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Nitrate supply and phytoplankton uptake kinetics in the euphotic layer of a Gulf Stream warm-core ring
019S...{)H9192 $5.00 + 0.00 Press pic
Deep-Sea Research, Vol. 39, Suppl. I. pp, S39J-S403. 1992. Printedin Great Britain.
© 1992 Pergamon
Nitrate supply and phytoplankton uptake kinetics in the euphotic layer of a Gulf Stream warm-core ring JAMES J. MCCARTHY, * CHRIS GARSIDE! and JOHN L. NEVINS* (Received 14 April 1989; ill revised [orm IS October 1990; accepted 23 October 1990) Abstract-Chemiluminescenl nitrate analysis was used in conjuctlon with 15N-labeled NOJ to assess the rates of NOi uptake by phytoplankton in warm-core ring 828. The relatively high precision of this method compared to conventional NOi analyses permits reliable estimates of NOi uptake in oligotrophic waters. Aggregation of uptake data from six profiles from 2 days of observation permitted the calculation of NO]' turnover times ranging from about 4 h near the surface to ISO h at the top of the nitracline. Turnover times in the euphotic zone and the observed half saturation constant of93 nmol kg- 1 for NOi uptake imply nitrogen limitation for these populations. Extrapolation from the linear portion of the kinetic curve revealed that a NOi threshold concentration of about 16 nmol kg-I was required for the initiation of uptake. These highly precise uptake measurements were used in a one-dimensional model to estimate the vertical flux of NOi. Maximum near-surface and deep-euphotic-zone eddy diffusivity values (K z ) were 3 x 10- 3 and 5 X 10- 4 m2 S-I , respectively, prior to a major storm. Following the storm K z values were substantially greater.
INTRODUCTION
INTHE open ocean the rate of primary production is usually assumed to be nutrient limited. Inorganic-combined nitrogen, phosphorous and silicate may all be severely depleted in the euphotic zone of oceanic regions, but most recent efforts to understand the regulation of oceanic production have focused on nitrogen (MCCARTHY and CARPENTER, 1983). The upward transport of NO.3 is the dominant process by which nitrogenous nutrient enters the wind-mixed layer, and the primary production arising from this process has been termed "new" production (DUGDALE and GOERING, 1967; EpPLEY and PETERSON, 1979). There is considerable uncertainty regarding the rates of various processes responsible for this flux and the manner in which they are regulated. The supply of NO.3 to the mixed layer occurs on scales that range from a more or less continuous molecular flux along the NO.3 gradient in the seasonal pycnocline to larger episodic fluxes associated with destabilization of the mixed layer, as in the case of upwelling or storm-induced deepening of the mixed layer. There has been a tendency to treat the more or less continuous molecular flux as a one-dimensional problem (d. PLAIT, 1984). This approach, which is similar to that of K-theory models that address the vertical • Harvard University, Museum of Comparative Zoology, 26 Oxford Street, Cambridge, MA 02138, U.S.A. t Bigelow Laboratory for Ocean Sciences, McKown Point, West Boothbay Harbor, ME Q.l575, U.S.A. S393
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flux of heat, assumes that transport of nutrients through the seasonal pycnocline to the mixed ldyer is analogous to molecular diffusion. However, some recent mixed-layer models that yield realistic results for the upper mixed layer fix K equal to zero in the thermocline, and thus ignore vertical diffusive processes in the thermocline altogether (MELLOR and DURBIN, 1975). With this latter approach physical processes (e.g. breaking internal waves) disrupt the thermocline and are responsible for the exchange of scalar properties. These episodic physical events are difficult to sample adequately for the purpose of estimating the upward vertical flux of nutrients. Biologists interested in the rates of NO)" consumption by plankton in the mixed layer have used the rates of NO)" uptake as determined by IsN-labeling experiments to estimate the upward diffusive flux of NO)" (KING and DEVOL, 1979; EpPLEY et al., 1979). One fundamental limitation of this approach is the resolution that can be achieved in determining NO)" concentrations in the lower portion of the mixed layer. In addition, although incubation experiments ideally would be executed with only a trace addition of IsNO)", this has not been practical for studies in oligotrophic waters, where the NO)" concentrations are below the limits of detection (typically 50-100 nmol kg -1 for automated nutrient analyses; Fox et al., 1984). In this situation the magnitude of the resultant IsN-enrichment of the substrate remains unknown within certain limits. For example, if a labeled addition is arbitrarily set as the equivalent of the limit of detection, the observed rate of uptake determined can only be known to within a factor of two, since the atom per cent (At%) enrichment of the labeled nutrient pool lies between 50 and 100. The precision of conventional mass spectrometry is typically under-utilized in these applications, thus it is the inability to quantify NO)" concentrations in the less than 50 nmol kg- 1 range that has limited the precision of estimates of NO)" uptake in oligotrophic waters. With the development of a chemiluminescent assay for NO)" in sea water that has a limit of detection of 2 nmol kg- 1 (GARSIDE, 1982), the opportunity now exists to study NO)" uptake in the mixed layer of oligotrophic waters with substantially increased precision. Experiments with IsN-labeled substrates were run as part of the Warm-Core Ring Program, and in the June 1982 study of ring 82B, the application of this high sensitivity NO)" method permitted exceptionally precise determination of NO)" uptake. The NO)" concentration data have been presented (GARSIDE, 1985), and this paper addresses the results of the uptake experiments. The physics, chemistry and biology of Gulf Stream warm-core ring 82B have been studied in considerable detail (cf. EVANS et al., 1985; SCHMITI and OLSON, 1985; WIEBE and McDoUGALL, 1986). This ring was formed in February 1982, and subsequently underwent a period of deep convective mixing that resulted in a 15.7°C thermostad extending from the surface to a depth of about 400 m (SCHMITI and OLSON, 1985). As a result of this mixing, NO)" concentrations in the thermostad increased to 5.5-6 Jimol kg- 1 (Fox et al., 1984). Between early May and mid-June, a strong pycnocline developed in the upper 30 m. At the time of our June study, near-surface NO)" concentrations were below the limits of detection for conventional automated nutrient analyses (Fox et al., 1984), but they significantly exceeded the detection limit of the chemiluminescent method. METHODS
The experiments described in this paper were conducted aboard the R. V. Knorr, cruise no. 095. Six samplings were made over two 24-h periods in ring 82B. Water was collected
Phytoplankton uptake in euphotic layer of a \vCR
