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Total dissolved nitrogen and phosphorus concentrations at US-JGOFS station ALOHA: Redfield reconciliation
Marine Chemistry, 41 (1993) 203-208 Elsevier Science Publishers B.V., Amsterdam
203
Total dissolved nitrogen and phosphorus concentrations at US-JGOFS Station ALOHA: Redfield reconciliation David M. Karl a'b, Georgia Tienb, John Dore" and Christopher D. Winna.b "Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA bDivision of Biological Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA (Received 8 July 1991; revision accepted 5 December 1991)
ABSTRACT Karl, D.M., Tien, G., Dore, J. and Winn, C.D., 1993. Total dissolved nitrogen and phosphorus concentrations at US-JGOFS Station ALOHA: Redfield reconciliation. Mar. Chem., 41: 203-208. Twenty-two water column profiles (0-4500 m) of total dissolved nitrogen (TDN) and phosphorus (TDP) concentrations have been collected over a 2-year period at the US-JGOFS time-series Station ALOHA. These data indicate that the N : P molar ratio (mean, 16.74; standard deviation, 2.95; n = 417) in the central Pacific Ocean is not significantly different from that predicted a priori based upon current models of the bioelemental composition of plankton. An examination of the literature and new data from our laboratory using direct high-temperature combustion methods for both TDP and TDN support the validity of historical oceanic analyses. Despite the substantially higher DOC values in oligotrophic Pacific Ocean surface waters, TDN (and consequently dissolved organic nitrogen [DON]) and TDP (and consequently dissolved organic phosphorus [DOP]) do not appear to have been underestimated.
INTRODUCTION
In 1934, A.C. Redfield began to investigate the quantitative relationships among the elemental composition of plankton, the distributions of dissolved organic and inorganic nutrients in surface waters and the rates of carbon (C), nitrogen (N) and phosphorus (P) cycling in the marine environment (Redfield, 1934, 1958). These pioneering efforts culminated in the now classic description of the 'Redfield-ratio' (Redfield et al., 1963) of particulate organic matter in natural waters. For the past 30 years, this fundamental relationship among the major bioelements has provided a theoretical framework for subsequent studies of organic matter production,
Correspondence to: D.M. Karl, University of Hawaii, SOEST[ Biological Oceanography Division, I000 Pope Road, Honolulu, HI 96822, USA.
0304-4203/93/$06.00
early diagenesis and nutrient regeneration rates in the marine environment. The Redfield ratio 'predicts' a mean compositional molar ratio of approximately 106:16 : 1, for C : N : P, respectively. Although deviations from the expected Redfield-ratio are known to occur in phytoplankton when grown under extreme or unusual conditions (e.g. Sakshaug and Holm-Hansen, 1977), the N : P ratio has been shown to be remarkably conservative in oxygenated marine environments. As a result, the Redfield-ratio concept has been invaluable in diverse studies of biological oceanography and marine chemistry, especially in studies of organic matter production and diagenesis. In recent years, however, the accuracy of historical measurements of dissolved organic C (DOC) and, more relevant to this study, dissolved organic N (DON) have been called into question (Suzuki et al., 1985; Sugimura and
© 1993 Elsevier Science Publishers B.V. All rights reserved
204
Suzuki, 1988). It has been suggested that there exists a substantial pool of previously undetected organic matter, especially in surface seawaters. The total dissolved N (TDN) pool in the Pacific Ocean, for example, is reported to be more than three times greater than had been previously determined by standard methods of quantitative analysis (Suzuki et al., 1985). Because of the implications of these new discoveries, especially to the general validity of our existing models of organic matter production and diagenesis (Williams and Druffel, 1988), these new claims need to be carefully evaluated by independent investigators. To date, independent confirmation of these elevated TDN values in surface waters of the ocean has not been achieved (see Walsh, 1989). If TDN concentrations in surface seawaters are actually 3-4 times greater than previously suspected, then one might also hypothesize the existence of a corresponding elevated pool of total dissolved phosphorus (TDP) in order to maintain an approximate 16 N : 1 P molar ratio, sensu Redfield. This would be predicted because the upwelled T D N : T D P ratio is known to be 16 : 1 and because neither the incorporation into particulate matter nor particle export ratios appear to deviate significantly from Redfield relationships (e.g. Martin et al., 1987; ChisweU et al., 1990). Consequently, one would predict a TDP pool of > 2.5 pM for the surface waters at 20°N, 137°E, a value much greater than currently measured. In this study we present the results of our initial attempts to detect this 'missing' pool of dissolved P by the application of a novel high-temperature combustion procedure. MATERIALS AND METHODS
Seawater samples (0-4500 m) were collected on approximately monthly intervals at the USJoint Global Ocean Flux Study (US-JGOFS) Hawaii Ocean Time-series (HOT) Station ALOHA (22°45'N, 158°W). Environmental conditions at this site are characteristic of the oligotrophic north Pacific Ocean gyre (Chiswell et al.,
D.M. KARL ET AL.
