°−28°N) was ~4.0−8.0 nmol/(L·d) (in terms of N), and thus much greater than that at low latitudes (~1.0−5.0 nmol/(L·d) (in terms of N) at ~5°−15°N) (Fig. 7a). Kitajima et al. (2009) also reported that Trichodesmium spp. were the dominant microplanktonic diazotroph, contributing high N2 fixation activity at 15°−28°N during their summer 2005 cruise, and Chen et al. (2019) demonstrated that Trichodesmium was the dominant N2 fixing diazotroph in the western tropical Pacific Ocean.
Zonally, N* generally decreased from the west (ranging from 4.0 μmol/L to −2.0 μmol/L in ~145°−165°E) to the east (−2.0 μmol/L to −8.0 μmol/L at ~155°W) along ~10°N, except for the high N* values (0−4.0 μmol/L) recorded in 2017 (Fig. 7b). This longitudinal N* distribution was consistent with the distinct horizontal negative N* distribution at 200 m in the Pacific Ocean, which decreased from 2.0−4.0 μmol/kg to −10.0−12.0 μmol/kg from ~150°E to ~140°W (Gruber and Sarmiento, 1997). In the present study, a N* minimum (as low as ~15.0 μmol/kg, WOCE data) was also associated with the oxygen minimum zone in the SPCW water mass of the tropical eastern North Pacific Ocean (~200−600 m, Fig. 3). This water mass was also identified by Gruber and Sarmiento (1997) (Fig. 5 in their study) with N* as low as −7.0 μmol/L at ~200−600 m. This characteristic was associated with the N* distribution which at intermediate depths where low O2 concentrations promoted denitrification processes and thus removed N (Kim et al., 2014). Again, the zonal depth-integrated Trichodesmium abundance demonstrated that the highest abundance (~18.0 filaments/L) coincided with the highest positive N* value at ~160°W, and the abundance decreased eastward thereafter. Taken together, these findings suggest that Trichodesmium spp. actively contributed to N2 fixation in the tropical North Pacific euphotic layer and that these cells were distributed with a distinct pattern of spatial variability.
It has been recognized that high N* in the tropical northwest Pacific Ocean might not only be associated with N2 fixation activity (Gruber and Sarmiento, 1997; Deutsch et al., 2001), but also with aeolian dust deposition which appeared to play a critical role in N2 fixation and the nitrogen budget (Karl et al., 2002; Kim et al., 2011, 2014; Kim and Kim, 2013). The latter studies suggested that the spatial pattern and absolute magnitude of the rate of increase of N* at mid-latitudes (20°−40°N) in the North Pacific agreed well with the atmospheric nitrogen deposition rates derived from an atmospheric transport model and observations. Aerosol nitrogen deposition in the tropical North Pacific was determined in the present study and in a prior cruise (April−May 2017) in which the sampling sites were close to the nutrient sampling stations (Figs 1 and 7a). As shown in Fig. 7a, within ~143°−146°E, the aerosol nitrogen deposition flux was relatively constant at ~3.0−5.0 μmol/(m2·d) between 5°−25°N in April−May 2017, and was enhanced at ~28°N with maxima of ~16.0 μmol/(m2·d), that agreed well with the high N* at 15°−28°N (Fig. 7a). Similar to the meridional consistence between nitrogen deposition and N* distributions, the longitudinal nitrogen deposition agreed well with the eastward decreasing N* distribution. Nitrogen deposition was extremely high in the western North Pacific, with a maximum of ~28.0 μmol/(m2·d) at ~130°E in July−August 2017, and then decreased markedly to ~6.0 μmol/(m2·d) at ~160°W (Fig. 7b). The agreement between meridional and zonal nitrogen deposition rates and the N* distribution, indicates that aerosol nitrogen deposition could also cause the observed DIN elevation and high N* signals in the upper layer of the tropical North Pacific.