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from six depths within the euphotic zone using a rosette sampler equipped with 30-1 Go-Flo bottles and it CTD. Samples were drawn for nutrient analyses and experiments were initiated to determine rates of NO] uptake using 15N-Iabeled NO]. Incubations of about 4-h duration were conducted under simulated ill sitlllight conditions and at ambient seasurface temperature. At the conclusion of an experiment a sample was drawn for a final NO] concentration and the particulate nitrogen was collected by filtration on Whatman GFIF glass fiber filters. Details regarding sample preparation and analysis for NO] are given in GARSIDE (1982, 1985), and details of the methods for determining the uptake of nitrogeneous nutrients including mass spectrometric analysis of 15N enrichment are described in MCCARTHY and NEVINS (1986). The precision of the NO] analysis is ±2 nmol kg- 1 for concentrations less than 200 nmol kg-I. Replicate determinations of 15Nuptake rates, which we express as p (equivalent to p(m) of DUGDALE and WILKERSON, 1986), for aliquots of the same sample typically fall within 5% of the mean (MCCARTHY et al., 1977 and unpublished data). RESULTS
Sampling dates, operation numbers and positions in ring 82B relative to ring center are given in Table 1. Particulate nitrogen and ambient concentration, final concentration, uptake rates and turnover times for NO] are also tabulated. In conducting our N uptake experiments where the ambient concentration of nitrogenous nutrient is at or below the detection limit of conventional analytical methods, we have customarily added the 15N-Iabeled substrate at a concentration equal to the limit of detection. As we have discussed above and elsewhere (MCCARTHY and NEVINS, 1986), this constrains the limits of uncertainty for the At% enrichment to within a factor of two. Figure 1 illustrates the range of uncertainty for NO] uptake which would have resulted for our samples collected at or above 22% 10 (i.e. the depth to which 22% surface incident irradiation penetrated) and where the ambient NO] concentrations were <100 nmol kg-I, had it been necessary to rely on conventional NO] analyses with a limit of detection of ±50 nmol kg-I. Since the 15N addition (30 nmol N kg " ! for the samples in Fig. 1) is accurately known, this was used as the minimum concentration for samples which would have had undetectable NO] as determined by conventional analyses. The populations occurring at these light levels and nutrient concentrations were probably not subjected to either light limitation or NO] uptake saturation (MAcIsAAC and DUGDALE, 1969), and thus were selected for this and the following treatment. Figure 2 demonstrates the improved resolution in calculations of p for the same experiments when NO] concentration data from the chemiluminescent method are used. Since p represents the average uptake rate during the course of the experiment and the sensitivity of the chemiluminescent method allows us to obtain precise estimates of the initial and final NO] concentration for each incubation, p has been plotted against the average experimental NO] concentration, [NO]], i.e. 15 ] + [NO]]r (1) [NO 3- ] = [NO]]a + [ NO] 2 ' where the subscripts a andj'refer to the ambient and final concentrations, respectively. In this case, the uncertainty in p due to the precision of the subtrate concentration measurements lies within the diameter of the data points.
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CaLLOS (1987; see also DUGDALE and WILKERSON, 1986) has pointed out that in an uptake'expcrirncnt in which only one form of N is labeled (e.g. NO J ) the rate of uptake calculated for that substrate will be underestimated to the extent that other forms of N (e.g. NHt and urea) are incorporated into the particulate pool (effectively diluting the 15N enrichment of the phytoplankton N). Calculations using estimates of NHt and urea uptake rates (McCarthy, unpublished data) obtained from incubations run simultaneously Table 1. Observations at the center ofring 828. J.D. = Julian day, hr = local time, r = distance from ring center, z = sampling depth, %10 = % ofincident sea-surface irradiance, PON = particulate organic nitrogen, [NOj l, = ambient NOj concentration, [NOjlr = final NOj concentration, t = NOj turnover time calculated using [NOJ 1 (see equation 1). See text for definition ofother terms
J.D.
he
r (km)
165
220-4
8.9
z %/0
(m)
PON (umol kg-I)
[NOjla (nn101 kg-I)
100 60 36 22 8 3
1. -4 8 11 19 29
2.2 2.3 2.2 2.5 1.6 1.6
180 360 280 570 710 3170
p
r
(nmol kg-I h- l )
(h) n.d. n.d.
11 12 30 3000
25.5 28.4 21.5 40.1 19.1 49.4
[NOjl! (nmol kg-I) n.d,
n.d.
9.8 8.9 27.1 62.9
166
0·B6 11.8
100 60 36 8 3
1 3 6 17 25
1.5 1.6 1.7 2.1 1.8
35 82 11 41 1270
16 18 13 29 1204
7.6 15.5 5.7 9.5 21.3
5.3 4.2 4.7 5.3 58.8
166
1105
100 60 36 22 8 3
1 4 5 10 16 25
1.7
2.2 2.2 3.2 2.6 2.5
43 35 62 199 600 2160
42 27 15 86 648 1922
12.0 7.0 9.3 21.5 9.3 13.4
4.8 5.8 5.8 7.3 68.7 153.4
8.4
171
0-455
16.2
100 36 22 8 3
1 5 11 17 24
1.2 1.2 1.2 2.0 2.8
18 41 55 129 144
10 9 19 79 179
4.5 5.4 6.0 3.7 6.1
6.4 8.9 8.7 32.2 28.9
171
1105
20.0
100 60 36 22 8 3
3 6
14 20 17 58 36 39
n.d. 6
5.6 4.2 4.8 9.1 7.8 5.8
n.d.
14 20 31
1.6 1.8 1.4 1.6 1.8 1.7
100 60 36 22 8 3
1 3 6 12 17 28
1.6 1.5 1.8 2.1 2.0 1.7
23 83 53 20 7 19
26 9 8 15 12 22
171
2157
25.4
10
10 11 48 14
3.4 11.6 9.3 4.4 3.3 3.9
5.1 5.9 5,4 7.3 7.2 11.6 5.3 4.9 7,4 7,4 9.1
Phytoplankton uptake in euphotic layer of a \vCR
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25
20
1: 15 'eD
:<:
Z "0
!
0-
10
5
o
Fig. 1. The range of estimates for NO.3 uptake rates resulting from a ±50 nmol uncertainty in [NO.3l for the 15N enrichments observed in samples collected at or above 22% 10 and where the ambient NO.3 concentration were <100 nmol kg", For each incubation, two closed circles (e) represent the extreme values with the true value lying on the line connecting them.
with the NO.3 experiments discussed here resulted in an increase of 4-8% in the rate of NO.3 uptake obtained for samples from the mixed layer. Thus, in this data set, the effect of alternative substrate uptake on calculated rates of NO.3 uptake is small, and has been ignored. However, several other potential sources of bias which may affect the accuracy of these data have not been eliminated. For instance, isotope dilution of the dissolved NO.3 pool (nitrification) may occur, affecting the estimates of NO.3 uptake in a manner analogous to NHt rernineralization (cf. GLlBERT, 1982; GARSIDE and GLlBERT, 1984). WARD et al. (1989) have clearly demonstrated the complexity of processes affecting 15N0.3 substrate concentrations. DISCUSSION
This paper demonstrates how the improved precision of 15N0.3 uptake rate measurements can be obtained by employing the chemiluminescent NO.3 assay, and shows how
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25
20 Y == O.27x - 4.31 ,2=0.76
1:
15
'eo
-'"
Z '0
1 o,
10
•
5
o o
w
~
w
W
[N03-] (nmol N kg -.I)
Fig. 2. NO)" uptake rate vs [NO.1] (see equation 1) calculated using data obtained with the chemilum inescent method for samples collected at or above 22% 10 and < 100 nmol kg-I ambient NO).
