Near-surface organic nitrogen in the Eastern Tropical Pacific Ocean

Deep-Sea Reaearc, h, 1971, Vol. 18, pp. 65 to 71. Pergamon Pre~. Printed in Great Britain.

Near-surface organic nitrogen in the Eastern Tropical Pacific Ocean*

WILLIAM H. THOMASt, EDWARD H. RENGER'~ and ANNE N. DODSON~

(Received 2 February 1970; in revised form 27 April 1970; accepted 28 April 1970)

Abstract--During the second EASTROPAC survey (August-September, 1967) 94 water samples from a depth of 10 m in the eastern tropical Pacific Ocean were analyzed for organic nitrogen by u.v. irradiation. Concentrations ranged from 3.4 to 13-8 pg-at./l, and were significantly lower in water that contained nitrate as compared with nitrate-free water. Organic N concentrations were higher than concentrations of inorganic N in nitrate-free water and organic N could be a significant source of N for phytoplankton flit was available to these organisms. Enrichment experiments suggest that organic N is not available to phytoplankton and it is probable that this material consists of relatively refractory compounds that are heterocyclic and of high molecular weight.

IN TI~ eastern tropical Pacific Ocean, nutrient-poor water can be distinguished from nutrient-rich water on the basis of the detectability of nitrate in near-surface samples (T~oMAS, 1969 and unpublished data). Rich areas include the equatorial upwelling zone, the Costa Rica Dome, and the Peru Current; water to the north and south of the equatorial upwelling zone and well offshore is typically lower in nutrients, particularly in nitrate. In poor water, enrichment experiments have given direct evidence that nitrogen is the main nutrient that limits phytoplankton growth, whereas in rich water phytoplankton are not nutrient-limited (Tno~u~s, 1969; 1970b).

It was hypothesized (THOMAS, 1966, 1970a) that in poor water nitrogen deficiency was alleviated by the presence of ammonium and amino-N (generally in concentrations < 1-0 #g-at./1.) as measured by the RICHAl~S and KLBTSCH (1964) method.

However, the possibility remained that deficiency was also alleviated by organic nitrogen other than the fraction of the amino-N measured by hypochlorite oxidation to nitrite. The purpose of this paper is to describe the magnitude of such organic nitrogen concentrations in this area.

During the second EASTROPAC Survey (August-September, 1967) 94 water samples were taken from a depth of 10 m. The area covered extended from the American coast west to l19°W and from 20°N to 10°S in offshore waters and to 15°S in waters immediately off Peru. The samples were frozen in plastic bags and analyzed ashore for organic nitrogen by u.v. irradiation (Am~_STRONG, WILLIAMS and ST lUC r ~ I ) , 1966). Irradiation of sea water with u.v. for 3 hr in the presence of excess oxygen--supplied by adding hydrogen peroxide to the sample--oxidized organic N and ammonium to nitrate. The latter was then measured by cadmium- copper reduction (STIUCr~A~q~ and PARSONS, 1968). The method was standardized by running known concentrations of nitrate in sea water through the cadmium--copper reducing columns and was also checked by irradiating sea water to which known

*Contribution from the Scripps Institution of Oceanography. tlmtitute of Marine Resources, Scripps Institution of Oceanography, University of California,

San Diego, La Jolla, California 92037.

65

66 WILLIAM H. THOMAS, EDWARD H. RENGER and ANNE N. DODSON

concentrat ions o f ~, 0~'-dipyridyl were added. Recoveries of this c o m p o u n d ranged

f rom 76 to 93% and the average recovery was 84%. These recoveries were lower

than we would have liked, and suggest that the values for organic N reported herein

are minimal. With longer i rradiat ion they might have been higher. Duplicate samples

were no t analyzed; STRICKLAND and PARSONS (1968) estimate the precision o f a single

measurement at :i: 0.25 #g-at./1. We corrected the nitrate obtained by irradiat ion

for nitrate and nitrite already present at 10 m, as measured at sea with the Technicon

Autoanalyzer (ARMSTRONG, STEARNS and STRICKLAND, 1967) or manual ly (STRICK-

LAND and PARSONS, 1968). We did not have a large enough sample to analyze for

a m m o n i u m at 10m in the same sample that was irradiated. Therefore, we have no t cor-

rected for this ni t rogenous substance. A m m o n i u m and labile am ino -N were measured

by the methods o f RICHARDS and KLETSCH (1964) in frozen samples at stations 30-60

miles away f rom the organic N stations. Our organic N samples were not filtered,

so our values include part iculate N that may have been oxidized to nitrate dur ing

irradiat ion. Part iculate carbon was measured by ROBERT OWEN (unpublished data)

dur ing cruises in this area in the two months immediately preceding and following

the cruises in which organic N was measured. F r o m these a m m o n i u m and part iculate

Table 1. Organic nitrogen concentrations at 10 m in poor (nitrate absent) water in the Eastern Tropical Pacific Ocean. EASTROPAC Expedition, August-September, 1967.

