Papers in Meteorology and Geophysics Vol. 28, No. 1, pp. 29-39. March 1977 On the Distribution of Nitrate Nitrogen in the Pacific Ocean by Takeshi Sagi Meteorological College, Kashiwa, Chiba Pref. (Received October 1, 1976) Abstract The desirable conditions for the operation of the Auto-Analyzer system were confirmed for the determination of nitrate in sea water. Based on nitrate data obtained up to the present, some characteristics were described with respect to the horizontal distributions on the isobaths and the vertical distributions along several meridians. 1. Introduction * Asahi -cho, Kashiwa, Chiba Pref., 277, Japan Since shipboard operations in determination of nitrate in sea water are tedious as compared with those of phosphorus and silicon, there have been few data available for nitrate in the Pacific. Up to the present, the general features of nitrate distribution in the Pacific have been little investigated. The technique for the photometric determination of nitrate in sea water has steadily progressed. Miyake (1938) and Kato et al. (1954) developed the reduction method of nitrate to nitrite with zinc powder, and Matida (1951) worked on the method of preparation of a reduced strychnine reagent for nitrate determination. Mullin et al. (1955) proposed the hydrazine copper reduction method. Until recently this method has been adopted in oceanographical observation for the determination of nitrate in sea water. All methods mentioned above, however, have some disadvantages in point of easiness, sensitivity and precision. The most promising procedure up to the present was the cadmium reduction method by Morris et al. (1963). An improvement on this reliable and sensitive procedure, the coppercadmium reduction method, was developed by Wood et al. (1967). In recent years, Auto- Analyzer systems for the determination of nitrate have been developed by many investigators (e. g. Henrikson, 1965 ; Strickland et al., 1968 ; Hager et al., 1972). It is well known that nitrate is the major component of nitrogen compounds in sea. water as an end product of the decomposition of organic matter. Ammonia and nitrite appear as intermediate products in the same process. However, ammonia dose not ordinarily occur in significant quantities below the photosynthetic layer, nor does nitrite appear to be more abundant than ammonia except in a region of extremely low oxygen content. In this paper, are described more satisfactory procedure developed by the present author for operating the Auto-Analyzer system for the determination of nitrate in sea water. Based on nitrate data obtained up to the present, some characteristics were described with respect to the horizontal distribution on the isobathes and the vertical distributions along several meridians. 2. Determination of nitrate in sea water The present author found out the desira-
30 T. Sagi Vol. 28, No. 1 ble conditions in operating the Auto-Analyzer system for the determination of nitrate in sea water. The automated system employed here is essentially after Wood et al. (1967). This automated procedure has been adopted routinely on oceanographic cruises by the Japan Meteorological Agency. Comparison between the usual manual and our automated method has revealed coincidence in the results obtained as to nitrate concentration. Reagents and procedures are as follows : Reagent A: dissolve 1 g sulfanilamide in 60 ml of 2 N hydrochloric acid and dilute to 500 ml with distilled water. Reagent B: dissolve 0.1 g N-(1-naphthyl) ethylenediamine hydrochloride in 500 ml of distilled water. This solution is stable for about two weeks. EDTA solution : dissolve 1 g EDTA-4Na in 11 of distilled water. Column for reduction : glass tubing used is 6 mm in inner diameter and 10-12 cm long. About 10 g of 20-50 mesh cadmium filings is cleaned with 2 N hydrochloric acid, rinsed with distilled water, washed a few times with 2% copper sulfate solution, and colloidal copper is rinsed away. Caution must be taken to prevent bubbles from generating in filling cadmium filings to column. Bottles containing reagents and a column are inserted in the flow system of the Auto- Analyzer as shown in Fig. 1. The column is conditioned with sea water containing Fig. 1. Flow diagram for the determination of nitrate in sea water by the Auto- Analyzer. about 20,ug at/i of nitrate for about 20 minutes. A devise was made to put sample and distilled water into the analyzer intermittently in the volume ratio of 2 : 3. In this way, 20 samples are treated in one hour. Standard solutions containing 20,ug at// of nitrate are added before and after each series of samples. Though the treatment of samples is not so rapid, the automated procedure is very convenient for shipboard operation. The amount of each sample is only less than 0.5 ml. 3. Samples Stations and sections relevant to this study are shown in Fig. 2. Table 1 is the list of the cruises referred to in this study. For determination of nitrate, the cadmium-copper reduction method by Wood Fig. 2. Map of stations and sections relevant to this study.