such precise measurements can be used to infer N03" uptake kinetic and physical mixing parameters. . While the addition of the 15N03" substrate necessarily perturbs the nutrient regime in the sample relative to illsitu conditions, if the uptake kinetics for a given population can be described, the ill situ uptake rate can be estimated by extrapolation to the ill situ nutrient concentration. The samples represented in Fig. 2 were collected at various times , places and depths in the euphotic zone near the center of ring 828 and there is no reason to expect that the species composition and physiological state of the phytoplankton in these samples were uniform. However, there is a high correlation (r 2 = 0.76) between the measured rates of N03" uptake and substrate concentration. This result implies that the populations sampled during this period had similar affinities for N03" over the range of concentrations plotted . Extrapolation of the geometric regression line in Fig. 2 results in an abscissal intercept of 16 nmol kg-I (±11 nmol kg- 1 at the 95% confidence level). The positive intercept may be an artifact of aggregating data from different populations, some other experimental bias or an initial nonlinear (sigmoidal) response (see P AASCHE , 1973) and/or a threshold for NO)"
Phytoplankton uptake in euphotic layer of a \vCR
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uptake may be characteristic for these populations. Potential biological mechanisms which could result "in a threshold for NO.3 uptake might include: (1) a minimum ambient concentration required for induction of enzymes associated with NO.3 uptake and reduction, (2) less than 100% efficiency in the transport of NO.3 across the cell membrane or the ability of the cell to retain intercellular NO.3 against the osmotic gradient, (3) nitrate efflux from phytoplankton cells in the presence of NHt (cf. DEANE-DRUMMOND, 1985; MORGAN et al., 1973). Threshold effects have been observed in continuous culture for the kinetics of SiOl uptake by diatoms (PAASCHE, 1973) and the uptake of several forms of organic carbon by a marine bacterium (JANNASCH and MANTELES, 1974). In the former case, it was shown that the threshold represented a rate of Si dissolution which had to be surpassed by the rate of SiOl uptake in order for net assimilation to occur. In the case of the marine bacterium, it was suggested that a minimum concentration of cells was required in order to modify the culture medium in such a way as to permit growth of the population. While neither of these mechanisms would apply to NO)' uptake, these studies provide applications of MichaelisMenten type kinetics to nutrient uptake rate data when diminished uptake efficiency is observed at low substrate concentrations. A form of the familiar kinetic equation may be used (PAASCllE, 1973): (2) where V is the observed uptake velocity, S, is the substrate concentration and So the "threshold" substrate concentration. The rate constants V max (maximal uptake rate) and K; (half saturation constant) may be obtained graphically from a linear form of equation
(2): (3) This is the Woolf transform which yields the most accurate rate constants of the several linear transforms in general use (WILKINSON, 1961; DOWD and RIGGS, 1965). We have chosen to substitute P (the nutrient transport rate) for V (the biomass specific velocity) in equation (3) because PN concentrations, while similar, were not uniform for these samples (see Table 1) and we have no independent estimates of phytoplankton N. Figure 3 presents the hyperbola obtained from a Woolf transformation of all the data from samples at or above 22% 10 , i.e. the data shown in Fig. 2, with the addition of three observations at> 100 nmol kg- 1 NO.3 concentration. So was set at 16 (see Fig. 2), and the geometric regression line for the data yielded rate constants of K; = 93 nmol kg- 1 and Pmax = 34 nmol kg- 1 h- 1 with a correlation coefficient, r = 0.80. Previous estimates of K, for NO.3 in oligotrophic waters range from 10-210 (mean = 110) nmol kg- 1 (MAcISAAC and DUGDALE, 1969). As these authors point out, K; is a function of the ratios of Vand V m ax (or by extension P and PmaJ; thus our value for K; is comparable to published values derived from V. Because NO.3 is typically the least-preferred inorganic nitrogen substrate for phytoplankton uptake (cf. EpPLEY and PETERSON, 1979; MCCARTHY et al., 1977; MCCARTlIY and NEVINS, 1986), NO.3 substrate concentrations well below the phytoplankton K; for NO.3 imply an insufficiency of more preferred forms of N for the phytoplankton populations in
5400
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x
...50
40
1:
•
30
...z
Pmax =:H,
'OIl
"0
~
20
Ks = 93, r2=0,6t
X
0-
X 10
o 2000
3000
iNOj1 (runoINkg") Fig, 3, The kinetic uptake curve estimated from a linear transformation of all the data collected at or above 22% 10 (e). The remaining data, collected deeper that 22% ~ where light limitation of NO] uptake would be more likely arc indicated (x),
warm-core ring 82B during the period of these observations, An alternative way of evaluating nutrient sufficiency is to compare the turnover time of the nutrient to the doubling time of the phytoplankton (GARSIDE, 1981). Calculated turnover times for NO.3 (lNO.3JlpN03) are presented in Table 1 andhave a range from just over 4 h in the upper mixed layer to as much as 150 h at the top of the nitracline. Given that doubling times for oceanic phytoplankton are typically greater than 24 h, the very much shorter NO.3 turnover times above the nitracline are indicative of N limitation. GARSIDE (1985) provided a simple one-dimensional model of diffusional NO.3 supply and substrate-dependent uptake to describe the vertical distribution of NO.3 in the upper 30-100 m of the water column. The model was used to relate the characteristic time-scale of the influence of biology on the NO.3 concentration (k) to the diffusivity required for NO.3 flux (K z ) . The values of kl K; estimated from the regression of NO.3 concentration on depth were 5 X 10- 3 to 5 x 102 m 2 (equation 7 , GARSIDE, 1985). From this data set we can estimate Ilk from the turnover time of the NO.3 pool (cf. equation 10, GARSIDE, 1985). The high and low values of K; calculated from these data for each water column are presented in Table 2. During the period J.D. 165-166, water-column stability increased with K; values falling within the range typical of near-surface and pycnocline values presented in Table 1 in DENMAN and GARGEIT (1983). Values on the morning of J.D. 171 in the pycnocline (25-30 m) were somewhat higher, and those in the afternoon were greater at both depths than typical values in the literature. Values could not be obtained for the late night
Phytoplankton uptake in euphotic layer of a \vCR
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Table 2. The vertical eddy dijfusivity; Kz ' calculated from the turnover time of the the NO; pool and the onedimensional model of NO; distribution and fluxes (GARSIDE, 1985). See text for details
Kz
Z (m)
(m 2 S-l)
220-l
8 29
3 x 10- 3 5 x 10- 4
166
0·B6
6 25
1 x 10- 3 8 x 10- 5
166
1105
4 25
1 x 10- 3 5 x 10- 5
171
0-l55
1 24
2 x 10- 3 4 x 10- 4
171
1105
6 31
3 x 10- 2 2 x 10- 2
171
2157
J.D.