1990), with low concentrations of inorganic nutrients in the surface waters (e.g. [ N O 3 - ] 25 nM), relatively deep nutricline depths (> 125 m) and low rates of export production (approximately 10-15gCm -2 year-l). Discrete water samples were collected using 12-I polyvinylchloride (PVC) bottles attached to an aluminum-framed rosette/CTD (SeaBird SBE-09 (Seabird Electronics, Bellevue, WA, USA)) package. Subsamples for nutrient determinations were collected directly from the PVC bottles into acid-washed, sample-rinsed 125ml polyethylene bottles. In preliminary investigations, we found no differences between filtered and unfiltered samples. This result is entirely consistent with the measured low concentrations of particulate N and P in the upper water column and with the low biomass of microorganisms. Thus, our total dissolved nutrient concentration results are the sum of dissolved and particulate pools, but because the latter comprise ~< 10% of the total, even in surface waters, we will refer to these pools as 'dissolved'. Routine measurements of TDN were obtained by UV-photooxidation (24h, 1200W lamp at 84 _+ 6°C) of the seawater sample (Armstrong et al., 1966), followed by autoanalysis of the hydrolysis products, [NO3- + NO2-] and ammonium (NH4+) using standard procedures (Strickland and Parsons, 1972). As an independent estimate of TDN, we also employed the high-temperature combustion and chemiluminescence detection method (Walsh, 1989), using a commercially available instrument (ANTEK model # 720). This method utilizes flash combustion of seawater at 1100°C under conditions of ultrapure O 2 saturation. Under these conditions, all nitrogen compounds are oxidized to NO. The NO reacts with ozone to form metastable NO2 and is finally measured by chemiluminescence. Routine measurements of TDP were also obtained by UV-photooxidation (2h, 1200W lamp at 84 + 6°C) of the seawater sample (Armstrong et al., 1966) followed by autoanalysis of the hydrolysis products using standard pro-
205
NITROGEN AND PHOSPHORUS CONCENTRATIONS
cedures (Strickland and Parsons, 1972). Certain P-containing compounds (e.g. inorganic polyphosphates) are not completely hydrolyzed under these reaction conditions. However, they generally are not a significant portion of TDP in seawater. As an independent estimate of TDP, we also employed the new magnesium-induced co-precipitation (MAGIC) method (Karl and Tien, 1992) for TDP determination. In this procedure, P-containing compounds are quantitatively removed from solution by in vitro brucite (Mg(OH)2) formation. One major advantage of MAGIC is that TDP compounds are transformed into a particulate phase which allows subsequent high-temperature dry ashing and oxidation of the hypothesized refractory P compounds. The particulate materials are collected by low-speed centrifugation and treated, sequentially, with hot 8 M HC1 followed by evaporation in vacuo, dry ashing (450°C, 3 h) and acid persulfate oxidation prior to soluble reactive P determinations (Strickland and Parsons, 1972).
TDN, pmol kg 1 10
0
20
30
40
0
50
'
1000 O0
I0
2O
3O
,~
SO
.
2000 able
E
ml
d
• •
mma •
o
"'I',' 3000
=~jhqm
4000
5000
RESULTS AND DISCUSSION
UV-photooxidation TDN and TDP profiles at Station A L O H A
An analysis of over 20 individual water column profiles of TDN (Fig. 1) and TDP (Fig. 2) collected at Station ALOHA and measured by UV-photooxidation during the period October 1988 to December 1990 indicates that our results are consistent with historical data from the central North Pacific gyre (e.g. Gundersen et al., 1976; Williams et al., 1980), with minimum surface water TDN and TDP concentrations of 5-7#molkg -~ and 0.2-0.4/~molkg -1 and middepth (02 minimum zone, 800-900 m) TDN and TDP maxima of 44-47 #mol kg-1 and 3.13.3 #mol kg-1, respectively. In surface waters (~< 100 m), DON and DOP dominate the TDN and TDP pools. In the case of nitrogen, DON consistently comprised > 90% of the TDN pool concentration. At greater depths, the inorganic fractions of the TDN and TDP pools become
Fig. 1. TDN concentration versus depth profiles for samples collected at Station ALOHA (22°45'N, 158°W) on 22 cruises during the period October 1988 to September 1990. Insert presents an expanded scale presentation for data in the upper 1000m of the water column.
increasingly more important due to nutrient regeneration processes. At 500 m, > 90% of the TDN and TDP can be attributed to the inorganic constituent pools (e.g. [ N O 3 - + NO2-] and [HPO42-], respectively), and at the nutrient maximum depth ( ~ 900m), approximately 95% of the total dissolved nutrient pools is inorganic. UV-photooxidation TDN." TDP relationships at Station A L O H A
The 417 individual sample analyses which comprise the data set presented in Figs. 1 and 2 were analyzed in order to evaluate the quantitative relationships between TDN and TDP pools. Consistent with expectations, this analysis revealed a highly significant linear relationship between TDN and TDP concentrations for the
206
D.M. KARL ET AL.