1977 Distribution of Nitrate in the Pacific 31 Table 1. List of cruises referred to in Fig. 2. Japan Meteorological Agency (1970-1974) : The results of marine meteorological and oceanographical observations No. 40, 42, 45, 46, and 52. Ocean Research Institute University of Tokyo: Oceanographic Data of KH 68-4 (Southern Cross Cruise) of the Hakuho Maru (1970). Preliminary Report of the Hakuho Maru Cruise KH 69-4 (IBP Cruise, 1970). Preliminary Report of the Hakuho Maru Cruise KII 70-1 (1971). Preliminary Report of the Hakuho Maru Cruise KH 70-2 (Great Bear Expedition, 1971). Oceanographic Data of KH 71-5 (Phoenix Expedition) of the Hakuho Maru (1973). Oceanographes au Centre O.R.S.T.O.M. de Noumea (1967) : Resultats des observations physicochimiques de la Croisieres " Bora 1", " Bora 2 ", " Bora 3 ", " Alize ", " Atoll " du N.O. "Coriolis ". SIC), WHOI, La Jolla, California (1969) : Physical and chemical data from the SCORPIO expedition in the South Pacific Ocean aboard Usns Eltanin Cruises 28 and 29. Japanese Oceanographic Data Center : Preliminary data report of CSK No. 36 Argo Scripps Institution of Oceanography USA (1967). Data report of CSK No. 176 Takuyo Hydrographic division, Maritime Safety Agency, Japan (1969). Preliminary data report No. 58 (1967) and No. 189 (1969) Nagasaki Maru, Faculty of Fisheries, Nagasaki University. et al. (1967) was used aboard the Ryofu Maru and Hakuho Maru, the method by Mullin et al. (1955) was used aboard the Kofu Maru and Nagasaki Maru and the cadmium reduction method by Morris et al. (1963) was used aboard the others.
32 T. Sagi Vol. 28, No. 1 4. Vertical distributions Each of the vertical distributions of nitrate, temperature and salinity shown in Fig. 3 is typical of the Antarctic, the Tropics, the Subtropics and the Subarctic. The concentration of nitrate is, in general, low in the surface layer and increases with depth to the maximum at a moderate depth (500-1500 m) and then decreases a little. But, in the Antarctic region, the concentration of nitrate deeper than about 1500 m increases with depth to the bottom. In waters near the surface, nitrate is depleted in the regions of the Tropics and the Subtropics where the thermocline develops remarkably in shallow layers, while contents higher than 20 pg at// are found in the Antarctic and the Subarctic regions. The depths and the concentration of the nitrate maximum are about 400 m and 45 pg. at/l, 500 m and 40 pg. at//, and 500 m and 35 fig at// respectively in the regions of the Subarctic, the Tropics or the Subtropics, and the Antarctic. The nitrate concentrations are higher in the northern region than in the Antarctic region with the exception of the surface layer. 5. Distribution in the euphotic layer The mechanism to generate nutrient salts in sea water is the oxidative decomposition of organic matter derived from marine organisms, and the mechanism to remove them is the uptake by biomass. Accordingly, the concentration of nutrient is, in general, lower in the euphotic layer and higher in the decomposition layer. Fig. 4 shows the distribution in the Pacific of the nitrate amount in sea water of the euphotic layer. Although the depth of the euphotic layer differs somewhat depending on location and season, it is here assumed to be approximately 50 m. What is shown in Fig. 4 is the total amount of nitrate in g. at unit contained in a water column 50 m deep and with a cross-section of unit area. In the surface layer where the mechanism to remove the nutrient is predominant, the total amount of nitrate is less than 0.25 g. at/m2 in most places, but in three places, i.e. to the south of the subantarctic region, the Subarctic region, and the eastern Equatorial region, it exceeds 0.5 g at/m2. It is well known that those three places are Fig. 3. Vertical distributions of nitrate in pg at/l, temperature in C, and salinity in 70.,