hr
165
sampling since a meaningful regression could not be fit to the NO.3 concentration data (GARsIDE, 1985). The increasing values on J.D. 171 leading to the breakdown of the vertical NO.3 structure may well have been the result of dissipation of energy from a storm on J.D. 170 that produced winds of90 km h- 1 and seas >7.5 m. Since both K; and k for these data can be calculated, it is also possible to calculate vertical NO.3 fluxes (equation 9, GARSIDE, 1985). Using average numbers for the two water columns sampled on J.D. 166 provides a calculated flux of NO.3 through 25 m of 1.7 mmol N m- 2day-l. GARSIDE (1985) estimated a minimum rate of water column (30 m) NO.3 depletion, which adjusted to 25 m amounts to 2.0 mmol N m- 2 day", providing a total NO.3 flux available to phytoplankton of 3.7 mmol N m- 2 day-I. The integrated experimental NO.3 uptake in the upper 25 m was 8 mmol N m- 2 day", while total nitrogen-based production calculated from 14C uptake data (G. L. HITCHCOCK, personal communication), assuming a C:N ratio of 6.6 atomic (REDFIELD, 1958) was 12.4 mmol N m- 2 day "'. With little redundancy in the data used for these calculations (only the 25 m 15N0.3 production number is used twice) there is remarkably good consistency among these estimates of vertical NO.3 fluxes. A similar calculation for data from a single station in the Sargasso Sea (using a pycnocline value of K; = 7 X 10- 5 m2 S-l) results in a model estimate of new production of 0.025 g C m 2 day-lor 9 g C m 2 y"". KING and DEVOL (1979) estimated K; from, data for NO.3 uptake by euphotic zone phytoplankton and the nitracline gradient at the base of the mixed layer. Although respected for its novelty, this approach is subject to the criticism that euphotic zone uptake and nitracline NO.3 flux estimated from the part of the nitracline where NO.3 concentrations can be measured using conventional methods are not necessarily coupled on the appropriate time-scales. Our approach and results differ significantly in that (1) the form
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of the N0 3 gradient is specified by a steady state model relating flux to uptake, (2) the gradient is calculated based on many N03 concentration data points in the upper nitracline and euphotic zone rather than a pair of values from deeper in the nitracline, (3) the N0 3 concentrations are measured precisely over the depth range where the important fluxes occur rather than at depths where non-zero conventional measurements can be made, (4) the N0 3 gradient represents an integration of recent fluxes and uptake on physical and biological time-scales. In addition, the estimate of K; and hence flux depends on the local turnover time of the N0 3 pool at a single depth in the upper nitracline, and not on integrated demand in the euphotic zone, so that the estimated flux can be tested independently against the integrated N0 3 uptake. Interest in documenting and studying the factors that regulate "new" production in the ocean has grown considerably since the importance of such understanding was put in a large-scale biogeochemical context by EpPLEY and PETERSON (1979). Without notable exception "new" production in the oceanic euphotic zone is equivalent to N03 supported primary production. Thus.when integrated over the appropriate time-scales there should be good agreement between the upward flux of N0 3 , "new" production, and downward flux of PN (and perhaps DON). The added precision brought to N03 uptake studies by the chemiluminescent method wiII greatly improve precision both in estimates of nitrate supply to the mixed layer and in measurements of the uptake rate of this nutrient by phytoplankton. As indicated above, both applications will be especially valuable in field studies designed to quantifying the interactions between physical and biogeochemical processes as they pertain to N03 driven primary production. Thus we are now ready for the first time to mount field campaigns in low nutrient waters with the expectation that highly accurate and precise determinations of "new" production can be attained. During the last decade many new' and improved methods have been introduced in related areas, which now make it possible to undertake large-scale studies of questions related to production and fate of biogenic particles in the sea. A 'global study of these processes is, for example, a primary objective of the Joint Ocean Flux Study (JGOFS, 1990). Acknoll'ledgemen/s-This work was supported by NSF grants OCE 80-22990, OCE 85-13920 (to J.J.M.) and OCE 82-00468 (to e.G.). Bigelow Laboratory contribution no. 91007.
REFERENCES COLLOS Y. (1987) Calculations of 15Nuptake rates by phytoplankton assimilating one or several nitrogen sources, Applied Radiation and Is%pes, 38, 439--447. DEANE-DRUmlO:-lD C. E. (1985) Regulation of nitrate uptake into Chara corallina cell via NHt stimulation of NOj efflux. Plant, Cell and Environment, 8,105-110. DEr-;~IAN K. L. and A. E. GARGETT (1983) Time and space scales of vertical mixing and advection of phytoplankton in the upper ocean. Limnology and Oceanography, 28, 801-815. DOWD J. E. and D. S. RIGGS (1965) A comparison of estimates of Michaelis-Menton kinetic constants from various linear transformations. Journal of Biological Chemistry, 240, 863-869. DUGDALE R. e. and J. J. GOERtl'G (1967) Uptake of new and regenerated forms of nitrogen in primary productivity. Limnology and Oceanography, 12, 196--206. DUGDALE R. e. and F. P. WiLKERSOl' (1986) The usc of 15N to measure nitrogen uptake in eutrophic oceans: experimental considerations. Limnology and Oceanography, 31, 673-
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EVANS R. H., K. S. BAKER, O. B. BROWN and R. C. S~lITH (1985) Chronology of warm-core ring 82-8. Journal of Geophysical Research, 90, 8803-8811. Fox M. F., P. F. BATES and D. R . KESTER (1984) Nutrient data for warm-core ring 82-8 from R.V. Knorr cruise 93. University of Rhode Island, Graduate School of Oceanography, Technical Report 84-1, Narragansett, R1. GARSIDE C. (1981) Nitrate and ammonium uptake in the apex of the New York Bight. Limnology and Oceanography, 26, 731-739. GARSIDE C . (1982) A chemiluminescent technique for the determination of nanomolar concentrations of nitrate, nitrite and nitrate, or nitrite alone in seawater. Marine Chemistry, 11, 159-167. GARSIDE C. (1985) The vertical distribution of nitrate in open ocean surface water. Deep-Sea Research, 32, 723732. GARSIDE C . and P. M. GLIDERT (1984) Computer modeling of ISN uptake and remineralization experiments. Limnology and Oceanography, 29,199-204. GLIBERT P. M. (1982) Regional studies of daily, seasonal and size fraction variability in ammonium remineralization. Marine Biology, 70, 209-222. JANNASCH H. W. and R. I. MANTELES (1974) Experimental bacterial ecology studied in continuous culture. Adl'Qnces in Microbial Physiology, 11, 165-212. JGOFS (1990) The Joint Global Ocean Flux Study, Science Plan. Scientific Committee on Oceanic Research, International Council of Scientific Unions, JGOFS Report No .5, Halifax, NS. Krxo F. D. and A. H. DEVOL (1979) Estimates of vertical eddy diffusion through the thermocline from phytoplankton nitrate