Distribution of nitrate, phosphate and silicate in the world oceans

Prog. Oceanog.

Vol. 31, pp.245-273, 1993.

0079 - 6611/93 $24.00

Printed in Great Britain.

1993 Pergamon Press Ltd

Distribution of nitrate, phosphate and silicate in the world oceans SYDNEYLEwxus~, MARGARITAE.

Co~r,P,Iorrr~,JOSEPHL. REID2, RAYMONDG. NAJJAR3 and ARNOLDMArCrVLA2

WOAA/NODC, 1825 Connecticut Avenue, Washington, D.C. 20235, USA eScripps Institution of Oceanography, University of California, San Diego, California, USA 3Program in Atmospheric and Ocean Sciences, Princeton University, Princeton, New Jersey 08542, USA Abstract This study describes the global horizontal distributions of the plant nutrients phosphate, nitrate and silicic acid, with depth, on a one-degree latitude-longitude grid. The source of the data is a subset of the National Oceanographic Data Center. Nutrients in surface waters axe enriched in upwelling and high latitude regions and axe generally depleted at mid-latitudes. The depletion at mid-latitudes is associated with the subtropical anticyclonic gyre systems. With increasing depth, the nutrient content increases in the water column. At depths of more than 1000m, the nutrient distributions axe associated with different water masses which have their own inherent characteristics.

CONTENTS |.

2. 3.

4.

5. 6.

Introduction Data Results and Discussion 3.1 Horizontal distributions of nitrate at standard depths 3.2 Horizontal distribution of phosphate at standard depths 3.3 Horizontal distribution of silicate Summary Acknowledgements References

245 246 254 254 256 257 272 272 272

1. INTRODUCTION Nutrient distribution s in the ocean basins are controlled by various inputs and losses to the ocean basins. Estimated annual nutrient fluxes for the world oceans (EMERY, ORR and RITTENBERG, 1955; WOLLAST, 1974) show small contributions from rivers and the atmosphere (less than 1% o f that required for oceanic primary production, HARRISON, 1980). The principal source o f nutrients to surface waters is upward vertical flux (diffusion and advection) and regeneration. In addition, nitrogen fixation (DUGDALE and GOERING, 19 67) and nitrification (HARRIS, 1961) m a y add to the nitrate supply. Nutrients are depleted in the surface waters through uptake by phytoplankton and sinking o f organisms and their skeletal remains. Grazing ofphytoplankton by zooplankton and elimination in fecal pellets also contributes to rapid removal o f nutrients from the photic zone to the deep ocean and sediments. 245

246

S. LEVITUSet

al.

Many studies have examined the distribution of nutrients in the ocean basins. A large number of them have focused on nitrate since it is considered to limit primary production in oligotrophic and temperate surface waters (RYTHERand DUNSTAN, 1981; EPPLEY,RENGERand HARRISON, 1979; DUGDALEand GOERING,1967) while phosphate is still available in waters where nitrate is depleted (ZENTARAand KAMYKOWSKI,1977). Silicate can be the limiting nutrient, especially in areas o fdiatomblooms(DUGDALE,1972; BRINK,JONES,VANLEEN,MOOEVS,STUART,STEVENSON, DUGDALEand HEBURN,1981). KAMYKOWSKIand ZENTARA(1985) described nitrate, silicic acid and the nitrate: silicic acid ratios in the upper ocean; METCALF(1969), MANN,COOTEand GARNER (1973), FRIEDERICHand CODISPOTI( 1979, 1981) studied silicates in the deep North Atlantic, Peru and northwest Africa; BROECKERand LI (1970) and BROECKERand PENG(1982) used nutrients to trace water masses in the deep ocean, and REID(1962, 1973) described phosphate distributions in the Pacific as a function of circulation. EDMOND, JACOBS, GORDON, MANTYLAand WEISS (1979) examined the sediment-water silica flux as a possible contributor to the deep silica maximums and MANTYLAand REID(1983)illustrate the distribution and spreading of silica in the bottom waters of the world oceans. KAWASEand SARMIENTO(1985) and FUKUMO~,MARTELand WUNSCH (1991) have presented nutrient analyses for the Atlantic Ocean based on data from expeditions during the 1970s and 1980s. GORDON, MOLINELLIand BAKER(1982) presented nutrient distributions for the Southern Ocean. The historical development of our understanding of nutrients and their distribution and relation to plankton is given by MILLS(1989). The findings from the above studies indicate a general depletion of nutrients in surface waters except for high latitudes and upwelling regions. ZENTARAand KAMYKOWSK~(1977) found an inverse relationship between temperature and nutrients. An increase in temperature is associated with a decrease in nutrients for the upper ocean. The same authors (ZENTARAand KAMYKOWSKI, 1981 ) concluded that differences in the global nutrient distributions are closely linked to circulation patterns. The general circulation of the oceans leads to an enrichment of nutrients in the North Pacific due to input of nutrient-rich deep waters, and a depletion in the North Pacific due to input of nutrient-rich deep waters, and a depletion in the North Atlantic due to loss of deep water (BROECKERand LI, 1970). This study describes the global horizontal distributions of the plant nutrients (phosphate, nitrate and silicic acid), with depth, based on a subset of the National Oceanographic Data Center (NODC) Station Data profiles. This data set has been used in several papers addressing circulation and nutrient distributions. 2. DATA The data used in this study were compiled at the Scripps Institute of Oceanography by J. Reid and collaborators. The stations have been selected by J. Reid and A. Mantyla to map large-scale patterns in the world oceans, as in REID(1981) and MANTYLAand REID(1983). The stations extend to the bottom and were not intended for seasonal or fine-scale studies. Most of the data were obtained from NODC, and the set is being augmented as new data become available. The set is available to other users. The use of a multi-year database, provided by di ffernt ships, is useful for describing large scale distributions and processes (LEVITUS,1982; WORTHINGTON,1981 ; REID,1965). LEVITUS(1982) discusses some of the biases which may occur as a result of analyzing composites of historical data. In addition, a possible bias may occur in using an all season database to look at properties which may have strong seasonal signals. Most expeditions to high latitudes are in the summer season, so a bias toward low nutrient values (due to uptake by phytoplankton) is apt to be found in the data.