EASTROPAC* Organic N Station Date Latitude L o n g i t u d e (#g-at./l.)

R.V. Thomas Washington: 45018

023 032 046 051 058 315 325 346 356 369 387

R.V. Undaunted: 46004

009 017 024 032 10O 108 116 132 145

R.V. Rockaway: 470O5

065 297 490 523

August

September

7 19 ° 15"5'N 118 ° 56"0'W 13.8 8 17 ° 41.0'N 119 ° 02"0'W 9"8 9 15 ° 10"5'N 119 ° 04"0'W 7.9

11 10 ° 51.5'N 119 ° 02"0'W 8.5 12 9 ° 10-0'N 118 ° 59"5'W 7.1 13 7 ° 25-0'N 118 ° 47-0'W 6'7 4 5 ° 35"0"N 111 ° 54'5'W 7"7 5 9 ° 02"0'N 111 ° 55"0'W 6.4 8 14 ° 38.0'N 109 ° 37"0'W 6.2 9 16 ° 53.0'N 107 ° 03.0"W 7.0

13 19 ° 05-5'N 105 ° 09.0'W 8.5 15 19 ° 58.5'N 111 ° 25"0'W 6.5

August 16 15 ° 48.5'N 10O ° 54.0'W 8.8 17 14 ° 50.5'N 102 ° 02.0'W 7-0 18 12 ° 53.0'N 103 ° 56-0'W 7.7 19 10 ° 46.0'N 105 ° 08.0'W 6.7 20 8 ° 07.0'N 105 ° 00-0'W 7.5

September 2 4 ° 48,0'N 98 ° 04.0"W 8.0 3 7 ° 35.0'N 98 ° 00.0'W 7-5 4 10 ° 25.0'N 98 ° 01.0'W 8.5 6 150 38.0'N 98 ° 01.0'W 7.5

16 8 ° 55.0'N 92 ° 02.0'W 8.2

August 2 6 ° 05.5'N 78 ° 21.5'W 8-3 6 1 ° 46.0'N 81 ° 58.0'W 8.4

September 4 4 ° 38.0'N 87 ° 57.5'W 8.6 19 3 ° 16.5'N 94 ° 41.0'W 10.9 22 13 ° 16.0'N 95 ° 0O.5'W 12.1

*Kesearch vessels from which samples were taken are indicated. The first two digits of the station number, i.e. "45, 46, and 4 7 ' , refer to the cruise numbers of the EASTROPAC cruise. The last three digits refer to the serially numbered station of the cruise.

Near-surface organic nitrogen in the Eastern Tropical Pacific Ocean 67

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68 WILLIAM H. THOMAS, EDWARD H. RENGER and ANNE N. ])ODSON

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Organic N, nitrate, and ammonium and labile amino-N at 10m along 119°W longitude in the eastern tropical Pacific Ocean.

C values it was possible to estimate the magnitude of corrections for ammonium and particulate N. These corrections are minor and are discussed below.

Organic N concentrations ranged from 3.4 to 13.8/~g-at./1. over the whoLe area (Tables 1 and 2). The mean concentration was 7.3/~g-at./1. The mean value in poor water where nitrate was not detected at 10 m ( < 0.1/~g-at./l. of NO3--N) was 8.2 ± 3.4/~g-at./1., the latter value being the 95 ~o limits; in rich water which contained up to 20.7/zg-at./l. of NOz--N, the mean value for organic N was 6.9 ± 3.4/zg-at./1. A ' t ' test showed that organic N concentrations in rich water differed statistically from those in poor water, although the difference was not great. The fact that organic N was lower in rich water was surprising. Nitrate-rich water was more productive of phytoplankton and contained more chlorophyll than nitrate-poor water (OWEN and ZEITZSCI-mL, unpublished data) and if organic N is derived from plant metabolic processes, one would expect to find more, not less, organic N in rich water than in poor water. The difference might be attributed to the fact that nitrate-containing (rich) water has been recently upwelled and this upweUed water from just below the surface is low in organic nitrogen. FRAGA (1966), WILLIAMS (1967) and HOLM-HANSEN, STmCKLAND and WILLIAMS (1966) show a slight reduction in organic N some tens of meters below the surface as compared with surface water. We have no vertical sections of organic N in the eastern tropical Pacific so this explanation remains conjucture.