uptake rates in the mixed layer of the eastern tropical Pacific. Limnology and Oceanography, 24, 645-651. MAcISAAC J. J. and R. C. DUGDALE (1969) The kinetics of nitrate and ammonium uptake by natural populations of marine phytoplankton. Deep-Sea Research, 16,45-57. MCCARTHY J. J. and E . CARPENTER (1983) Nitrogen cycling in ncar-surface waters of the open ocean. In: Nitrogen in the marine environment, E. J. CARPENTER and D . G. CAPONE, editors, Academic Press, New York, pp. 487-512. MCCARTIlY J. J. and J. L. NEVINS (1986) Sources of nitrogen for primary production in warm-core rings 79-E and 81-D . Limnology and Oceanography, 31,690-700. MCCARTHY J. J., R. W . TAYLOR and J. L. TAFT (1977) Nitrogenous nutrition of the plankton in the Chesapeake Bay. 1. Nutrient availability and phytoplankton preferences. Limnology and Oceanography, 22, 996-1011. MELLOR G. L. and P. A. DURDI:-l (1975) The structure and dynamics of the ocean su~face mixed layer. Journal of Physical Oceanography, 5, 718-728. MORGAN A. A ., R. J . YOLK and W . A . JACKSON (1973) Simultaneous influx and eflluxofnitrate during uptake by perennial rye-grass. Plant Physiology, 51, 267-272. PAASCIIE E. (1973) Silicon and the ecology of marine plankton diatoms: 1. Thalassiosira pseudonana (Cyclotella nan a) grown in a chernostat with silicate as limiting nutrient. Marine Biology, 19, 117-126. PLATT T. (1984) Thermodynamics of the pelagic ecosystem: elementary closure conditions for biological production in the open ocean. In: Flows of energy and materials in marine ecosystems, M. J. R. FASHAM, editor, Plenum Press, New York, pp . 49-84. REDFIELD A. C. (1958) The biological control of chemical factors in the environment. American Scientist, 46, 205-221. . SCHMITT R. W. and D. 8. OLSO:-l (1985) Wintertime convection in warm-core rings: thermocline ventilation and the formation of mesoscale lenses. Journal of Geophysical Research, 8, 8823-8837. WARD B. B., K. A. KILPATRICK, E. H . REI'GER and R. W. EpPLEY (1989) Biological nitrogen cycling in the nitracline. Limnology and Oceanography, 34, 493-514. WtEBE P. H. and T. J. McDOUGALL (1986) (Guest editors) Warm-Core Rings . Studies of their physics, chemistry and biology. Deep-Sea Research, 33 . WILKINSON G. N. (1961) Statistical estimations in enzyme kinetics. Biochemical Iournal 80,324-332.
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Deep-Sea Research, Vol. 39, Suppl. I. pp, S39J-S403. 1992. Printedin Great Britain.
© 1992 Pergamon
Nitrate supply and phytoplankton uptake kinetics in the euphotic layer of a Gulf Stream warm-core ring JAMES J. MCCARTHY, * CHRIS GARSIDE! and JOHN L. NEVINS* (Received 14 April 1989; ill revised [orm IS October 1990; accepted 23 October 1990) Abstract-Chemiluminescenl nitrate analysis was used in conjuctlon with 15N-labeled NOJ to assess the rates of NOi uptake by phytoplankton in warm-core ring 828. The relatively high precision of this method compared to conventional NOi analyses permits reliable estimates of NOi uptake in oligotrophic waters. Aggregation of uptake data from six profiles from 2 days of observation permitted the calculation of NO]' turnover times ranging from about 4 h near the surface to ISO h at the top of the nitracline. Turnover times in the euphotic zone and the observed half saturation constant of93 nmol kg- 1 for NOi uptake imply nitrogen limitation for these populations. Extrapolation from the linear portion of the kinetic curve revealed that a NOi threshold concentration of about 16 nmol kg-I was required for the initiation of uptake. These highly precise uptake measurements were used in a one-dimensional model to estimate the vertical flux of NOi. Maximum near-surface and deep-euphotic-zone eddy diffusivity values (K z ) were 3 x 10- 3 and 5 X 10- 4 m2 S-I , respectively, prior to a major storm. Following the storm K z values were substantially greater.
INTRODUCTION
INTHE open ocean the rate of primary production is usually assumed to be nutrient limited. Inorganic-combined nitrogen, phosphorous and silicate may all be severely depleted in the euphotic zone of oceanic regions, but most recent efforts to understand the regulation of oceanic production have focused on nitrogen (MCCARTHY and CARPENTER, 1983). The upward transport of NO.3 is the dominant process by which nitrogenous nutrient enters the wind-mixed layer, and the primary production arising from this process has been termed "new" production (DUGDALE and GOERING, 1967; EpPLEY and PETERSON, 1979). There is considerable uncertainty regarding the rates of various processes responsible for this flux and the manner in which they are regulated. The supply of NO.3 to the mixed layer occurs on scales that range from a more or less continuous molecular flux along the NO.3 gradient in the seasonal pycnocline to larger episodic fluxes associated with destabilization of the mixed layer, as in the case of upwelling or storm-induced deepening of the mixed layer. There has been a tendency to treat the more or less continuous molecular flux as a one-dimensional problem (d. PLAIT, 1984). This approach, which is similar to that of K-theory models that address the vertical • Harvard University, Museum of Comparative Zoology, 26 Oxford Street, Cambridge, MA 02138, U.S.A. t Bigelow Laboratory for Ocean Sciences, McKown Point, West Boothbay Harbor, ME Q.l575, U.S.A. S393
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flux of heat, assumes that transport of nutrients through the seasonal pycnocline to the mixed ldyer is analogous to molecular diffusion. However, some recent mixed-layer models that yield realistic results for the upper mixed layer fix K equal to zero in the thermocline, and thus ignore vertical diffusive processes in the thermocline altogether (MELLOR and DURBIN, 1975). With this latter approach physical processes (e.g. breaking internal waves) disrupt the thermocline and are responsible for the exchange of scalar properties. These episodic physical events are difficult to sample adequately for the purpose of estimating the upward vertical flux of nutrients. Biologists interested in the rates of NO)" consumption by plankton in the mixed layer have used the rates of NO)" uptake as determined by IsN-labeling experiments to estimate the upward diffusive flux of NO)" (KING and DEVOL, 1979; EpPLEY et al., 1979). One fundamental limitation of this approach is the resolution that can be achieved in determining NO)" concentrations in the lower portion of the mixed layer. In addition, although incubation experiments ideally would be executed with only a trace addition of IsNO)", this has not been practical for studies in oligotrophic waters, where the NO)" concentrations are below the limits of detection (typically 50-100 nmol kg -1 for automated nutrient analyses; Fox et al., 1984). In this situation the magnitude of the resultant IsN-enrichment of the substrate remains unknown within certain limits. For example, if a labeled addition is arbitrarily set as the equivalent of the limit of detection, the observed rate of uptake determined can only be known to within a factor of two, since the atom per cent (At%) enrichment of the labeled nutrient pool lies between 50 and 100. The