TDP, pmol kg "~ 0
0.5
1
1.5
2
2.5
0
50
3.5
3
4O
I
t,bt, P - l,,mmml~,, 1000
~2oi O E
Za I--0.8
1
1.5
2
•5
3
;
[
2o
3.8
10
2000 0
E
II"
w:mm .% • ,Wop
3000
I P 10001
1'
1.'5
2'
215
i 3
3.5
TOP, pmol kg"*
Ih I •
4000
0.5'
|
5000
Fig. 2. TDP concentration versus depth profiles for samples collected at Station A L O H A (22°45'N, 158°W) as in Fig. 1.
entire water column (Fig. 3). The arithmetic mean (and standard deviation) T D N : TDP ratio of 16.74 ___ 2.95 (n = 417), is remarkably consistent with the N : P ratio of 16 predicted from Redfield stoichiometry. Although there is more scatter in the surface water values, a statistical analysis of T D N : T D P ratios for the 0-200 m depth interval ( T D N : T D P = 18.56 [-I-3.26]; n = 196) was not significantly different (at the 99% confidence interval) from the total data base. We believe these ratios are accurate, especially below 200 m where the total dissolved nutrient pools are dominated by inorganic species. The consistency of the TDN : TDP ratio in the oligotrophic Pacific Ocean is in agreement with historical models of nutrient recycling. If, as Suzuki et al. (1985) report, DON concentrations in surface waters are up to an order of magnitude higher (,-~ 40-50/tM) than currently measured by either wet oxidation or UV-photooxidation
Fig. 3. Regression analysis of TDN vs. TDP for individual water samples collected at Station ALOHA. The linear regression statistics are: T D N (~molkg -t) = 13.867 [+0.874] TDP (/~molkg -z) + 1.413 [+0.874], r z = 0.997, n = 417; values in brackets indicate the standard error of the slope and intercept estimates. The mean T D N : T D P value from this data set is 16.74 + 2.95. The difference between the arithmetic mean value and slope of the regression line is a consequence of the non-zero y-intercept and the fact that the data are clustered near the origin.
methods, then it would be impossible to obtain our results unless there was also an undetectable and 'missing fraction' of TDP which had precisely the same N : P ratio as the readily oxidizable fractions. This seems unlikely.
Direct comparisons using UV-photooxidation and high-temperature combustion Direct comparisons of TDN and TDP concentrations derived from conventional UVphotooxidation methods and the new high temperature combustion techniques (i.e. T D N after Walsh, 1989; TDP, after Karl and Tien, 1992) show that the concentrations of T D N and TDP estimated by the conventional and modern techniques are indistinguishable for surface seawater samples collected from station ALOHA (Table 1). Although the data base on HTC-TDN is admittedly limited, it is important to emphasize that our results are consistent with the more extensive data base collected during the VERTEX program (Walsh, 1989) and with the Station ALOHA DON intercomparison results derived
NITROGEN AND PHOSPHORUS CONCENTRATIONS
207
TABLE 1 Surface mixed-layer depth T D N and TDP concentrations measured during three Spring 1991 cruises to Station ALOHA. TDP was measured, independently, by M A G I C and UV-photooxidation. TDN was measured, independently, by HTC and UV-photooxidation Cruise
Date
Sample depth or intervaP (m)
HTC TDN b
UV-PO TDN c
MAGIC TDP ~
UV-PO TDP ~
HOT-24 DOC/DON cruised HOT-26
5-9 March April
0-62 (7) 10
7.11 (±1.84)
0.230 0.241 (±0.084)
0.240 0.20 e (_+0.01)
6-10 May
0--43 (3)
-
4.25 5.41 e ( ± 0.084) 5.06
0.159
0.292
a number in parentheses indicates number of discrete depths where samples were collected and analyzed. b Our value in this table is indistinguishable from the consensus value T D N concentration derived from the 12 independent laboratories participating in the NSF D O C / D O N intercomparison exercise. c Average concentration for mixed-layer as determined by 'trapezoid-rule' integration of measured T D N or TDP concentrations over the depth intervals indicated. All values are in units of/~mol I-~ . d NSF-sponsored intercomparison cruise organized by J. Hedges, University of Washington. e UV-PO intercomparison data courtesy of Ted Walsh.
from 12 independent laboratories (Hedges et al., 1993). Furthermore, we are fairly confident that no naturally occurring P compounds have escaped hydrolysis (to SRP) by our MAGIC-TDP procedure. Because of its ability to concentrate TDP from seawater, MAGIC allows for much more destructive digestion/hydrolysis conditions than is possible for aqueous samples. The stepwise hydrolysis (8 M HC1), oxidation (dry ashing at 450°C) and final hydrolysis/oxidation (hot persulfate digestion) MAGIC-TDP protocol is employed for maximum recovery of SRP, minimal volatilization losses and minimal interference from arsenic. Although we have not yet attempted these analyses, it is worth noting that because the MAGIC procedure provides an opportunity to concentrate TDP from seawater (up to 100-fold), it is possible that the total P concentrations we report herein can eventually be confirmed by alternate methods of analysis which do not require total P-compound hydrolysis (e.g. emission spectroscopy or neutron activation). SUMMARY
The validity of using direct measurements of TDP as a means of constraining seawater TDN (and DON) pool concentrations relies upon the assumption that: (1) the T D N : T D P ratio is
identical to that hypothesized for particulate matter (Redfield et al., 1963) and (2) the TDP estimates are accurate. Implicit in this logic is the suggestion that measurements of near surface water TDP can be made with greater accuracy than direct measurements of TDN. With regard to the second assumption, it is difficult to envision problems of incomplete oxidation of organic-P if temperatures are kept between 450 and 500°C (Harwood and Hattingh, 1973). At higher temperatures, the formation of pyrophosphates and metaphosphates is possible. However, even if these non-reactive products were formed in samples, the MAGIC postashing persulfate digestion step would quantitatively convert all polyphosphates to SRP. In conclusion, surface seawaters at Station ALOHA appear to have elevated values of DOC relative to historical data (i.e. [DOC] = 100120 #mol kg-~ compared with historical values of 30-40 #mol kg- ~) but DON concentrations that are not substantially different from previous measurements. This conclusion is based upon our data from the Hawaii Ocean Time-series program, and is also the consensus opinion of the participants of the DOC-DON intercomparison workshop. Future research should focus on the molecular nature of this 'extra' DOC and the dynamics of production and removal.