Nitrate, phosphate and silicate in the world oceans

247

Figures 1-3 show the data distribution of nitrate, phosphate and silicate at 0m, 1000m and 2000m respectively. These maps are useful in identifying biases in the data analysis due to missing data or few values. Generally, these maps show that phosphate has been more heavily sampled than silicate and nitrtae. Large gaps are observed in the data collected from the South Atlantic and part of the Pacific Ocean for nitrate and silicate, and the South Indian Ocean for nitrate. Only stations that reach close to the bottom and had high quality temperature and salinity data were chosen. The North Atlantic has been sampled more often compared to other ocean basins. All of these maps show a lack of data in the northern polar regions; therefore our analyses in latitudes 80-90°N have been masked out. Table 1 lists the number of observations at each standard level discussed in this paper for each nutrient. The original data set that met the selection criteria contained a total of 7880 stations. About 80% of these stations contained valid nutrient data. We screened the data records for extreme values. Thirty-two phosphate data above 4.5~mol 11, forth-two nitrate data above 60lamol !l , and two silicate data above 300~tmol 1-1 were deleted. TABLE 1: Number of observations, with depth, of nitrate, phosphate and silicate. Depth (m)

Nitrate

Phosphate

0 10 20 30 50 75 100 125 150 200 250 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1750 2000 2500 3000 3500 4000 4500 5000 5500

2688 2109 2199 2061 2182 2192 2742 2708 2930 2892 2980 2965 2918 2827 2784 2739 2691 2681 2357 2388 1991 2033 1870 2048 2566 2423 2255 2001 1642 1278 887 497 171

4304 3120 3361 3001 3309 3377 4071 3923 4508 4464 4545 4494 4406 4378 4406 4301 4269 4330 3747 3691 3162 3242 2910 3193 4082 3863 3586 3141 2563 2010 1339 729 240

Silicate 3635 2658 2791 2594 2818 2865 3600 3516 3932 3906 4032 4055 4026 3944 3924 3841 3810 3763 3287 3234 2715 2794 2563 2976 3671 3455 3189 2809 2296 1783 1200 678 220

248

S. LEvrrus et al.

We used a total of 6171 stations containing 144,523 phosphate observations, 96,154 nitrate observations and 132,463 silicate observations. We then linearly interpolated these data to thirtythree standard depths. To interpolate to a given standard depth, we required one observation to lie above the standard depth and one to lie below the standard depth, both within a given distance from the standard depth. The standard depths and the maximum distance allowed for the interpolation are given in Table 2. The standard depths are those used in the atlas of Levitus (1982) and include the thirty NODC standard depths. Interpolation was not performed at the surface. Rather, the closest observation to the surface less than 9m deep was taken to represent the surface value. Some data were "lost" in the interpolation because some observations were too far from a standard level, or there were other closer observtions to a standard level. The data were binned into one-degree squares to facilitate handling and filtering procedures.