The difference between poor and rich water is delineated in Fig. 1 which is a plot of nitrogenous fractions at 10 m along the 119°W meridian. Ammonium and labile amino-N remained nearly constant, at ~ 0"5 #g-at./1., with latitude. The difference between rich and poor water is defined by the nitrate plot; nitrate is at zero concentration from 20°N to about 7°N and increases to > 10 ~g-at./1. at 5°S. Organic N is slightly higher in poor water than in rich water, a result that agrees with the overall statistical findings.

Our mean value of 7.3 #g-at./1. is similar to concentrations reported below the surface yet in the euphotic zone by HOLM-HANSEN, STRICKLAND and WILLIAMS (1966) or WILLIAMS (1967) either off southern California or offPeru. Organic N was measured by them in the same manner (by u.v. irradiation) but their samples were filtered while ours were not. Our values were also similar to those reported by FRAGA (1966) for dissolved organic N in the western Indian Ocean, which were measured by a different method. In the north Atlantic, values ranging from 2.8 to 28/~g-at./1. were found by DUURSMA (1961). ,

The mean value for particulate carbon in 10 m samples collected on two other EASTROPAC cruises was 51-5/,g/q. (OWEN, unpublished data). Using a mean C /N ratio of 5 (by atoms) for particulate matter, as suggested by FRAGA (1966) for particulate matter in the Indian Ocean (his Table 4 data for near surface measurements), one can convert the mean particulate C to particulate N. The mean value for particulate N is thus about 0.8/~g-at./1. or about 10~o of our mean value for organic N obtained by irradiation of unfiltered samples. This value might be even less if a C/N ratio of 10:11 was used such as is characteristic of N-deficient phytoplankton. Most of the organic N that we measured was dissolved. The mean ammonium and amino-N concentration at nearby stations was 0-66/,g-at./1., so any correction for these fractions would be small. Such a correction was not applied to our data. Most of the organic N must be dissolved and consist of relatively high molecular weight compounds.

In poor water where nitrate is absent and ammonium and amino-N are present in concentrations generally less than 1.0/~g-at./1., the amount of organic N present is large relative to these other sources (approximately 8/~g-at./1.--see Fig. 1). There- fore, the organic N could be a significant source of nitrogen for phytoplankton if it were utilized. Present evidence, as follows, indicated that little of this nitrogen is utilized:

(1) When natural populations from poor water in the eastern tropical Pacific were not enriched with either nitrate or ammonium, little or no chloropl'ffll was formed upon prolonged incubation in natural light (THOMAS, 1969, 1970b).

(2) In enrichment experiments, the yield of chlorophyll formed was linearly related to the amount of nitrate or ammonium added or present initially, and the yield curve extrapolated to about 0.6/,g-at./1. of inorganic N (THOMAS, 1970b). The extrapolation indicates that some of the measured ammonium and amino-N was not utilized, and surely little of the approximately 8/~g-at./1. of organic N present was used.

(3) If this material were available to phytoplankton, it would not be present in such high concentrations relative to nitrate and ammonium concentrations.

HARVEY (1957) showed that organic N was not utilized by Phaeodactylum tri- cornutum or Chlorella stigmatophora. Growth was proportional to the inorganic N supplied. HARVEY speculated that the water must contain organic N, but made no

70 WILUAM H. THOMAS, EDWARD H. RENOER and ANNE N. DODSON

measurements. Recently, STRICKLAND, HOLM-HANSEN, EPPLEY and LINN (1969) measured changes in nitrogen fractions in sea water in which three phytoplankton species were cultured. Inorganic N fractions decreased, and organic N decreased slightly and then increased as the cultures grew; organic N was certainly utilized only to a limited extent.