1977 Distribution of Nitrate in the Pacific 33 6. Distribution on the isobaths Fig. 4. Distribution of nitrate amount in g-at/m2 in the water column above the depth of 50 m. the more active sites of vertical mixing or upwelling. It is presumed that water in the decomposition layer is brought into the euphotic layer in considerably large quan- tities and the supply of nitrate surpasses the removal of nitrate in those places. The distribution of nitrate at the depth of 100 m is shown in Fig. 5-a. The great part of the Subtropical and the Tropical regions enclosed by the anticyclonic circulation is occupied by waters containing nitrate less than 5 pg at/i. In regions north of the Kuroshio Extension, nitrate-rich waters containing more than 5 pg at// are distributed. Nitrate concentration increases to more than 20 pg. at// towards the seas adjacent to the Kurile and the Aleutian Islands. In the eastern Equatorial region, a tongue of water whose nitrate content is more than 25pg at// extends west from the coast of Central America along latitude 10 N. In the western Equatorial region, the nitrate content is more or less uniform at about 5 pg. at//. The distribution of nitrate at the depth of 500 m is shown in Fig. 5-b. Near the central part of the Subtropical region the low values less than 20 pg at// are observed particularly in the south of Japan is found a concentration less than 15 pg at//. Off the coasts of Kurile and Aleutian Islands, and off California, nitrate concentrations higher than 40 pg at// are found. Nitrate-rich waters higher than 35 pg. at// occupy a large area of the eastern Pacific. (5-a)
34 T. Sagi Vol. 28, No. 1 (5-b) (5-c) Fig. 5. Distributions of nitrate in trg at// at the depth of 100 m (a), 500 m (b), and 1000 m (c). The concentration of nitrate in the upper layer above the depth of 500 m is related to the oceanic circulation at the surface. It is low in the Subtropical region enclosed by the anticyclonic circulation and high in the Subarctic region influenced by the coastal upwelling and cyclonic circulation. It is also high off Central America and in the Equatorial region where upwelling occurs. At the depth of 1000 m (Fig. 5-c) no remarkable difference of nitrate concentration exists between the subtropical and the subarctic region. In the eastern North Pacific and in the area from the east of Japan to
1977 Distribution of Nitrate in the Pacific 35 Kamchatka, the nitrate concentrations are slightly more than 40 pg at//. In other regions, they are less than 40 pg at//, which may be due to the influence by the South Pacific water. Waters containing nitrate lower than 40 pg at// appear along the Aleutian Islands. 7. Meridional distributions Here, the meridional distributions of nitrate from the surface to the bottom in the Pacific are described. Locations of the five sections (A-E line) are shown in Fig. 6. Fig. 6. Location of five long sections relevant to this study. 