TABLE 2: Standard depths and interpolation distances Level 1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

Depth (m) 0

10 20 30 50 75 100 125 150 200 250 300 400 500 600 700 800 900 1000 i100 1200 1300 1400 1500 1750 2000 2500 3000 3500 4000 4500 5000 5500

Interpolation *

25 25 25 25 25 50 50 100 100 200 200 200 200 200 200 200 200 200 200 200 200 200 250 500 500 500 500 500 500 500 500 500

*NODC criterion is any observtions shallower than 9m can be used as the "surface" observation

N i t r a t e , p h o s p h a t e a n d s i l i c a t e in t h e w o r l d o c e a n s

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A statistical check was performed to eliminate some of the bad or unrepresentative data, as follows. For each one-degree square containing data, the mean and standard deviation of all of the data lying within 1000km was computed. If there were ten or more data within this region, then all data lying more than three standard deviations from the mean were deleted. We often found that outliers at one depth were present at many other depths. Thus, if five or more mean values in a water column of a given one-degree square were removed in the statistical check, then all of the data in that water column were deleted. This greatly reduced unrealistic features in the objectively analyzed fields. After this procedure some outliers still remained. We then subjectively deleted the one-degree squares that contained outliers. The data were then objectively analyzed using the procedure described by LEVITUS(1982). Since the scheme analyzes an anomaly field, as noted by LEVITUS,some small negative values were produced in regions of low nutrient distributions. These negative values have been changed to zero.

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3. RESULTSA N D DISCUSSION (Figures 4-30 appear at the end of this section on pages 258-271) 3.1 Horizontal distributions of nitrate at standard depths

The availability ofnitrate will determine the extent of new primary production in the surface waters (DUGDALEand GOERING,1967). Nitrate is consumed in the photic zone by phytoplankton and as a result most of the surface waters in the world oceans are depleted in nitrate, except for the high latitudes and eastern tropical Pacific ocean, as shown in Fig.4. Thehighest surface concentrations are found south of the Subantarctic Convergence. Upwelling occurs primarily at the center of the divergent cyclonic gyres in the Weddeil and Ross Seas (GORDON 1971). Much of the flow along the Antarctic coast is westward which results in downwelling at the coast. The surface nitrate patterns show the highest concentrations are mostly well north of the coast with lower values around the Antarctic coastline. These results are consistent with what we might expect based on the circulation patterns around the Antarctic continent. Nitrate is available in surface waters of the subarctic gyre in the northern Pacific. REID(1962) described this gyre to be a zone of divergence and characteristically high in nutrients. In this area, the concentration of nitrate increases from east to west along the Pacific basin with the highest nitrate concentration found around the Sea of Okhotsk and the Bering Sea. The northeastern Pacific basin is characterized by a cyclonic gyre in the Gulf of Alaska (UDA, 1963). The center of this gyre entrains nutrient-rich deep water, which coupled with a short bloom, leads to the persistence of nitrate in these waters (ANDERSON,PARSONSand STEPHENS,1969). Surface nitrate values in the northern high latitudes are considerably lower than in the Antarctic region. Low amounts of nitrate are also found in the surface waters of the Norwegian and North Sea. Upwelling regions are characterized by the upward movement of deep waters which replace nutrient-depleted surface waters. Upweiling occurs mainly at eastern ocean boundaries, in the high latitude cyclonic gyres and the equatorial belt (REID, BRINTON, FLEMINGER, VENRICK and McGOWAN, 1978; BROEEKERand PENG, 1982). The Peru upwelling region has surface nitrate concentrations ranging from 10~tmol I'l in the coastal waters decreasing with distance from shore. This region extends to about 180°W in the equatorial Pacific. With increasing depth, nitrate concentrations in the subarctic and subantarctic waters increase. At 50m depth (shown in Fig.5.), the Arabian Sea, coastal waters of Western South America, equatorial Africa (20°N to 20°S) and the southwest coast of the United States, have nitrate values increasing to 10~tmol i-I and decreasing with distance from shore. The presence of nitrate in these subsurface waters could be due to vertical advection, either by upwelling or divergence. Regeneration may be taking place if the water quality is such that not much light penetrates to these depths thereby limiting phytoplankton uptake of nitrate. The same probably applies to the upwelling regions of California, northwest Africa and Peru since the upweiling of water occurs in depths less than 250m (BENNEKOM, 1978; SMITHand CODISPOTI,1979; BROECKERand PENG 1982). The equatorial Pacific has higher nitrate values than the equatorial Atlantic. As depth increases from 50 to 150m the new features evident in the 50m depth figure begin to expand. At 75m (not shown), nitrate is present in the Indian Ocean, all along the west coast of America, from Cape Hatteras north in the Atlantic and along the equator in the Atlantic. The distribution of nitrate at 150m depth is notable for the areas containing low nitrate. Figure 6 shows the nutrient-poor subtropical gyres in the Pacific and Atlantic separated by nitrate-rich waters in

