New production and photosynthetic rates within and outside a cyclonic mesoscale eddy in the North Pacific subtropical gyre

Pergamon

PII: So967-0637(96)00022-2

Deep-Sea Research I. Vol. 43. No. 6, pp. 917-936, 1996 Copyright 8 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0967-0637/96$15.00+0.00

New production and photosynthetic rates within and outside a cyclonic mesoscale eddy in the North Pacific subtropical gyre COLLEEN

B. ALLEN,*

JOTA

KANDAT

and

EDWARD

(Received 12 April 1995; in revisedform 20 November 1995; accepted

A. LAWS*

11December 1995)

Abstract-Photosynthetic rates, autotrophic nitrogen assimilation, and the uptake and assimilation of nitrate were measured at three stations in the center of, near the edge of, and outside of a cyclonic mesoscale eddy near the island of Hawaii. Areal photosynthetic rates near the edge and at the center of the eddy were 31% and 66% higher, respectively, than outside the eddy. Nitrate uptake was equated to the drawdown of nitrate in incubated samples. Nitrate assimilation was calculated from the incorporation into particulate matter of “N derived from K”NOs. Ambient nitrate concentrations were about 8nM, and uptake of nitrate was undetectable above the nitracline. Addition of 55 nM Kr5N0s spikes to samples taken above the nitracline stimulated an uptake and assimilation of nitrate which averaged 20-24 nM day-‘. The stimulated nitrate uptake occurred at comparable rates during the day and night, but almost all stimulated assimilation occurred at night. Much of the stimulated nitrate uptake which occurred during the day was apparently excreted in a reduced form, most but not all of which was assimilated at night. In deeper samples that contained greater than 350nM nitrate, almost all assimilation of nitrate occurred during the day. Nitrate uptake outside the eddy and the upward diffusion of nitrate outside the eddy were estimated to be l& 15 mg N mm2 day-‘. Thef-ratio was estimated to be 0.2 outside the eddy and 0.8 at the center of the eddy. Relative growth rates were about 70% outside the eddy and either nutrient-saturated or nearly were about 32% higher at the center of so within the eddy. Area1 chlorophyll a (chl a) concentrations the eddy than outside. The differences in photosynthetic rates between the three stations could be largely explained by the differences in relative growth rates and chl a biomass. In samples taken from below the top of the nitracline, dark assimilation of i4C increased calculated photosynthetic rates by 25-35%, the percentage increase being positively correlated with nitrate concentration. No comparable dark assimilation was observed in samples taken from above the nitracline. This dark assimilation may reflect the release and subsequent uptake of carbon assimilated by phytoplankton exposed to recently upwelled water. Copyright 0 1996 Elsevier Science Ltd

INTRODUCTION The oligotrophic realm constitutes more than 75% of the surface waters of the world’s oceans. Within this vast area nitrogen is generally agreed to be the nutrient that most limits primary production (Thomas, 1966, 1969, 1970a,b; Smith et al., 1986; Howarth, 1988). This realization has motivated numerous efforts to better understand nitrogen dynamics and the extent to which the growth rates and production of phytoplankton are limited by nitrogen in oligotrophic marine waters (MacIsaac and Dugdale, 1969; Eppley et al., 1971, 1973, 1977; McCarthy and Goldman, 1979; Glibert and McCarthy, 1984; Kanda et al., 1985, 1988; Sahlsten, 1987; Price and Harrison, 1988; Ward et al., 1989; Eppley and Koeve, 1990; Palenik and Morel, 1990; Harrison et al., 1992). A seminal paper on the subject of marine

* School of Ocean and Earth Science and Technology, University of Hawaii Oceanography Department, 1000 Pope Road, Honolulu, HI 96822, U.S.A. t Faculty of Liberal Arts and Sciences, Shizuoka University, 836 Ohya, Shizuoka-shi, Shizuoka 422, Japan. 917