208 ACKNOWLEDGMENTS W e t h a n k the Hawaii O c e a n Time-series ( H O T ) p r o g r a m scientists, especially D a l e Hebel a n d R i c a r d o Letelier, f o r their help in s a m p l e collection a n d d a t a m a n a g e m e n t , T e d W a l s h for his advice o n the A n t e k i n s t r u m e n t a n d f o r allowing us to use his D O C / D O N i n t e r c o m p a r i s o n d a t a in o u r T a b l e 1, a n d Lisa L u m for the p r e p a r a t i o n o f text a n d figures. M o s t o f the d a t a used in this m a n u s c r i p t were o b t a i n e d as p a r t o f the H O T p r o g r a m , o n a c o n t r a c t u a l basis, t h r o u g h the U . H . A n a l y t i c a l Services o r g a n i z a t i o n , T e d Walsh, m a n a g e r a n d analyst. W e appreciate a n d a c k n o w l e d g e his diligence a n d expertise. This research was s u p p o r t e d , in part, by N a t i o n a l Science F o u n d a t i o n g r a n t O C E 8 8 - 0 0 3 2 9 a w a r d e d to D M K a n d C D W . REFERENCES Armstrong, F.A.J., Williams, P.M. and Strickland, J.D.H., 1966. Photo-oxidation of organic matter in seawater by ultraviolet radiation, analytical and other applications. Nature, 211: 481-483. Chiswell, S., Firing, E., Karl, D., Lukas, R. and Winn, C., 1990. Hawaii Ocean Time-series Data Report l, 19881989. School of Ocean and Earth Science and Technology Technical Report # 1, University of Hawaii. Gundersen, K.R., Corbin, J.S., Hanson, C.L., Hanson, M.L., Hanson, R.B., Russell, D.J., Stollar, A. and Yamada, O., 1976. Structure and biological dynamics of the oligotrophic ocean photic zone off the Hawaiian Islands. Pac. Sci., 30: 45-68. Harwood, J.E. and Hattingh, W.H.J., 1973. Colorimetric methods of analysis of phosphorus at low concentrations in water. In: E.J. Grittith, A. Beeton, J.M. Spencer and D.T. Mitchell (Editors), Environmental Phosphorus Handbook. John Wiley, New York, pp. 289-301. Hedges, J., Bergamaschi, B.A. and Benner, R., 1993. Com-
D.M. KARL ET AL.
parative analyses of DOC and DON in natural waters. Mar. Chem., 41: 121-134. Karl, D.M. and Tien, G., 1992. MAGIC: A sensitive and precise method for measuring dissolved phosphorus in aquatic environments. Limnol. Oceanogr., in press. Martin, J.H., Knauer, G.A., Karl, D.M. and Broenkow, W.W., 1987. VERTEX: Carbon cycling in the Northeast Pacific. Deep-Sea Res., 34: 267-285. Redfield, A.C., 1934. On the proportions of organic derivatives in sea water and their relation to the composition of plankton. In: James Johnstone Memorial Volume. University Press of Liverpool, pp. 176-192. Redfield, A.C., 1958. The biological control of chemical factors in the environment. Am. Sci. 46: 205-221. Redfield, A.C., Ketchum, B.H. and Richards, F.A., 1963. The influence of organisms on the composition of seawater. In: M.N. Hill (Editor), The Sea, Ideas and Observations on Progress in the Study of the Seas, Vol. 2, Interscience, NY, pp. 26-77. Sakshaug, E. and Holm-Hansen, O., 1977. Chemical composition of Skeletonema costatum (Grev.) Cleve and Pavlova (Monochrysis) lutheri (Droop) Green as a function of nitrate-, phosphate-, and iron-limited growth. J. Exp. Mar. Biol. Ecol., 29: 1-34. Strickland, J.D.H. and Parsons, T.R., 1972. A Practical Handbook of Seawater Analysis. Fisheries Research Board of Canada, Ottawa, 167: 310. Sugimura, Y. and Suzuki, Y., 1988. A high temperature catalytic oxidation method of non-volatile dissolved organic carbon in seawater by direct injection of liquid samples. Mar. Chem., 24: 105-131. Suzuki, Y., Sugimura, Y. and Itoh, T., 1985. A catalytic oxidation method for the determination of total nitrogen dissolved in seawater. Mar. Chem., 16: 83-97. Walsh, T.W., 1989. Total dissolved nitrogen in seawater: a new-high-temperature combustion method and a comparison with photo-oxidation. Mar. Chem., 26: 295311. Williams, P.M. and Druffel, E.R.M., 1988. Dissolved organic matter in the ocean: Comments on a controversy. Oceanogr. Mag., 1: 14-17. Williams, P.M., Carlucci, A.F. and Olson, R., 1980. A deep profile of some biologically important properties in the central North Pacific gyre. Oceanol. Acta, 3: 471-476.