Nitrate, phosphate and silicate in the world oceans

255

the tropics. The source of the nitrate along the equator probably results from the upwelling of waters since the concentrations increase from east to west across the basins. The only source of nitrate to the gyres in subsurface waters is either vertical diffusion, upwelling, or horizontal advection from surrounding waters (REID et al, 1978). The South Indian Ocean is also surrounded by nitrate-rich waters in the Circumpolar region, the Bay of Bengal, and the Arabian Sea. The gyres in the South Indian, Pacific and Atlantic are anticyclonic which characteristically are low in nutrients as a result of the downwelling of relatively nutrient poor upper ocean water (REID, et al, 1978). Figure 7 shows the nitrate distribution at 250m depth. This is much the same as that found at 150m except the nitrate levels are higher and the regions of high nitrate in the Gulf of Alaska and Bay of Bengal have intensified. The tongue of high nitrate values in the equatorial Pacific now extends all across the basin. The nitrate distribution at 500m, shown in Fig.8, is notable because the highest values are found in several locations, the eastern equatorial Pacific, western coast of Africa, and northern Pacific, which are all areas of surface upwelling. Nitrate concentrations are fairly homogeneous in the subantarctic wters, equatorial waters and the northern North Pacific. The subtropical gyres of the North pacific and the North Atlantic still show distinct areas of lower nitrate values, with minima in the South Indian and North Atlantic oceans. Nutrient-enriched waters have been pushed to the Bay of Bengal in the North Indian and to the subpolar Pacific ocean. At 1000m (Fig.9), the structure of the subtropical gyres are discernible in the Southern Hemisphere but not the Northern Hemisphere. This is especially evident in the North Pacific and South Atlantic. Nitrate distributions can be divided into three categories: low (less than 221.tmol 1-l) in the North Atlantic and south west Indian Ocean; intermediate (22-38~tmol 1-1)extending from the equatorial Atlantic to the Southern ocean, the rest of the Indian Ocean and the southern Pacific; high (values exceed 38~tmol 1-1) concentrations are found in the equatorial Pacific extending northward. Notable in this figure is the larger amount of nitrate found in the eastern Pacific than the western Pacific. Already in this depth, we are seeing the concentrating of nitrate in the North pacific Basin. Figure 10 shows nitrate at 1500m. This figure is similar to the distributions shown in the previous figure except that nitrate concentrations have decreased in the 30°N to 30°S belt. This is due to the intrusion ofnutrient-poorNorth Atlantic Deep Water (NADW) which flows south over a depth range of 1500-4000m (MANN et al, 1973). Also notable is the increase in nitrate in the southwest Indian Ocean. Nitrate distributions at 2000m are shown in Fig. 11. At this depth, the highest nitrate values are found in the North Pacific subpolar waters with the concentrations ranging from 42-46ktmol 11. Nitrate decreases with decreasing latitude in the Pacific. The low nitrate subantarctic waters (3040~tmol 1-l) have penetrated into the South Pacific, Indian, and South Atlantic waters. There are patches of lower nitrate in the Indian Ocean, particularly off the southeast coast of Africa and off the east Antarctic coastal area. The Atlantic Ocean is characterizedby Antarctic type concentrations which extends to about 50°S; a band of intermediate values (24-30~tmol 1-l) and lower values extending from 25°S into the northern latitudes. The distribution of nitrate at 3000m is similar to that at 2000m (shown in Fig. 12) except the band of intermediate nitrate values has narrowed in the Atlantic and the high values in the Pacific subpolar waters have disappeared and been replaced by the 30-36mmol 1-1waters. The North Indian, at this depth, is characterized by a tongue of circumpolar water extending from the south (WYRTKI,1971 ). In addition, the Bay of Bengal sediment fan serves as a major nutrient source to the bottom waters of the North Indian (BROECKER,TOGGWEILERand TAKAHASHI,1980).