918

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nitrogen dynamics is that of Dugdale and Goering (1967) in which the distinction is made between new primary production, which is associated with allochthonous sources of nitrogen, and regenerated primary production, which is associated with autochthonous or recycled nitrogen. In almost all cases field work has involved the use of “N-labeled substrates to follow the pathways and transformations of nitrogen species. Not infrequently the interpretation of the results of these studies has been confounded by one or more of the following problems: (i) isotope dilution of the substrate atom per cent due to recycling of substrate with the background isotope ratio (Glibert et al., 1982; Harrison, 1983; Harrison and Harris, 1986); (ii) difficulty in measuring the very low concentrations of substrate in oligotrophic ocean surface waters (Eppley et al., 1977); (iii) the need to include urea and amino acid uptake as well as ammonium uptake in the calculation of regenerated production (Eppley and Peterson, 1979; Palenik and Morel, 1990); (iv) artifacts created by the recycling of “N back into the substrate pool in long incubations (Laws et al., 1985); (v) bottle confinement problems including depletion of substrate in long incubations (Goldman et al., 1981); (vi) perturbations created by addition of “N-labeled substrates at concentrations which are not small compared to ambient concentrations (Glibert and Goldman, 1981); (vii) assimilation of added substrates by bacteria (Laws et al., 1985; Wheeler and Kirchman, 1986; Eppley and Renger, 1992; Harrison et al., 1992); and (viii) the unexplained disappearance of 15N from incubation bottles (Laws, 1984; Ward et al., 1989). Techniques developed during the past 12-l 3 years have enabled most, if not all, of these problems to be overcome or at least circumvented. Particularly important has been the chemiluminescent method for measuring nitrate concentrations developed by Garside (1982). The technique has a sensitivity of better than 1 nM with currently available instrumentation. With the use of Garside’s method nitrate concentrations in the surface waters of the oligotrophic ocean have been found to be roughly lo-20 nM (Garside, 1985; Chiswell et al., 1990) and hence are well above the limit of detection of the method. The mystery of the disappearance of 15N from incubation bottles appears to have been resolved by Bronk and Glibert (1994) and Bronk et al. (1994). They concluded that, “In oceanic, coastal, and estuarine environments, an average of 2541% of the dissolved inorganic nitrogen (NH4+ and N03-) taken up by phytoplankton is released as dissolved organic nitrogen (DON)” (Bronk et al., 1994, p. 1843). Problems associated with bottle confinement of microbial populations have been largely overcome with the use of clean sampling and incubation methods (Fitzwater et al., 1982; Williams and Robertson, 1989). Finally, the problem of accounting for all forms of phytoplankton nitrogen uptake and of distinguishing photosynthetic from heterotrophic nitrogen assimilation can be circumvented by measuring the rate of 14C-labeled inorganic carbon incorporation into protein (Ditullio and Laws, 1983). The present study was motivated in part by a desire to utilize these developments to reexamine the conventional wisdom concerning nitrogen utilization by phytoplankton in the oligotrophic ocean. The study took advantage of the presence of mesoscale eddies that are consistently found in the lee of the island of Hawaii in the Hawaiian Islands. The majority of the eddies, and those for which the most data have been gathered, are cyclonic features with diameters of 50-l 50 km characterized by divergent surface flows with resultant doming of the thermal structure and localized upwelling of nutrients into the euphotic zone (Patzert, 1969; Falkowski et al., 1991). Our study compared photosynthetic rates and phytoplankton nitrogen utilization within, near the perimeter of, and outside of one of these eddies.

New production

and photosynthetic

MATERIALS

rates of a mesoscale

919

eddy

AND METHODS

The presence and position of a cyclonic eddy in the lee of the island of Hawaii was initially determined from an aerial expendable bathythermograph (AXBT) survey conducted on 29 August 1989. The center of the eddy on that date was located at approximately 20”05’N, 156”3O’W(Fig. 1). The field studies were carried out from 6-12 September 1989, aboard the R.V. Kilu. Sampling was conducted along three transects as shown in Fig. 1. Measurements were made at selected stations for temperature and salinity using an Applied Microsciences model 12 conductivity-temperaturedepth (CTD) meter. These measurements were used to determine the approximate center and area1 extent of the eddy. The detailed biological studies reported here were carried out at Stas 2,6, and 10, with Sta. 2 being outside the eddy, Sta. 6 within but near the edge of the eddy, and Sta. 10 at the approximate center of the eddy. Samples of water at these three stations were collected about 1 h before dawn using a rosette of twelve 30-l Niskin bottles fitted with a Seacast model SBE-19 CTD meter. The original central rubber cord and o-rings in the Niskin bottles were replaced with silicone rubber equivalents as suggested by Williams and Robertson (1989). Duplicate Niskin bottles were tripped at each of six depths, which were estimated to correspond to visible light levels of lOO%, 33%, 15%, 8%, 1% and 0.1% of the flux of visible light quanta at the surface. A depth profile of the flux of visible light quanta was made at approximately noon on the same day using a LI-COR model 185-A submarine quantum sensor. Seven clear 4.5-l polycarbonate incubation bottles were filled with seawater from each depth. One of the bottles received no added isotopes. A second bottle was spiked with 3.7 x 10’ becquerels of NaHr4C03. K15N03 was added to the remaining five bottles in an amount sufficient to increase the nitrate concentration in most cases by 55 nM. For samples from the 0.1% light level the K1’N03 addition was either 250 nM (Stas 2 and 6) or 500 nM (Sta. 10). At Sta. 10 the K”NOs addition to the 1% light level bottle was IOOnM. The