precision of conventional mass spectrometry is typically under-utilized in these applications, thus it is the inability to quantify NO)" concentrations in the less than 50 nmol kg- 1 range that has limited the precision of estimates of NO)" uptake in oligotrophic waters. With the development of a chemiluminescent assay for NO)" in sea water that has a limit of detection of 2 nmol kg- 1 (GARSIDE, 1982), the opportunity now exists to study NO)" uptake in the mixed layer of oligotrophic waters with substantially increased precision. Experiments with IsN-labeled substrates were run as part of the Warm-Core Ring Program, and in the June 1982 study of ring 82B, the application of this high sensitivity NO)" method permitted exceptionally precise determination of NO)" uptake. The NO)" concentration data have been presented (GARSIDE, 1985), and this paper addresses the results of the uptake experiments. The physics, chemistry and biology of Gulf Stream warm-core ring 82B have been studied in considerable detail (cf. EVANS et al., 1985; SCHMITI and OLSON, 1985; WIEBE and McDoUGALL, 1986). This ring was formed in February 1982, and subsequently underwent a period of deep convective mixing that resulted in a 15.7°C thermostad extending from the surface to a depth of about 400 m (SCHMITI and OLSON, 1985). As a result of this mixing, NO)" concentrations in the thermostad increased to 5.5-6 Jimol kg- 1 (Fox et al., 1984). Between early May and mid-June, a strong pycnocline developed in the upper 30 m. At the time of our June study, near-surface NO)" concentrations were below the limits of detection for conventional automated nutrient analyses (Fox et al., 1984), but they significantly exceeded the detection limit of the chemiluminescent method. METHODS
The experiments described in this paper were conducted aboard the R. V. Knorr, cruise no. 095. Six samplings were made over two 24-h periods in ring 82B. Water was collected
Phytoplankton uptake in euphotic layer of a \vCR
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from six depths within the euphotic zone using a rosette sampler equipped with 30-1 Go-Flo bottles and it CTD. Samples were drawn for nutrient analyses and experiments were initiated to determine rates of NO] uptake using 15N-Iabeled NO]. Incubations of about 4-h duration were conducted under simulated ill sitlllight conditions and at ambient seasurface temperature. At the conclusion of an experiment a sample was drawn for a final NO] concentration and the particulate nitrogen was collected by filtration on Whatman GFIF glass fiber filters. Details regarding sample preparation and analysis for NO] are given in GARSIDE (1982, 1985), and details of the methods for determining the uptake of nitrogeneous nutrients including mass spectrometric analysis of 15N enrichment are described in MCCARTHY and NEVINS (1986). The precision of the NO] analysis is ±2 nmol kg- 1 for concentrations less than 200 nmol kg-I. Replicate determinations of 15Nuptake rates, which we express as p (equivalent to p(m) of DUGDALE and WILKERSON, 1986), for aliquots of the same sample typically fall within 5% of the mean (MCCARTHY et al., 1977 and unpublished data). RESULTS
Sampling dates, operation numbers and positions in ring 82B relative to ring center are given in Table 1. Particulate nitrogen and ambient concentration, final concentration, uptake rates and turnover times for NO] are also tabulated. In conducting our N uptake experiments where the ambient concentration of nitrogenous nutrient is at or below the detection limit of conventional analytical methods, we have customarily added the 15N-Iabeled substrate at a concentration equal to the limit of detection. As we have discussed above and elsewhere (MCCARTHY and NEVINS, 1986), this constrains the limits of uncertainty for the At% enrichment to within a factor of two. Figure 1 illustrates the range of uncertainty for NO] uptake which would have resulted for our samples collected at or above 22% 10 (i.e. the depth to which 22% surface incident irradiation penetrated) and where the ambient NO] concentrations were <100 nmol kg-I, had it been necessary to rely on conventional NO] analyses with a limit of detection of ±50 nmol kg-I. Since the 15N addition (30 nmol N kg " ! for the samples in Fig. 1) is accurately known, this was used as the minimum concentration for samples which would have had undetectable NO] as determined by conventional analyses. The populations occurring at these light levels and nutrient concentrations were probably not subjected to either light limitation or NO] uptake saturation (MAcIsAAC and DUGDALE, 1969), and thus were selected for this and the following treatment. Figure 2 demonstrates the improved resolution in calculations of p for the same experiments when NO] concentration data from the chemiluminescent method are used. Since p represents the average uptake rate during the course of the experiment and the sensitivity of the chemiluminescent method allows us to obtain precise estimates of the initial and final NO] concentration for each incubation, p has been plotted against the average experimental NO] concentration, [NO]], i.e. 15 ] + [NO]]r (1) [NO 3- ] = [NO]]a + [ NO] 2 ' where the subscripts a andj'refer to the ambient and final concentrations, respectively. In this case, the uncertainty in p due to the precision of the subtrate concentration measurements lies within the diameter of the data points.
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CaLLOS (1987; see also DUGDALE and WILKERSON, 1986) has pointed out that in an uptake'expcrirncnt in which only one form of N is labeled (e.g. NO J ) the rate of uptake calculated for that substrate will be underestimated to the extent that other forms of N (e.g. NHt and urea) are incorporated into the particulate pool (effectively diluting the 15N enrichment of the phytoplankton N). Calculations using estimates of NHt and urea uptake rates (McCarthy, unpublished data) obtained from incubations run simultaneously Table 1. Observations at the center ofring 828. J.D. = Julian day, hr = local time, r = distance from ring center, z = sampling depth, %10 = % ofincident sea-surface irradiance, PON = particulate organic nitrogen, [NOj l, = ambient NOj concentration, [NOjlr = final NOj concentration, t = NOj turnover time calculated using [NOJ 1 (see equation 1). See text for definition ofother terms
J.D.
he
r (km)
165
220-4
8.9
z %/0
(m)
PON (umol kg-I)
[NOjla (nn101 kg-I)
100 60 36 22 8 3
1. -4 8 11 19 29
2.2 2.3 2.2 2.5 1.6 1.6
180 360 280 570 710 3170
p
r
(nmol kg-I h- l )
(h) n.d. n.d.
11 12 30 3000
25.5 28.4 21.5 40.1 19.1 49.4
[NOjl! (nmol kg-I) n.d,
n.d.
9.8 8.9 27.1 62.9
166
0·B6 11.8
100 60 36 8 3
1 3 6 17 25
1.5 1.6 1.7 2.1 1.8
35 82 11 41 1270
16 18 13 29 1204
7.6 15.5 5.7 9.5 21.3
5.3 4.2 4.7 5.3 58.8
166
1105
100 60 36 22 8 3
1 4 5 10 16 25
1.7
2.2 2.2 3.2 2.6 2.5
43 35 62 199 600 2160
42 27 15 86 648 1922
12.0 7.0 9.3 21.5 9.3 13.4
4.8 5.8 5.8 7.3 68.7 153.4
8.4
171
0-455
16.2
100 36 22 8 3
1 5 11 17 24
1.2 1.2 1.2 2.0 2.8
18 41 55 129 144
10 9 19 79 179
4.5 5.4 6.0 3.7 6.1
6.4 8.9 8.7 32.2 28.9
171
1105
20.0
100 60 36 22 8 3
3 6
14 20 17 58 36 39
n.d. 6
5.6 4.2 4.8 9.1 7.8 5.8
n.d.