256

S. LEvrruset al.

3.2 Horizontal distribution of phosphate at standard depths The distribution of phosphate is very similar to that of nitrate since their pathways through the water column are similar. Comparison of vertical profiles of phosphate and nitrate indicate that the phosphate is regenerated somewhat faster or at least in more shallow depths than nitrate. Phosphate distributions in the world oceans will be described only briefly, with emphasis placed on the differences between nitrate and phosphate. Figure 13 describes surface phosphate distributions. Phosphate is most abundant in the surface waters surrounding the Antarctic continent where pockets of high phosphate (concentrations greater than 2~tmol 1-1)are found. High phosphate content is also found in the Pacific subpolar gyres and the Peru upwelling region. The signal from the Peru upwelling region is as well defined for phosphate as it is for nitrate. Concentrations are higher in this upwelling area than in adjacent waters and reach a maximum of0.81~mol 1-1in the coastal region. The Subarctic Pacific has concentrations starting at 0.41xmol 1"l increasing to 1.4ktmol 1-1 with increasing latitude and higher values in the western section of the subarctic Pacific basin. The features of the Pacific subarctic gyre are clearly defined, even in surface waters. Unlike nitrate, most of the surface waters in the oceans contain some phosphate. The North Atlantic has phosphate ranging from 0.1-0.4~tmol 1-1 with the highest amounts found in the Labrador andNorth Seas. The South Atlantic concentrations exceed 2.0~tmol !-1. Phosphate is also present in most coastal areas with the exception of eastern Asia and Australia. It is depleted in the North and South Pacific and North Atlantic waters at mid-latitudes. The above distribution s become more sharply defined as phosphate content increases with depth as indicated by Fig. 14. This distribution is similar to that ofnitrate. At 50m, the concentrations have increased to 2.2~tmol i-I in the Antarctic region, and some phosphate is now present in the Central Pacific and coastal Australian waters. The upwelling areas off the western coasts of Africa and South and Central America are apparent at this depth. At 150m (Fig. 15), some differences emerge between nitrate and phosphate, particularly in the North Indian ocean and the equatorial waters. The North Indian is characterized by an area of high phosphate content (>2ktmol I-l), in the Arabian Sea. Nitrate values are only high for a small area in the Bay of Bengal. These differences could be due to either loss of nitrate by denitrification in this area, or that phosphate is being regenerated faster or at shallower waters than nitrate. It is more likely that denitrification is the cause for these distributions since the oxygen content is quite low in this area and the conditions are favorable for nitrate reduction. Figure 15 also shows the Pacific subtropical gyres defined more clearly than for nitrate at this depth. Lowest concentrations are found in the subtropical gyres of the Pacific and North Atlantic oceans. The phosphate distribution follows a pattern similar to nitrate in that higher phosphate is found in the subpolar regions but less in the North Atlantic than North Pacific. The Indian ocean has lowest phosphate values in its subtropical gyre (18°S-30°S). The eastern Pacific and Atlantic equatorial waters have a higher phosphate content than the western portion of the basins due to upwelling in the eastem basins. In the tropics of the Atlantic and Pacific higher values are found in the central and eastern parts of the basin. These higher values are associated with the waters above the main thermocline in these regions. The permanent thermocline slopes down from east to west across these basins. The 250m (Fig. 16) distribution is quite similar to the 150m distribution. The major difference occurs in the tropics of the Atlantic and Paci fic where high phosphate tongues at 250m depth extend across the entire width of each basin. The phosphate distribution at 500m is presented in Fig.17. This figure bears a strong resemblance to the 500m nitrate distribution so we will not discuss specific features.

Nitrate, phosphate and silicate in the world oceans

257

Phosphate and nitrate distributions at 1000m are also very similar. The distribution at this depth, shown in Fig.18, is an increase in phosphate in the North Atlantic, Pacific, and Indian Oceans compared to the 500m values. At this depth, phosphate values are twice as high in the Indian Ocean and three times as high in the Pacific when compared to the North Atlantic. The influence of the AAIW is clearly seen in the North Atlantic as evidenced by the northeastward extension of the 1.2 contour. There is little change in phosphate distributions between 1500-3000m. At 1500m (Fig. 19), phosphate has a more uniform distribution, throughout the ocean basins, than nitrate. Like nitrate, there is a phosphate minimum in the Atlantic due to the influx of NADW. Concentrations around Antarctica and the South Pacific and Indian oceans are fairly homogeneous and increase with increasing latitude. Figures 20 and 21 show phosphate distributions at 2000 and 3000m. Phosphate has further decreased in the Atlantic ocean suggesting that at 3000m this is deep Circumpolar Water. There is also a minimum found between 2000 to 3000m in the North Pacific.

3.3 Horizontal distribution of silicate The distribution of silicate in the world oceans is slightly different from that of nitrate and phosphate since its pathway through the water column is different. Silicate is a nutrient essential only to siliceous-bearing plankton such as diatoms and radiolarians. Therefore, near depletion of this nutrient will occur where these plankton bloom, primarily in cold waters (DUGDALE, 1972; BRINK etal, 1981). Silica is regenerated at different rates and by different processes than phosphate and nitrate and at present there is uncertainty about the factors that control the marine silica budget (CHES~R, 1990, p.552). Generally, though, silicate has similar distribution patterns as nitrate and phosphate in surface waters; patterns which appear strongly determined by the oceanic circulation. Surface silicate distributions are shown in Fig.22. Silicate is generally depleted in surface waters of the Atlantic Ocean and Indian Ocean north of 45°S and the Pacific Ocean south of 45°N. It is found in small amounts along the coastal waters of southeast Asia, New Guinea and the Bay of Bengal. The subpolar waters of the North Pacific have silicate content increasing from east to west across the basin reaching a maximum along the Kamchatka and Oyashio currents. The circumpolar waters around Antarctica exhibit large silicate content near the continent, particularly in the Weddell Sea. The Weddell Sea exhibits the highest silicate content of any of the surface waters of the world ocean. Silicate distributions change slowly in the upper 50m of the ocean. The distribution at 50m, as illustrated in Fig.23, closely resembles the surface waters. There is an increase in concentrations in the Southern ocean and in the west coasts of northern Africa, South America, Central America and the southern United States, all of these regions being upwelling areas. At 150m (Fig.24), a tongue of silicate with concentrations around 20ktmol 1"1can be seen along the equatorial waters of the Indian and Pacific oceans. The silicate-poor nature of the waters of the central gyres is quite clear. The Weddell Sea still contains the highest amounts of silicate. At 250m (Fig.25), the silicate distributions are quite similar to those found at 150m. One difference is that at 250m depth the silicate-poor subtropical waters are more distinctly separated by relatively silicate-rich equatorial waters. The amount of silicate in the Pacific and Indian equatorial regions is much greater than that found in the equatorial Atlantic. The circumpolar waters of the Antarctic are still characterized by large amounts of silicate around the whole continent. The subarctic waters of the Pacific contain 2-3 times more silicate than the subarctic Atlantic. The maximum values of silica (110ktmoi l-l) for the entire world ocean is found in the northwest North Pacific and the Weddell Sea.