1) Along A line (137 E, Fig. 7-a) A meridional section at 137 E from Japan to New Guinea is shown in Fig. 7-a. Nitrate distribution in this section was described previously in detail (Sagi, 1970). A sharp gradient at about 33 N indicates the eastward Kuroshio. The contour becomes steeply shallow from 20 N to about 7 N.) The boundary between the North Equatorial Current and the Equatorial Counter Current is located at about 7-8 N. Around this region, waters containing nitrate ranging from 30 to 35 pg at// are widely distributed between 200 m and 800 m in depth. The maximum values, i.e. more than 40 pg. at//, are found at about the depth of 1000 m and approximately coincide with the isopicnal plane of cr, 27.3. In this section, nitrate-rich waters containing more than 40 pg. at// occupy the north of 22 N and seem to decrease gradually towards the south. In the region south of the Equatorial Counter Current, the distribution of nitrate at intermediate depths does not coincide with those of the isopicnal plane, which fact suggests the existence of a water mass with different nitrate content to the south of the Equatorial Counter Current. 2) Along B line (155 E, Fig. 7-b) A meridional section at 155 E from the Kuriles to the Coral Sea is shown in Fig. 7-b. Observations of the northern and the southern half of the section were made in May 1969 and in June 1972, respectively. The Kuroshio front and the Oyashio front are situated at about 34 N and 40 N. In the sea north of the Oyashio front, remarkably high contents of nitrate are observed at the surface. In this region, waters containing nitrate higher than 40 pg at// are widely distributed between 200 m and 2500 m in depth. Waters containing nitrate more than 40 pg at// are found on the isopicnal plane of c ft27.3 and spread out from around the Kuriles to 18 N in the south. In the regions from 40 N to 5 N, the contours of the isopicnal plane are parallel to those of nitrate concentration. On the other hand, in the region north of the Oyashio front and south of 5 N, the contours of the isopicnal plane are not parallel to those of nitrate concentration. It means that the relatively uniform water in the Subtropical region comes into contact with water masses with different nitrate content at around 5 N and the Oyashio front. 3) Along C line (170 W, Fig. 7-c) A meridional section at 170 W from the Aleutian Islands to the Antarctic Ocean is shown in Fig. 7-c. Observations in the northern and the southern part of the C line were made aboard the Hakuho Maru of the Ocean Research Institute of Tokyo University in the Great Bear Expedition (KH 70-2) and in the Southern Cross Cruise (KH 68-4), respectively. In the Subarctic region to the north of 43 N and the Antarctic Ocean to
36 T. Sagi Vol, 28, No. 1 (7-a) (7-b) the south of 51 S, nitrate higher than 10 pg. at// is distributed up to the surface. The boundary between the North Equatorial Current and the Equatorial Counter Current is located at about 10 N. The contour is most shallow and a strong thermocline develops in this area. In the South Pacific waters shallower than 500 m, the contour is deepest at about 23 S. Waters containing nitrate more than 40 pg at// are also found in the North Pacific on the isopicnal plane of a 27.3 in this section and spread out from the Subarctic to the Tropical region at about 10 N, while, in the region from the Antarctic to the Antarctic convergence, waters containing nitrate about 35 pg at// are uniformly distributed between 200 m to 2000 m in depth. 4) Along D line (Fig. 7-d) A section from off the Alaska to the Antarctic is shown in Fig. 7-d. Observations of the northern and the southern part of the section were made aboard the Hakuho Maru in the Great Bear Expedition (KH 70-2) and in the Phoenix Expedition (KH 71-5). In the Subarctic region to the north of 46 N,
1977 Distribution of Nitrate in the Pacific 37 (7-c) (7-d) (7-e) Fig. 7. Meridional distributions of nitrate in pg.at// at the A line (a), B line (b), C line (c), D line (d), and E line (e) in Fig. 6.