FRAGA and VIVES (1961) suggested that a fraction of the dissolved organic N in the Bay of Vigo, Spain, entered into biochemical cycles and was related to the phytoplankton production; a larger fraction was inert and was not affected by biological processes. In the poor water of the tropical Pacific Ocean, most of the dissolved organic N must be part of this inert fraction since the evidence suggests that little of it is utilized by phytoplankton. The organic N must be quite refractory and at present we can only speculate on its nature--perhaps the nitrogen is in the heterocyclic form, and consists mainly of compounds of high molecular weight.

Acknowledgements--This investigation is part of the Scripps Tuna Oceanography Research Program and is a result of the EASTROPAC Expedition, a cooperative survey of the physical, chemical, and biological oceanography of the eastern tropical Pacific Ocean. The work was supported in part by National Science Foundation Grants GB-5541 and GB-8618 and in part by Contract Nos. 14-17- 0007-742 and 14-17-0007-963 between the U.S. Bureau of Commercial Fisheries and the Institute of Marine Resources. We are grateful to Mrs. STEPI-IANm WILLIAMS for data calculations and analysis, to the U.S. Coast Guard for collecting samples during an EASTROPAC cruise aboard the R.V. Rockaway, and to Mr. ROBERT OWEN of the U.S. Bureau of Commercial Fisheries for providing data on chlorophyll and particulate carbon in these waters.

REFERENCES

ARMSTRONG F. A. J., P. M. WILLIAMS and J. D. H. STRICKLAND (1966) Photooxidation of organic matter in sea water by ultraviolet radiation, analytical and other applications. Nature, Lond., 211,481--483.

ARMSTRONQ F. A. J., C. R. STEARNS and J. D. H. STRICKLAND (1967) The measurement of upwelling and subsequent biological processes by means of the Technicon Autoanalyzer and associated equipment. Deep-Sea Res., 14, 381-389.

DUURSMA E. K. (1961) Dissolved organic carbon, nitrogen, and phosphorus in the sea. Netherlands d. Sea Res., 1, 1-148.

FRAGA F. (1966) Distribution of particulate and dissolved nitrogen in the western Indian Ocean. Deep-Sea Res., 13, 413-425.

FRAGA F. and F. VIVES (1961) La descomposici6n de la materia organica nitrogenada en la mar. Inv. Pesq., 19, 65-80.

HARVEY H. W. (1957) Bio-assay of nitrogen available to two species of phytoplankton in an off-shore water, d. mar. biol. Ass. U.K., 36, 157-160.

HOLM-HANSEN O., J. D. H. STRICKLAND and P. M. WILLIAMS (1966) A detailed analysis of biologically important substances in a profile off southern California. Limnol. Oeeanogr., 11, 548-561.

RICHAROS F. A. and R. A. KLETSCH (1964) The spectrophotometric determination of ammonia and labile amino compounds in fresh and sea water by oxidation to nitrite. In: Recent Researches in the Fields of Hydrosphere, Atmosphere, and Nuclear Geochemistry, editors Y. MIYAKE and T. KOYAMA, Maruzen, Tokyo, 65-81.

STalCKLAND J. D. H. and T. R. PA~ONS (1968) A practical handbook of sea-water analysis. Bull. Fish. Res. Bd. Can., 167, 1-311.

STRICKLAND J. D. H., O. HOLM-HANsEN, R. W. EPPLEY and R. J. Ln'~N (1969) The use of a deep tank in plankton ecology--I. Studies of the growth and composition of phytoplankton at low nutrient levels. Limnol. Oceanogr., 14, 23-34.

THOMAS W. H. (1966) Surface nitrogenous nutrients and phytoplankton in the northeastern tropical Pacific Ocean. Limnol. Oceanogr., 11, 393--400.

THOMAS W. H. (1969) Phytoplankton nutrient enrichment experiments off Baja California and in the eastern equatorial Pacific Ocean. J. Fish. Res. Bd. Can., 26, 1133-1145.


THOMAS W. H. (1970a) On nitrogen deficiency in tropical oceanic phytoplankton: Photo- synthetic parameters in rich and poor Pacific water. Limnol. Oceanogr. (in press).

THOMAS W. H. (1970b) Effect of ammonium and nitrate concentrations on chlorophyll increases in natural tropical Pacific phytoplankton populations. Limnol. Oceanogr. (in press).

WILLIAMS P. M. (1967) Sea surface chemistry: organic carbon and organic and inorganic nitrogen and phosphorus in surface films and subsurface waters. Deep-Sea Res., 14, 791-800.

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Near-surface organic nitrogen in the Eastern Tropical Pacific Ocean