Taken together, the field atmospheric nitrogen deposition rate was (7.85±6.51) μmol/(m2·d) (in terms of N) (Fig. 7a), which agreed well with the model result of 5.5−16.4 μmol/(m2·d) (in terms of N) (Kim et al., 2014). The field N2 fixation rate was (0.60±0.96) nmol/(L·d) (in terms of N) (Kitajima et al., 2009), which could be estimated as (120±192) μmol/(m2·d) (in terms of N) in the upper 200 m. Such an estimation was in the same order with the results from Zhang et al. (2019b). Thus, the N2 fixation rate was larger than atmospheric deposition per unit area. On the other hand, it has been reported that modeled N deposition was ~7.0 Tg/a in the North Pacific Ocean, ~7.5 Tg/a in the North Central Pacific Ocean, and ~6.9 Tg/a in Equatorial Pacific Ocean (Okin et al., 2011). Geometric mean depth-integrated N2 fixation rates in tropical North Pacific Ocean were ~20−120 μmol/(m2·d) and the North Pacific Ocean area was thus assumed to be ~8.9×1013 m2 (Luo et al., 2012), the N2 fixation flux was calculated to be ~9.1−54.6 Tg/a. It seemed again that N2 fixation contribution was higher relative to atmospheric deposition.
Moreover, the rate of aeolian FeDI flux displayed a similar pattern to that of nitrogen deposition (Fig. 7a), remaining constant at ~10.0 nmol/(m2·d) within 5°−25°N in April−May 2017 and increasing to ~110.0 nmol/(m2·d) at ~28°N, again in good agreement with the elevated N* distributions at high latitudes. Zonally, the aerosol FeDI was consistent with nitrogen deposition, decreasing from ~42.0 nmol/(m2·d) in the western ocean to ~8.0 nmol/(m2·d) in the eastern ocean (Fig. 7b). Although nitrogen deposition may be attributable to the high N* in the study area, aeolian FeDI deposition might play an important synchronous role in the distribution of Trichodesmium spp. and thus strongly influence their N2 fixation activity in the tropical North Pacific Ocean.
4 Conclusions
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Based on the WOCE dataset and results of the 2017 in situ cruises, the present study identified nutrient characteristics associated with hydrological features across the large tropical North Pacific Ocean area, and investigated meridional and zonal N* distributions, finding the highest positive N* value (2.0−6.0 μmol/L) in the northwest region (~15°−25°N, ~155°−165°E) (a N*>2.0 μmol/L is considered an indicator of N2 fixation activity). Trichodesmium spp. were the dominant contributor to N2 fixation during the sampling period and their depth-integrated abundance was the highest in the northwest Pacific Ocean, where high N2 fixation rate has been observed. Results of this study confirmed that such spatial variation in the N* index may not only be induced by N2 fixation, but also by aeolian nitrogen and iron deposition. The former may elevate nitrogen concentrations and thus N* in the upper water column, and the latter may play an important role in the distribution of diazotrophs and enhanced N2 fixation.
Although N* distributions in this study were roughly consistent with the distributions of N-fixing cyanobacteria and those of aerosol nitrogen and iron deposition, the field-determined meridional and zonal N2 fixation activity and abundance and distribution of specific diazotrophs are still not adequately quantified. Furthermore, the tropical North Pacific Ocean is oligotrophic, with nutrient concentrations at the nanomolar per litre level, and it is critical to determine DIN/DIP ratios and the related N* in the upper column. Additionally, the importance of aeolian dust deposition, including that of nitrogen and iron in elevating the N* signal, remain poorly definition. Taken together, results of this study highlight the variability in the meridional and zonal N2 fixation. Additional field studies are needed, however, to improve on the spatial characterization of N2 fixation and associated nitrogen budgets.
Acknowledgements
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This study was partially supported by the numerous National Science Foundation grants under the WOCE program. We thank all the scientists and technicians involved in data collection and analysis of the high quality data from the WOCE program. We also thank the captain and crew of the R/V Xiangyanghong 03 and R/V Kexue for their assistance while sampling at sea.
Funding
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  • The Eastern Pacific Environment Monitoring and Protection Project under contract No. DY135-E2-5-02; the Global Change and Air-Sea Interaction Project; the National Natural Science Foundation of China under contract No. 42103077; the Natural Science Foundation of Fujian Province of China under contract No. 2020J05077.
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Appendix
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Year 2022 volume 41 Issue 11
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doi: 10.1007/s13131-021-1918-8
  • Receive Date：2021-05-11
  • Online Date：2025-11-21
  • Published：2022-11-25
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  • Received：2021-05-11
  • Accepted：2021-07-03
Funding
The Eastern Pacific Environment Monitoring and Protection Project under contract No. DY135-E2-5-02; the Global Change and Air-Sea Interaction Project; the National Natural Science Foundation of China under contract No. 42103077; the Natural Science Foundation of Fujian Province of China under contract No. 2020J05077.
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    1 Third Institute of Oceanography, Ministry of Natural Resources, Xiamen 361005, China

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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