38 T. Sagi Vol. 28, No. 1 the Subantarctic region to the south and the Equatorial region, surface waters containing nitrate higher than 5 pg. at// are found. Waters containing nitrate more than 40 pg. at// are traced from the Subarctic region to the Equatorial region along the isopicnal plane of cr, 27.3. In the Antarctic Ocean, nitrate contents are rather uniform ranging from 25 to 35 pg. at// from the surface to the bottom. 5) Along E line (Fig. 7-e). A section from the coast of Mexico to the Antarctic Ocean is shown in Fig. 7-e. Observation on this section was made aboard the Hakuho Maru in the Phoenix Expedition (KH 71-5). In the Equatorial region and the region to the south of the Subantarctic convergence, nitrate-rich water of more than 10 pg.at/i exists at the surface. Waters containing nitrate more than 40 pg at// are found between 600 m and 2000 m in depth, and spread out to the south about 24 S along the isopicnal plane of at 27.3. In the coastal areas to the north of 10 N, waters containing nitrate lower than 30 pg at// appear between 200 m and 500 m in depth. These waters coincide with waters containing dissolved oxygen lower than 0.2 m1/1 (Tsubota, 1973). It is considered that nitrate in sea water was reduced by denitrification. The amount of reduced nitrate was estimated between 13 and 14 pg. at// (Cline et al., 1972). Antarctic Intermediate waters which sink in the Antarctic convergence spread out to the north between 400 m and 700 m in depth. In all sections, there is nitrate-rich surface water in the region -to the south of the Subantarctic convergence, in the Subarctic region, and in the eastern Equatorial region. Waters containing nitrate higher than 40 pg at// are found about 1000 m in depth associated with the isopicnal plane of at 27.3. The southern extremities of the zones, which indicate nitrate concentrations higher than 40 pg at//, are located respectively at 22 N, 18 N, 10 N, 5 S, and 24 S along A line, B line, C line, D line, and E line. The southern extremities of the higher concentration zone move gradually to the south as the observational line moves from west to east. Acknowledgements : The author wishes to express his hearty thanks to Dr. Y. Miyake, Geochemistry Research Association, for his kind advice and suggestions. He also appreciates the kindness with which Dr. Y. Sugiura, Meteorological Research Institute, read this manuscript and gave valuable advice to him. He also wishes to thank Prof. Y. Horibe and Dr. H. Tsubota, Ocean Research Institute University of Tokyo, for collecting the data. He is indebted to Messrs. T. Akiyama, T. Yura, and K. Kimura, Japan Meteorological Agency, for their cooperation in determination of nitrate. References Cline, J. D. and F. A. Richards, 1972 : Oxygen deficient conditions and nitrate reduction in the eastern Tropical North Pacific Ocean. Limnol. Oceanogr., 17, 885-900. Hager, S. W., E. A. Atlas, L. I. Gordon, A. W. Mantyla and P. K. Park, 1972 : A comparison at sea of manual and Auto-analyzer analyses of phosphate, nitrate and silicate. Limnol. Oceanogr., 17, 931-937. Henrikson A., 1965: An automatic method for determining nitrate and nitrite in fresh and saline waters. Analyst, 90, 83-88. Kato, T., H. Okinaka, and K. Sakai, 1954: Colorimetric determination of nitrates 2. 3. A new method using aniline. Bunseki Kagaku, 3, 231-236. Matida, Y., 1951: New method of preparation of reduced strychnine reagent for the nitrate determination in sea water. Bull. Chem. Soc. Japan, 24, 254-257. Miyake, Y., 1938 : A critical study on the methods of water analysis (9), A new colorimetric analysis of nitrate. J. Met. Soc. Japan, 16, 1, 1-4. Morris, A. W. and J. P. Riley, 1963 : The determination of nitrate in sea water. Anal. Chim. Acta, 29, 272-279. Mullin, J. B. and J. P. Riley, 1955 : The spectrophotometric determination of nitrate in natural waters with particular reference to sea water. Anal. Chim. Acta, 12, 464-480. Sagi, T., 1970 : On the distribution of nitrate nitrogen in the western North Pacific Ocean. Oceanogr. Mag., 22, 63-74.
1977 Distribution of Nitrate in the Pacific 39 Strickland, J. D. H. and T. R. Parsons, 1968 : Automated nutrient analysis in "A practical handbook of sea water analysis", Fish. Res. Bd. Can., Bull., 167, 119-128. Tsubota, H., 1973: Preliminary Report of the Hakuho Maru Cruise KH 71-5 (Phoenix Expedition) (1973). Wood, E. D., F. A. J. Armstrong, and F. A. Richards, 1967: Determination of nitrate in sea water by cadmium-copper reduction to nitrite. J. Mar. Biol. Ass. U.K., 47, 23-31.