14 20 31
1.6 1.8 1.4 1.6 1.8 1.7
100 60 36 22 8 3
1 3 6 12 17 28
1.6 1.5 1.8 2.1 2.0 1.7
23 83 53 20 7 19
26 9 8 15 12 22
171
2157
25.4
10
10 11 48 14
3.4 11.6 9.3 4.4 3.3 3.9
5.1 5.9 5,4 7.3 7.2 11.6 5.3 4.9 7,4 7,4 9.1
Phytoplankton uptake in euphotic layer of a \vCR
S397
25
20
1: 15 'eD
:<:
Z "0
!
0-
10
5
o
Fig. 1. The range of estimates for NO.3 uptake rates resulting from a ±50 nmol uncertainty in [NO.3l for the 15N enrichments observed in samples collected at or above 22% 10 and where the ambient NO.3 concentration were <100 nmol kg", For each incubation, two closed circles (e) represent the extreme values with the true value lying on the line connecting them.
with the NO.3 experiments discussed here resulted in an increase of 4-8% in the rate of NO.3 uptake obtained for samples from the mixed layer. Thus, in this data set, the effect of alternative substrate uptake on calculated rates of NO.3 uptake is small, and has been ignored. However, several other potential sources of bias which may affect the accuracy of these data have not been eliminated. For instance, isotope dilution of the dissolved NO.3 pool (nitrification) may occur, affecting the estimates of NO.3 uptake in a manner analogous to NHt rernineralization (cf. GLlBERT, 1982; GARSIDE and GLlBERT, 1984). WARD et al. (1989) have clearly demonstrated the complexity of processes affecting 15N0.3 substrate concentrations. DISCUSSION
This paper demonstrates how the improved precision of 15N0.3 uptake rate measurements can be obtained by employing the chemiluminescent NO.3 assay, and shows how
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25
20 Y == O.27x - 4.31 ,2=0.76
1:
15
'eo
-'"
Z '0
1 o,
10
•
5
o o
w
~
w
W
[N03-] (nmol N kg -.I)
Fig. 2. NO)" uptake rate vs [NO.1] (see equation 1) calculated using data obtained with the chemilum inescent method for samples collected at or above 22% 10 and < 100 nmol kg-I ambient NO).
such precise measurements can be used to infer N03" uptake kinetic and physical mixing parameters. . While the addition of the 15N03" substrate necessarily perturbs the nutrient regime in the sample relative to illsitu conditions, if the uptake kinetics for a given population can be described, the ill situ uptake rate can be estimated by extrapolation to the ill situ nutrient concentration. The samples represented in Fig. 2 were collected at various times , places and depths in the euphotic zone near the center of ring 828 and there is no reason to expect that the species composition and physiological state of the phytoplankton in these samples were uniform. However, there is a high correlation (r 2 = 0.76) between the measured rates of N03" uptake and substrate concentration. This result implies that the populations sampled during this period had similar affinities for N03" over the range of concentrations plotted . Extrapolation of the geometric regression line in Fig. 2 results in an abscissal intercept of 16 nmol kg-I (±11 nmol kg- 1 at the 95% confidence level). The positive intercept may be an artifact of aggregating data from different populations, some other experimental bias or an initial nonlinear (sigmoidal) response (see P AASCHE , 1973) and/or a threshold for NO)"
Phytoplankton uptake in euphotic layer of a \vCR
S399
uptake may be characteristic for these populations. Potential biological mechanisms which could result "in a threshold for NO.3 uptake might include: (1) a minimum ambient concentration required for induction of enzymes associated with NO.3 uptake and reduction, (2) less than 100% efficiency in the transport of NO.3 across the cell membrane or the ability of the cell to retain intercellular NO.3 against the osmotic gradient, (3) nitrate efflux from phytoplankton cells in the presence of NHt (cf. DEANE-DRUMMOND, 1985; MORGAN et al., 1973). Threshold effects have been observed in continuous culture for the kinetics of SiOl uptake by diatoms (PAASCHE, 1973) and the uptake of several forms of organic carbon by a marine bacterium (JANNASCH and MANTELES, 1974). In the former case, it was shown that the threshold represented a rate of Si dissolution which had to be surpassed by the rate of SiOl uptake in order for net assimilation to occur. In the case of the marine bacterium, it was suggested that a minimum concentration of cells was required in order to modify the culture medium in such a way as to permit growth of the population. While neither of these mechanisms would apply to NO)' uptake, these studies provide applications of MichaelisMenten type kinetics to nutrient uptake rate data when diminished uptake efficiency is observed at low substrate concentrations. A form of the familiar kinetic equation may be used (PAASCllE, 1973): (2) where V is the observed uptake velocity, S, is the substrate concentration and So the "threshold" substrate concentration. The rate constants V max (maximal uptake rate) and K; (half saturation constant) may be obtained graphically from a linear form of equation
(2): (3) This is the Woolf transform which yields the most accurate rate constants of the several linear transforms in general use (WILKINSON, 1961; DOWD and RIGGS, 1965). We have chosen to substitute P (the nutrient transport rate) for V (the biomass specific velocity) in equation (3) because PN concentrations, while similar, were not uniform for these samples (see Table 1) and we have no independent estimates of phytoplankton N. Figure 3 presents the hyperbola obtained from a Woolf transformation of all the data from samples at or above 22% 10 , i.e. the data shown in Fig. 2, with the addition of three observations at> 100 nmol kg- 1 NO.3 concentration. So was set at 16 (see Fig. 2), and the geometric regression line for the data yielded rate constants of K; = 93 nmol kg- 1 and Pmax = 34 nmol kg- 1 h- 1 with a correlation coefficient, r = 0.80. Previous estimates of K, for NO.3 in oligotrophic waters range from 10-210 (mean = 110) nmol kg- 1 (MAcISAAC and DUGDALE, 1969). As these authors point out, K; is a function of the ratios of Vand V m ax (or by extension P and PmaJ; thus our value for K; is comparable to published values derived from V. Because NO.3 is typically the least-preferred inorganic nitrogen substrate for phytoplankton uptake (cf. EpPLEY and PETERSON, 1979; MCCARTHY et al., 1977; MCCARTlIY and NEVINS, 1986), NO.3 substrate concentrations well below the phytoplankton K; for NO.3 imply an insufficiency of more preferred forms of N for the phytoplankton populations in
5400
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et al,
x
...50
40
1:
•
30
...z
Pmax =:H,
'OIl
"0
~
20
Ks = 93, r2=0,6t
X
0-
X 10
o 2000
3000
iNOj1 (runoINkg") Fig, 3, The kinetic uptake curve estimated from a linear transformation of all the data collected at or above 22% 10 (e). The remaining data, collected deeper that 22% ~ where light limitation of NO] uptake would be more likely arc indicated (x),
warm-core ring 82B during the period of these observations, An alternative way of evaluating nutrient sufficiency is to compare the turnover time of the nutrient to the doubling time of the phytoplankton (GARSIDE, 1981). Calculated turnover times for NO.3 (lNO.3JlpN03) are presented in Table 1 andhave a range from just over 4 h in the upper mixed layer to as much as 150 h at the top of the nitracline. Given that doubling times for oceanic phytoplankton are typically greater than 24 h, the very much shorter NO.3 turnover times above the nitracline are indicative of N limitation. GARSIDE (1985) provided a simple one-dimensional model of diffusional NO.3 supply and substrate-dependent uptake to describe the vertical distribution of NO.3 in the upper 30-100 m of the water column. The model was used to relate the characteristic time-scale of the influence of biology on the NO.3 concentration (k) to the diffusivity required for NO.3 flux (K z ) . The values of kl K; estimated from the regression of NO.3 concentration on depth were 5 X 10- 3 to 5 x 102 m 2 (equation 7 , GARSIDE, 1985). From this data set we can estimate Ilk from the turnover time of the NO.3 pool (cf. equation 10, GARSIDE, 1985). The high and low values of K; calculated from these data for each water column are presented in Table 2. During the period J.D. 165-166, water-column stability increased with K; values falling within the range typical of near-surface and pycnocline values presented in Table 1 in DENMAN and GARGEIT (1983). Values on the morning of J.D. 171 in the pycnocline (25-30 m) were somewhat higher, and those in the afternoon were greater at both depths than typical values in the literature. Values could not be obtained for the late night
Phytoplankton uptake in euphotic layer of a \vCR
S401
Table 2. The vertical eddy dijfusivity; Kz ' calculated from the turnover time of the the NO; pool and the onedimensional model of NO; distribution and fluxes (GARSIDE, 1985). See text for details
Kz
Z (m)
(m 2 S-l)
220-l
8 29
3 x 10- 3 5 x 10- 4
166
0·B6
6 25
1 x 10- 3 8 x 10- 5
166
1105
4 25
1 x 10- 3 5 x 10- 5
171
0-l55
1 24
2 x 10- 3 4 x 10- 4
171
1105
6 31
3 x 10- 2 2 x 10- 2
171
2157
J.D.