203
Total dissolved nitrogen and phosphorus concentrations at US-JGOFS Station ALOHA: Redfield reconciliation David M. Karl a'b, Georgia Tienb, John Dore" and Christopher D. Winna.b "Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA bDivision of Biological Oceanography, School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA (Received 8 July 1991; revision accepted 5 December 1991)
ABSTRACT Karl, D.M., Tien, G., Dore, J. and Winn, C.D., 1993. Total dissolved nitrogen and phosphorus concentrations at US-JGOFS Station ALOHA: Redfield reconciliation. Mar. Chem., 41: 203-208. Twenty-two water column profiles (0-4500 m) of total dissolved nitrogen (TDN) and phosphorus (TDP) concentrations have been collected over a 2-year period at the US-JGOFS time-series Station ALOHA. These data indicate that the N : P molar ratio (mean, 16.74; standard deviation, 2.95; n = 417) in the central Pacific Ocean is not significantly different from that predicted a priori based upon current models of the bioelemental composition of plankton. An examination of the literature and new data from our laboratory using direct high-temperature combustion methods for both TDP and TDN support the validity of historical oceanic analyses. Despite the substantially higher DOC values in oligotrophic Pacific Ocean surface waters, TDN (and consequently dissolved organic nitrogen [DON]) and TDP (and consequently dissolved organic phosphorus [DOP]) do not appear to have been underestimated.
INTRODUCTION
In 1934, A.C. Redfield began to investigate the quantitative relationships among the elemental composition of plankton, the distributions of dissolved organic and inorganic nutrients in surface waters and the rates of carbon (C), nitrogen (N) and phosphorus (P) cycling in the marine environment (Redfield, 1934, 1958). These pioneering efforts culminated in the now classic description of the 'Redfield-ratio' (Redfield et al., 1963) of particulate organic matter in natural waters. For the past 30 years, this fundamental relationship among the major bioelements has provided a theoretical framework for subsequent studies of organic matter production,
Correspondence to: D.M. Karl, University of Hawaii, SOEST[ Biological Oceanography Division, I000 Pope Road, Honolulu, HI 96822, USA.
0304-4203/93/$06.00
early diagenesis and nutrient regeneration rates in the marine environment. The Redfield ratio 'predicts' a mean compositional molar ratio of approximately 106:16 : 1, for C : N : P, respectively. Although deviations from the expected Redfield-ratio are known to occur in phytoplankton when grown under extreme or unusual conditions (e.g. Sakshaug and Holm-Hansen, 1977), the N : P ratio has been shown to be remarkably conservative in oxygenated marine environments. As a result, the Redfield-ratio concept has been invaluable in diverse studies of biological oceanography and marine chemistry, especially in studies of organic matter production and diagenesis. In recent years, however, the accuracy of historical measurements of dissolved organic C (DOC) and, more relevant to this study, dissolved organic N (DON) have been called into question (Suzuki et al., 1985; Sugimura and
© 1993 Elsevier Science Publishers B.V. All rights reserved
204
Suzuki, 1988). It has been suggested that there exists a substantial pool of previously undetected organic matter, especially in surface seawaters. The total dissolved N (TDN) pool in the Pacific Ocean, for example, is reported to be more than three times greater than had been previously determined by standard methods of quantitative analysis (Suzuki et al., 1985). Because of the implications of these new discoveries, especially to the general validity of our existing models of organic matter production and diagenesis (Williams and Druffel, 1988), these new claims need to be carefully evaluated by independent investigators. To date, independent confirmation of these elevated TDN values in surface waters of the ocean has not been achieved (see Walsh, 1989). If TDN concentrations in surface seawaters are actually 3-4 times greater than previously suspected, then one might also hypothesize the existence of a corresponding elevated pool of total dissolved phosphorus (TDP) in order to maintain an approximate 16 N : 1 P molar ratio, sensu Redfield. This would be predicted because the upwelled T D N : T D P ratio is known to be 16 : 1 and because neither the incorporation into particulate matter nor particle export ratios appear to deviate significantly from Redfield relationships (e.g. Martin et al., 1987; ChisweU et al., 1990). Consequently, one would predict a TDP pool of > 2.5 pM for the surface waters at 20°N, 137°E, a value much greater than currently measured. In this study we present the results of our initial attempts to detect this 'missing' pool of dissolved P by the application of a novel high-temperature combustion procedure. MATERIALS AND METHODS
Seawater samples (0-4500 m) were collected on approximately monthly intervals at the USJoint Global Ocean Flux Study (US-JGOFS) Hawaii Ocean Time-series (HOT) Station ALOHA (22°45'N, 158°W). Environmental conditions at this site are characteristic of the oligotrophic north Pacific Ocean gyre (Chiswell et al.,
D.M. KARL ET AL.