258

S. LEVlTUSet

al.

At 500m depth, shown in Fig.26, the North Pacific concentrates more silicate so that the maximum silicate value (150~tmol 1"1) of the entire world ocean is found in the high latitudes of the northwest Pacific. The Weddeil Sea silicate values are nearly constant with depth. Little change is found in the North Atlantic which, except for the equatorial area, has very low silicate content. Silicate distributions in intermediate and deep waters are determined by the source of water with its own inherent nutrient characteristics, the rate of renewal, and the amount of silicate raining down from surface waters (METCALF, 1969). Figure 27 shows silicate at 1000m. The southern waters of the Indian, Pacific and Atlantic show an increase in silicate content at 1000m compared to the 500m distribution. At this 1000m depth-level, the waters of the Northwest Pacific have the highest silicate content of any part of the world ocean. The North Atlantic still has low silicate content. The figure for 1500m (Fig.28) strikingly illustrates the enrichment of the North Pacific and North Indian and the silicate poor waters found in the North Atlantic. At this depth low nutrient values continue to characterize the North Atlantic as was observed for upper levels. A tongue of low silicate extends from the western Atlantic to the east along the Equator. With increasing depth, the difference between the upper NADW and Pacific and Indian Deep Water is clearly shown. BROECKERand LI (1970) found the enrichment of silicate in the Pacific Intermediate Depth Water (PIDW) to be five times that of the North Atlantic Deep Water (NADW). Figures 29 and 30 illustrate the silicate distribution at 2000 and 3000m. Unlike nitrate and phosphate, silicate shows a progressive increase from 1500 to 3000m in the North Atlantic. This increase could be due to advection of high nutrient abyssal waters (MANTYLAand REID, 1983; REID, NOWLINand PATZERT, 1977).

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Nitrate, phosphate and silicate in the world oceans

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30W

LONGITUDE Fig.9. Annual mean nitrate at 1000m depth.

30E

60

90

120

150E

180

150W

120

90

0

-60

-30

w

t7 -EQ

I-

5 =30

-60

90S-

-90S 30E

60

90

120

150E

180

150W

120

90

LONGITUDE

Fig. 10. Annual mean nitrate at 1500m depth.

60

30W

262

S. LEVlTUSet al.

30E

60

90

120

150E

180

150W

120

90

60

30W

0 90N

90N

60

,60

30

"3O

EQ

"EQ

30

-30

60-

-60

IJJ a I--

-90S

90S" 30E

60

90

120

150E

180

150W

120

90

60

30W

LONGITUDE

Fig. 11. Annual mean nitrate at 2000m depth.

30E

60

90

120

150E

180

150W

120

90

60

30W

90N

0 90N

60'

60

30"

30

EQ

EQ

30'

30

60'

60

ILl

a

I-m

I.-

90S'

90S 30E

90

120

150E

180

150W

120

90

LONGITUDE

Fig. 12. Annual mean nitrate at 3000m depth.

60

30W

Nitrate, phosphate and silicate in the world oceans

30E

60

90

120

150E

180

150W

120

90

263

60

30W

0 90N

90N-

60"

60

30'

30

EQ'

EQ

LU

£3 I--

30 ¸

30

60

60

90S

90S 30E

60

90

120

150E

180

150W

120

90

60

30W

60

30W

LONGITUDE

Fig. 13. Annual mean phosphate at the sea surface.

30E

60

90

120

150E

180

150W

120

90

0

90N

90N-

60"

60

30-

30

EQ

EQ

30

3O

60,

6O

u.I a I-

90S"

90S 30E

60

90

120

150E

180

150W

120

90

LONGITUDE

Fig. 14. Annual mean phosphate at 50m depth.