hr
165
sampling since a meaningful regression could not be fit to the NO.3 concentration data (GARsIDE, 1985). The increasing values on J.D. 171 leading to the breakdown of the vertical NO.3 structure may well have been the result of dissipation of energy from a storm on J.D. 170 that produced winds of90 km h- 1 and seas >7.5 m. Since both K; and k for these data can be calculated, it is also possible to calculate vertical NO.3 fluxes (equation 9, GARSIDE, 1985). Using average numbers for the two water columns sampled on J.D. 166 provides a calculated flux of NO.3 through 25 m of 1.7 mmol N m- 2day-l. GARSIDE (1985) estimated a minimum rate of water column (30 m) NO.3 depletion, which adjusted to 25 m amounts to 2.0 mmol N m- 2 day", providing a total NO.3 flux available to phytoplankton of 3.7 mmol N m- 2 day-I. The integrated experimental NO.3 uptake in the upper 25 m was 8 mmol N m- 2 day", while total nitrogen-based production calculated from 14C uptake data (G. L. HITCHCOCK, personal communication), assuming a C:N ratio of 6.6 atomic (REDFIELD, 1958) was 12.4 mmol N m- 2 day "'. With little redundancy in the data used for these calculations (only the 25 m 15N0.3 production number is used twice) there is remarkably good consistency among these estimates of vertical NO.3 fluxes. A similar calculation for data from a single station in the Sargasso Sea (using a pycnocline value of K; = 7 X 10- 5 m2 S-l) results in a model estimate of new production of 0.025 g C m 2 day-lor 9 g C m 2 y"". KING and DEVOL (1979) estimated K; from, data for NO.3 uptake by euphotic zone phytoplankton and the nitracline gradient at the base of the mixed layer. Although respected for its novelty, this approach is subject to the criticism that euphotic zone uptake and nitracline NO.3 flux estimated from the part of the nitracline where NO.3 concentrations can be measured using conventional methods are not necessarily coupled on the appropriate time-scales. Our approach and results differ significantly in that (1) the form
S402
J. J. MCCARTIIY e/ al.
of the N0 3 gradient is specified by a steady state model relating flux to uptake, (2) the gradient is calculated based on many N03 concentration data points in the upper nitracline and euphotic zone rather than a pair of values from deeper in the nitracline, (3) the N0 3 concentrations are measured precisely over the depth range where the important fluxes occur rather than at depths where non-zero conventional measurements can be made, (4) the N0 3 gradient represents an integration of recent fluxes and uptake on physical and biological time-scales. In addition, the estimate of K; and hence flux depends on the local turnover time of the N0 3 pool at a single depth in the upper nitracline, and not on integrated demand in the euphotic zone, so that the estimated flux can be tested independently against the integrated N0 3 uptake. Interest in documenting and studying the factors that regulate "new" production in the ocean has grown considerably since the importance of such understanding was put in a large-scale biogeochemical context by EpPLEY and PETERSON (1979). Without notable exception "new" production in the oceanic euphotic zone is equivalent to N03 supported primary production. Thus.when integrated over the appropriate time-scales there should be good agreement between the upward flux of N0 3 , "new" production, and downward flux of PN (and perhaps DON). The added precision brought to N03 uptake studies by the chemiluminescent method wiII greatly improve precision both in estimates of nitrate supply to the mixed layer and in measurements of the uptake rate of this nutrient by phytoplankton. As indicated above, both applications will be especially valuable in field studies designed to quantifying the interactions between physical and biogeochemical processes as they pertain to N03 driven primary production. Thus we are now ready for the first time to mount field campaigns in low nutrient waters with the expectation that highly accurate and precise determinations of "new" production can be attained. During the last decade many new' and improved methods have been introduced in related areas, which now make it possible to undertake large-scale studies of questions related to production and fate of biogenic particles in the sea. A 'global study of these processes is, for example, a primary objective of the Joint Ocean Flux Study (JGOFS, 1990). Acknoll'ledgemen/s-This work was supported by NSF grants OCE 80-22990, OCE 85-13920 (to J.J.M.) and OCE 82-00468 (to e.G.). Bigelow Laboratory contribution no. 91007.
REFERENCES COLLOS Y. (1987) Calculations of 15Nuptake rates by phytoplankton assimilating one or several nitrogen sources, Applied Radiation and Is%pes, 38, 439--447. DEANE-DRUmlO:-lD C. E. (1985) Regulation of nitrate uptake into Chara corallina cell via NHt stimulation of NOj efflux. Plant, Cell and Environment, 8,105-110. DEr-;~IAN K. L. and A. E. GARGETT (1983) Time and space scales of vertical mixing and advection of phytoplankton in the upper ocean. Limnology and Oceanography, 28, 801-815. DOWD J. E. and D. S. RIGGS (1965) A comparison of estimates of Michaelis-Menton kinetic constants from various linear transformations. Journal of Biological Chemistry, 240, 863-869. DUGDALE R. e. and J. J. GOERtl'G (1967) Uptake of new and regenerated forms of nitrogen in primary productivity. Limnology and Oceanography, 12, 196--206. DUGDALE R. e. and F. P. WiLKERSOl' (1986) The usc of 15N to measure nitrogen uptake in eutrophic oceans: experimental considerations. Limnology and Oceanography, 31, 673-
Phytoplankton uptake in euphotic layer of a WCR
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