1990), with low concentrations of inorganic nutrients in the surface waters (e.g. [ N O 3 - ] 25 nM), relatively deep nutricline depths (> 125 m) and low rates of export production (approximately 10-15gCm -2 year-l). Discrete water samples were collected using 12-I polyvinylchloride (PVC) bottles attached to an aluminum-framed rosette/CTD (SeaBird SBE-09 (Seabird Electronics, Bellevue, WA, USA)) package. Subsamples for nutrient determinations were collected directly from the PVC bottles into acid-washed, sample-rinsed 125ml polyethylene bottles. In preliminary investigations, we found no differences between filtered and unfiltered samples. This result is entirely consistent with the measured low concentrations of particulate N and P in the upper water column and with the low biomass of microorganisms. Thus, our total dissolved nutrient concentration results are the sum of dissolved and particulate pools, but because the latter comprise ~< 10% of the total, even in surface waters, we will refer to these pools as 'dissolved'. Routine measurements of TDN were obtained by UV-photooxidation (24h, 1200W lamp at 84 _+ 6°C) of the seawater sample (Armstrong et al., 1966), followed by autoanalysis of the hydrolysis products, [NO3- + NO2-] and ammonium (NH4+) using standard procedures (Strickland and Parsons, 1972). As an independent estimate of TDN, we also employed the high-temperature combustion and chemiluminescence detection method (Walsh, 1989), using a commercially available instrument (ANTEK model # 720). This method utilizes flash combustion of seawater at 1100°C under conditions of ultrapure O 2 saturation. Under these conditions, all nitrogen compounds are oxidized to NO. The NO reacts with ozone to form metastable NO2 and is finally measured by chemiluminescence. Routine measurements of TDP were also obtained by UV-photooxidation (2h, 1200W lamp at 84 + 6°C) of the seawater sample (Armstrong et al., 1966) followed by autoanalysis of the hydrolysis products using standard pro-
205
NITROGEN AND PHOSPHORUS CONCENTRATIONS
cedures (Strickland and Parsons, 1972). Certain P-containing compounds (e.g. inorganic polyphosphates) are not completely hydrolyzed under these reaction conditions. However, they generally are not a significant portion of TDP in seawater. As an independent estimate of TDP, we also employed the new magnesium-induced co-precipitation (MAGIC) method (Karl and Tien, 1992) for TDP determination. In this procedure, P-containing compounds are quantitatively removed from solution by in vitro brucite (Mg(OH)2) formation. One major advantage of MAGIC is that TDP compounds are transformed into a particulate phase which allows subsequent high-temperature dry ashing and oxidation of the hypothesized refractory P compounds. The particulate materials are collected by low-speed centrifugation and treated, sequentially, with hot 8 M HC1 followed by evaporation in vacuo, dry ashing (450°C, 3 h) and acid persulfate oxidation prior to soluble reactive P determinations (Strickland and Parsons, 1972).
TDN, pmol kg 1 10
0
20
30
40
0
50
'
1000 O0
I0
2O
3O
,~
SO
.
2000 able
E
ml
d
• •
mma •
o
"'I',' 3000
=~jhqm
4000
5000
RESULTS AND DISCUSSION
UV-photooxidation TDN and TDP profiles at Station A L O H A
An analysis of over 20 individual water column profiles of TDN (Fig. 1) and TDP (Fig. 2) collected at Station ALOHA and measured by UV-photooxidation during the period October 1988 to December 1990 indicates that our results are consistent with historical data from the central North Pacific gyre (e.g. Gundersen et al., 1976; Williams et al., 1980), with minimum surface water TDN and TDP concentrations of 5-7#molkg -~ and 0.2-0.4/~molkg -1 and middepth (02 minimum zone, 800-900 m) TDN and TDP maxima of 44-47 #mol kg-1 and 3.13.3 #mol kg-1, respectively. In surface waters (~< 100 m), DON and DOP dominate the TDN and TDP pools. In the case of nitrogen, DON consistently comprised > 90% of the TDN pool concentration. At greater depths, the inorganic fractions of the TDN and TDP pools become
Fig. 1. TDN concentration versus depth profiles for samples collected at Station ALOHA (22°45'N, 158°W) on 22 cruises during the period October 1988 to September 1990. Insert presents an expanded scale presentation for data in the upper 1000m of the water column.
increasingly more important due to nutrient regeneration processes. At 500 m, > 90% of the TDN and TDP can be attributed to the inorganic constituent pools (e.g. [ N O 3 - + NO2-] and [HPO42-], respectively), and at the nutrient maximum depth ( ~ 900m), approximately 95% of the total dissolved nutrient pools is inorganic. UV-photooxidation TDN." TDP relationships at Station A L O H A
The 417 individual sample analyses which comprise the data set presented in Figs. 1 and 2 were analyzed in order to evaluate the quantitative relationships between TDN and TDP pools. Consistent with expectations, this analysis revealed a highly significant linear relationship between TDN and TDP concentrations for the
206
D.M. KARL ET AL.