60

30W

264

S. LEvrrus et al.

30E

60

90

120

150E

180

150W

120

90

60

30W

0 60N

90N-

60-

.60

30-

30

EQ-

EQ

30-

.30

60-

-60

111 o I~

i.--

5

-60S

90S" 30E

60

90

120

150E

180

150W

120

90

60

30W

60

30W

LONGITUDE

Fig.15. Annual mean phosphate at 150m depth.

30E

60

90

120

150E

180

150W

120

90

0

90N

90N

60

60

3O

30

EQ

EQ

30

30

60

60

LU

E3

9OS

.60S 30E

60

90

120

150E

180

150W

120

90

LONGITUDE

Fig. 16. Annual mean phosphate at 250m depth.

60

30W

Nitrate, phosphate and silicate in the world oceans

30E

60

90

120

150E

180

150W

120

90

265

60

30W

0 90N

90N-

60-

60

30-

30

EQ-

EQ

30-

30

60-

60

UJ 0 I--

i-

5

90S-

90S 30E

60

90

120

150 E

180

150W

120

90

60

30W

60

30W

LONGITUDE Fig. 17. Annual mean phosphate at 500m depth.

30E

60

90

120

150E

180

150W

120

90

90N-

0 90N

60-

-60

30-

-30

EQ-

"EQ

30-

"30

60-

-60

LU 0 I--

5

90S-

-90S 30E

60

90

120

150E

180

150W

120

90

LONGITUDE

Fig. 18. Annual mean phosphate at 1000m depth.

60

30W

266

S. LEvrrus et al.

30E

60

90

120

150E

180

150W

120

90

60

30W

0 ~N

9ON-

60.

-60

30'

-30

EQ

-EQ

30

-30

60

60

I..LI 0 I--

i-

5

90S

90S 30E

60

90

120

150E

180

150W

120

90

60

30W

60

30W

LONGITUDE Fig. 19. Annual mean phosphate at 1500m depth.

30E

60

90

120

150E

180

150W

120

90

0 90N

-60

.30 U.I E3 t-

I

-EQ

F-

5

-30

-60

-90S 30E

60

90

120

150E

180

150W

120

90

LONGITUDE

Fig.20. Annual mean phosphate at 2000m depth.

60

30W

Nitrate, phosphate and silicate in the world oceans

30E

60

90

120

150E

180

150W

120

90

267

60

30W

90N-

0

90N

60.

60

30.

30

EQ

EQ

30

30

60

60

UJ E3 I-I--

5

9os

90S 30E

60

90

120

150E

180

150W

120

90

60

30W

60

30W

LONGITUDE

Fig.21. Annual mean phosphate at 3000m depth.

30E

60

90

120

150E

180

150W

120

90

90N-

-90N

60-

-60

30-

"30

UJ O P"

EQ-

"EQ

30-

30

60-

60

9OS-

90s I Cu

1 ~ul::

1@o

150W

120

90

LONGITUDE

Fig.22. Annual mean silicate at the sea surface.

60

30W

268

S. LEvrrvs et al.

30E

60

90

120

150E

180

150W

120

90

60

30W

0 90N

-60

"30

I.U

E3 I.-

"EQ

5 30

-60

.90S 30E

60

90

120

150E

180

150W

120

90

60

30W

LONGITUDE

Fig.23. Annual mean silicate at 50m depth.

30E

60

90

120

150E

180

150W

120

90

60

30W

90N -

0 90N

60-

-60

30-

-30

EQ-

-EQ

30-

-30

60-

60

UJ

a

J'--

i-

5

goS-

g0S 30E

60

gO

120

150E

180

150W

120

90

LONGITUDE

Fig.24. Annual mean silicate at 150m depth.

60

30W

Nitrate, phosphate and silicate in the world oceans

269

30E

60

90

120

150E

180

150W

120

90

60

30W

0

30E

60

90

120

150E

180

150W

120

90

60

30W

0

111 o I-

LONGITUDE

Fig.25. Annual mean silicate at 250m depth.

30E

60

90

120

150E

30E

60

90

120

150E

180

150W

120

90

60

30W

180

150W

120

90

60

30W

uJ t7 I-

5

LONGITUDE

Fig.26. Annual mean silicate at 500m depth.

0

270

S. LEVITUSet aL

30E

60

90

120

150E

180

150W

120

90

60

30W

0 90N

9ON-

60"

60

30 ¸

30

EO

EQ

3O

30

60

-60

111 0 ~-

i-

5

-90S

90S 30E

60

90

120

150E

180

150W

120

60

60

30W

LONGITUDE

Fig.27. Annual mean silicate at 1000m depth.

30E

60

90

120

150E

180

150W

120

90

60

30W

90N-

90N

60-

-60

30.

-30

EQ"

-EQ

30 ¸

-30

60,

-60

uJ 0 I-

5

90S.