TDP, pmol kg "~ 0
0.5
1
1.5
2
2.5
0
50
3.5
3
4O
I
t,bt, P - l,,mmml~,, 1000
~2oi O E
Za I--0.8
1
1.5
2
•5
3
;
[
2o
3.8
10
2000 0
E
II"
w:mm .% • ,Wop
3000
I P 10001
1'
1.'5
2'
215
i 3
3.5
TOP, pmol kg"*
Ih I •
4000
0.5'
|
5000
Fig. 2. TDP concentration versus depth profiles for samples collected at Station A L O H A (22°45'N, 158°W) as in Fig. 1.
entire water column (Fig. 3). The arithmetic mean (and standard deviation) T D N : TDP ratio of 16.74 ___ 2.95 (n = 417), is remarkably consistent with the N : P ratio of 16 predicted from Redfield stoichiometry. Although there is more scatter in the surface water values, a statistical analysis of T D N : T D P ratios for the 0-200 m depth interval ( T D N : T D P = 18.56 [-I-3.26]; n = 196) was not significantly different (at the 99% confidence interval) from the total data base. We believe these ratios are accurate, especially below 200 m where the total dissolved nutrient pools are dominated by inorganic species. The consistency of the TDN : TDP ratio in the oligotrophic Pacific Ocean is in agreement with historical models of nutrient recycling. If, as Suzuki et al. (1985) report, DON concentrations in surface waters are up to an order of magnitude higher (,-~ 40-50/tM) than currently measured by either wet oxidation or UV-photooxidation
Fig. 3. Regression analysis of TDN vs. TDP for individual water samples collected at Station ALOHA. The linear regression statistics are: T D N (~molkg -t) = 13.867 [+0.874] TDP (/~molkg -z) + 1.413 [+0.874], r z = 0.997, n = 417; values in brackets indicate the standard error of the slope and intercept estimates. The mean T D N : T D P value from this data set is 16.74 + 2.95. The difference between the arithmetic mean value and slope of the regression line is a consequence of the non-zero y-intercept and the fact that the data are clustered near the origin.
methods, then it would be impossible to obtain our results unless there was also an undetectable and 'missing fraction' of TDP which had precisely the same N : P ratio as the readily oxidizable fractions. This seems unlikely.
Direct comparisons using UV-photooxidation and high-temperature combustion Direct comparisons of TDN and TDP concentrations derived from conventional UVphotooxidation methods and the new high temperature combustion techniques (i.e. T D N after Walsh, 1989; TDP, after Karl and Tien, 1992) show that the concentrations of T D N and TDP estimated by the conventional and modern techniques are indistinguishable for surface seawater samples collected from station ALOHA (Table 1). Although the data base on HTC-TDN is admittedly limited, it is important to emphasize that our results are consistent with the more extensive data base collected during the VERTEX program (Walsh, 1989) and with the Station ALOHA DON intercomparison results derived
NITROGEN AND PHOSPHORUS CONCENTRATIONS
207
TABLE 1 Surface mixed-layer depth T D N and TDP concentrations measured during three Spring 1991 cruises to Station ALOHA. TDP was measured, independently, by M A G I C and UV-photooxidation. TDN was measured, independently, by HTC and UV-photooxidation Cruise
Date
Sample depth or intervaP (m)
HTC TDN b
UV-PO TDN c
MAGIC TDP ~
UV-PO TDP ~
HOT-24 DOC/DON cruised HOT-26
5-9 March April
0-62 (7) 10
7.11 (±1.84)
0.230 0.241 (±0.084)
0.240 0.20 e (_+0.01)
6-10 May
0--43 (3)
-
4.25 5.41 e ( ± 0.084) 5.06
0.159
0.292
a number in parentheses indicates number of discrete depths where samples were collected and analyzed. b Our value in this table is indistinguishable from the consensus value T D N concentration derived from the 12 independent laboratories participating in the NSF D O C / D O N intercomparison exercise. c Average concentration for mixed-layer as determined by 'trapezoid-rule' integration of measured T D N or TDP concentrations over the depth intervals indicated. All values are in units of/~mol I-~ . d NSF-sponsored intercomparison cruise organized by J. Hedges, University of Washington. e UV-PO intercomparison data courtesy of Ted Walsh.
from 12 independent laboratories (Hedges et al., 1993). Furthermore, we are fairly confident that no naturally occurring P compounds have escaped hydrolysis (to SRP) by our MAGIC-TDP procedure. Because of its ability to concentrate TDP from seawater, MAGIC allows for much more destructive digestion/hydrolysis conditions than is possible for aqueous samples. The stepwise hydrolysis (8 M HC1), oxidation (dry ashing at 450°C) and final hydrolysis/oxidation (hot persulfate digestion) MAGIC-TDP protocol is employed for maximum recovery of SRP, minimal volatilization losses and minimal interference from arsenic. Although we have not yet attempted these analyses, it is worth noting that because the MAGIC procedure provides an opportunity to concentrate TDP from seawater (up to 100-fold), it is possible that the total P concentrations we report herein can eventually be confirmed by alternate methods of analysis which do not require total P-compound hydrolysis (e.g. emission spectroscopy or neutron activation). SUMMARY
The validity of using direct measurements of TDP as a means of constraining seawater TDN (and DON) pool concentrations relies upon the assumption that: (1) the T D N : T D P ratio is
identical to that hypothesized for particulate matter (Redfield et al., 1963) and (2) the TDP estimates are accurate. Implicit in this logic is the suggestion that measurements of near surface water TDP can be made with greater accuracy than direct measurements of TDN. With regard to the second assumption, it is difficult to envision problems of incomplete oxidation of organic-P if temperatures are kept between 450 and 500°C (Harwood and Hattingh, 1973). At higher temperatures, the formation of pyrophosphates and metaphosphates is possible. However, even if these non-reactive products were formed in samples, the MAGIC postashing persulfate digestion step would quantitatively convert all polyphosphates to SRP. In conclusion, surface seawaters at Station ALOHA appear to have elevated values of DOC relative to historical data (i.e. [DOC] = 100120 #mol kg-~ compared with historical values of 30-40 #mol kg- ~) but DON concentrations that are not substantially different from previous measurements. This conclusion is based upon our data from the Hawaii Ocean Time-series program, and is also the consensus opinion of the participants of the DOC-DON intercomparison workshop. Future research should focus on the molecular nature of this 'extra' DOC and the dynamics of production and removal.
208 ACKNOWLEDGMENTS W e t h a n k the Hawaii O c e a n Time-series ( H O T ) p r o g r a m scientists, especially D a l e Hebel a n d R i c a r d o Letelier, f o r their help in s a m p l e collection a n d d a t a m a n a g e m e n t , T e d W a l s h for his advice o n the A n t e k i n s t r u m e n t a n d f o r allowing us to use his D O C / D O N i n t e r c o m p a r i s o n d a t a in o u r T a b l e 1, a n d Lisa L u m for the p r e p a r a t i o n o f text a n d figures. M o s t o f the d a t a used in this m a n u s c r i p t were o b t a i n e d as p a r t o f the H O T p r o g r a m , o n a c o n t r a c t u a l basis, t h r o u g h the U . H . A n a l y t i c a l Services o r g a n i z a t i o n , T e d Walsh, m a n a g e r a n d analyst. W e appreciate a n d a c k n o w l e d g e his diligence a n d expertise. This research was s u p p o r t e d , in part, by N a t i o n a l Science F o u n d a t i o n g r a n t O C E 8 8 - 0 0 3 2 9 a w a r d e d to D M K a n d C D W . REFERENCES Armstrong, F.A.J., Williams, P.M. and Strickland, J.D.H., 1966. Photo-oxidation of organic matter in seawater by ultraviolet radiation, analytical and other applications. Nature, 211: 481-483. Chiswell, S., Firing, E., Karl, D., Lukas, R. and Winn, C., 1990. Hawaii Ocean Time-series Data Report l, 19881989. School of Ocean and Earth Science and Technology Technical Report # 1, University of Hawaii. Gundersen, K.R., Corbin, J.S., Hanson, C.L., Hanson, M.L., Hanson, R.B., Russell, D.J., Stollar, A. and Yamada, O., 1976. Structure and biological dynamics of the oligotrophic ocean photic zone off the Hawaiian Islands. Pac. Sci., 30: 45-68. Harwood, J.E. and Hattingh, W.H.J., 1973. Colorimetric methods of analysis of phosphorus at low concentrations in water. In: E.J. Grittith, A. Beeton, J.M. Spencer and D.T. Mitchell (Editors), Environmental Phosphorus Handbook. John Wiley, New York, pp. 289-301. Hedges, J., Bergamaschi, B.A. and Benner, R., 1993. Com-
D.M. KARL ET AL.
parative analyses of DOC and DON in natural waters. Mar. Chem., 41: 121-134. Karl, D.M. and Tien, G., 1992. MAGIC: A sensitive and precise method for measuring dissolved phosphorus in aquatic environments. Limnol. Oceanogr., in press. Martin, J.H., Knauer, G.A., Karl, D.M. and Broenkow, W.W., 1987. VERTEX: Carbon cycling in the Northeast Pacific. Deep-Sea Res., 34: 267-285. Redfield, A.C., 1934. On the proportions of organic derivatives in sea water and their relation to the composition of plankton. In: James Johnstone Memorial Volume. University Press of Liverpool, pp. 176-192. Redfield, A.C., 1958. The biological control of chemical factors in the environment. Am. Sci. 46: 205-221. Redfield, A.C., Ketchum, B.H. and Richards, F.A., 1963. The influence of organisms on the composition of seawater. In: M.N. Hill (Editor), The Sea, Ideas and Observations on Progress in the Study of the Seas, Vol. 2, Interscience, NY, pp. 26-77. Sakshaug, E. and Holm-Hansen, O., 1977. Chemical composition of Skeletonema costatum (Grev.) Cleve and Pavlova (Monochrysis) lutheri (Droop) Green as a function of nitrate-, phosphate-, and iron-limited growth. J. Exp. Mar. Biol. Ecol., 29: 1-34. Strickland, J.D.H. and Parsons, T.R., 1972. A Practical Handbook of Seawater Analysis. Fisheries Research Board of Canada, Ottawa, 167: 310. Sugimura, Y. and Suzuki, Y., 1988. A high temperature catalytic oxidation method of non-volatile dissolved organic carbon in seawater by direct injection of liquid samples. Mar. Chem., 24: 105-131. Suzuki, Y., Sugimura, Y. and Itoh, T., 1985. A catalytic oxidation method for the determination of total nitrogen dissolved in seawater. Mar. Chem., 16: 83-97. Walsh, T.W., 1989. Total dissolved nitrogen in seawater: a new-high-temperature combustion method and a comparison with photo-oxidation. Mar. Chem., 26: 295311. Williams, P.M. and Druffel, E.R.M., 1988. Dissolved organic matter in the ocean: Comments on a controversy. Oceanogr. Mag., 1: 14-17. Williams, P.M., Carlucci, A.F. and Olson, R., 1980. A deep profile of some biologically important properties in the central North Pacific gyre. Oceanol. Acta, 3: 471-476.