-90S 30E

60

90

120

150E

180

150W

120

90

LONGITUDE Fig.28. Annual mean silicate at 1500m depth.

60

30W

Nitrate, phosphate and silicate in the world oceans

30E

60

90

120

150E

180

150W

120

90

271

60

30W

0

90N

90N -

60-

-60

30-

30

EQ-

EQ

30-

-30

60

-60

LU a I'-I--

5

.9os

90S30E

60

120

150E

180

150W

120

90

60

30W

LONGITUDE

Fig.29. Annual mean silicate at 2000m depth.

30E

60

90

120

150E

180

150W

120

90

60

30W

90N

0 90N

60 ¸

.60

30.

30

EQ.

"EQ

30.

30

60-

60

UJ D I'-" I--"

5

90S-

9os 30E

60

90

120

150E

180

150W

120

90

LONGITUDE

Fig.30. Annual mean silciate at 3000m depth.

60

30W

272

S. LEVITUSet al.

4. SUMMARY Our objectively analyzed fields describe the general oceanic patterns o f nutrient distributions. Nutrients in surface waters are found in upwelling and high latitude regions and are generally depleted at mid-latitudes. Phosphate is usually present in surface waters, unlike nitrate and silicte. With increasing depth, nutrients increase in the water column and their distributions in the mixed layer are primarily determined by the biochemical processes operating at these depths. At depths o f more than 1000m, the nutrient distributions are associated with different water masses which have their own inherent characteristics. At these depths, we also note the relatively high concentration o f nutrients in the North Pacific and North Indian oceans and the relatively low concentrations in the Atlantic ocean. 5. ACKNOWLEDGEMENTS The analyses of the nutrient data was supported by the NOAA Climate and Global Change Program. The comments from E. Collins, C. McClain and ofthree anonymous reviewers are greatly appreciated. 6. REFERENCES ANDERSON,G.C., T.R. PARSONSand K. STEPHENS(1969) Nitrate distribution in the subarctic Northeast Pacific Ocean. Deep-Sea Research, 16, 329-334. BENNEKOM,A.J. (1978) Nutrients on and offthe Guyana Shelfrelated to upwelling and Amazon outflow. FA 0 Fish Rep, 233-253. BPONK,K.H., B.J. JONES,J.C. VANLEEN,C.K.N. MOOEVS,D.W. STUART,M.R. STEVENSON,R.C. DUGDALEand G.W. HEBURN( 198 I) Physical and biological structure and variability in an upwelling center offPeru near 15°S during March 1977. In: Coastal Upwelling, F.A. R/CHARDS,editor, American Geophysical Union, Washington DC, 473-495. BROECKER,W.S. and YUAN-HUILI (1970) Interchange of water between the major oceans. Deep-Sea Research, 25, 3545-3552. BROECKER,W.S. and T.H. PENG(1982) Tracers in the Sea. Columbia University, New York, 690pp. Broecker, W.S., J.R. Toggweiler and T. TAKArtASHI(1980) The Bay of Bengal - a major nutrient source for the deep Indian Ocean. Earth Planet. Sci. Lett. 49,506-512. CHESTER,R. (1990) Marine Geochemistry. Unwin Hyman, Ltd. London. DUGDALE,R.C. (1972) Chemical oceanography and primary productivity in upwelling regions. Geoforum, 2, 4761. DUGDALE,R.C. and J.J. GOERING(1967) Uptake of new and regenerated forms of nitrogen in primary productivity. Limnology and Oceanography, 12, 196-206. EDMOND,J.M., S.S. JACOBS,A.L. GORDON,A.W. MANTYLAand R.F. WEISS(1979) Water column anomalies in dissolved silica over opaline pelagic sediments and the origin of the deep silica maximum. Journal of Geophysical Research, 84(C 12), 7809-7826. EMERY,K.O., W.L. ORR and J.C. RJTrENBERG(1955) Nutrient budgets in the ocean. In: Essays in the Natural Sciences in honor of Captain Allan Hancock. University of California Press, Los Angeles, pp. 147-157. EPPLEY, R.W., E.H. RENGERand W.G. HARRISON(1979) Nitrate and phytoplaakton production in Southern California coastal waters. Limnology and Oceanography, 24, 483-494. EPPLEY,R.W. and B.J. PETERSON(1979) The flux of particulate organic matter to the deep ocean and its relation to planktonic new production. Nature, London, 282,677-680. FRIEDERICH,G.E. and L.A. CODISFO'n(1979) On some factors influencing dissolved silicon distribution over the Northwest African Shelf. Journal of Marine Research, 37, 337-353. FRIEDERICH,G.E. and L.A. CODISPOTI(1981) The effects of mixing and regeneration on the nutrient content of upwelling waters offPeru. In: Coastal Upwelling, F.A. RICHnRDS,editor, American Geophysical Union, Washington DC, 221-227.

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