156”W

I

I

Fig. 1. Surface isotherms derived from CTD profiles at Stas 2, 6, and 10 (closed circles) and at additional stations (open circles) during the period 6-12 September, 1989. The contour interval is 0.2x

920

C. B. Allen et al.

bottles were incubated in an on-deck incubator system which simulated both the intensity and spectral characteristics of the submarine light at the targeted light levels (Laws et al., 1990). Temperature in the incubator was maintained by a continuous flow of surface seawater. The bottles spiked with NaHi4C0s were sampled at sunset and again at the following sunrise. Triplicate 100 ml aliquots were filtered onto glass fiber (Whatman GF/F) filters and stored (- 20°C) in scintillation vials. The filters were later processed to determine the total assimilation of inorganic carbon and the percentage of the 14C incorporated into protein (Ditullio and Laws, 1983). The activity of the 14C spike in each bottle was checked by measuring the activity in a 1 ml aliquot which was added to a scintillation vial containing 0.2 measurements were made by ml of phenethylamine (Iverson et al., 1976). All radioactivity liquid scintillation counting using Aquasol (DuPont) as the fluor on a Searle Analytic Delta 300 liquid scintillation counter. Quench corrections were made using the external standard ratio. Activities were converted to assimilation rates using the equations in Strickland and Parsons (1972) assuming the specific alkalinity of the seawater to be 0.126 (Riley and Chester, 1971). Autotrophic nitrogen assimilation was estimated by dividing the protein carbon assimilation rate by 2.8 g C g-i N (Ditullio and Laws, 1986). The five bottles which received K1’N03 additions were processed in serial order at 0, 6, 12, 18, and 24 h after the start of the incubations at each simulated light level. The contents of entire bottles were used for the i5N assays in order to obtain sufficient particulate nitrogen and nitrate for mass spectrometric analysis. At each time point, 2 1 of water from the appropriate bottle from each depth were filtered through a precombusted (500°C) GF/F filter. The initial - 200 ml of the filtrate were used to rinse the collection flasks. Following this rinse, 125 ml and 1 1 aliquots of the filtrate was collected and frozen (-20°C) for later analysis of the nitrate concentrations and nitrate 15N atom per cent excess (APE), respectively. To facilitate mass spectrometric analysis, carrier KNOs was added to the l-1 filtrates from the bottles that had received 55 nM spikes at Stas 2 and 6 in an amount sufficient to increase the nitrate concentration by 500 nM. A similar amount of carrier K14N03 was added to the 1 I filtrate from the surface water sample at Sta. 10. After 2 1 of water had been filtered, the filters were rinsed with 0.2 pm-filtered surface seawater and frozen for later determination of the particulate nitrogen 15N APE. The bottles that received no additions were sampled at 0,6, 12, 18, and 24 h after the start of the incubations to determine the time course of nitrate concentrations. For the nitrate analyses, 150 ml aliquots were taken from each bottle and filtered through precombusted GF/F filters. The first 25 ml of the filtrate was used to rinse the collection flasks. The remaining 125 ml was retained and frozen for later analysis. Nitrate analyses were carried out using the chemiluminescent procedure of Garside (1982) with an Antek model 703C nitrogen oxides analyzer. The instrument was modified slightly to maximize its sensitivity and the reproducibility of results. Modifications included the addition of a vacuum line to the system and two external needle valves to balance gas flow. The internal line constrictors provided with the instrument gave poor control of flow rate. Balancing the pressure in the system between the vacuum and the argon flow proved critical to the performance of the instrument. Sulfanilamide was added to each sample as described by Garside (1982) to prevent the conversion of nitrite to NO. Hence the nitrate concentrations reported here are indeed nitrate concentrations and not nitrate plus nitrite. The amount of 15N in the nitrate in bottles spiked with K”NOs was determined using an extraction procedure adapted from Olson (198 1) and Horrigan and Capone (1985). Briefly,

New production

and photosynthetic

rates of a mesoscale

eddy

921

the nitrate was reduced to nitrite using cadmium
(1)

where PN is the concentration of particulate nitrogen and APEPN and APEN~, are the atom per cent excess of the PN and nitrate, respectively (Glibert et al., 1982). Samples for total chlorophyll a (chl a) analysis were taken at the start of the incubations by filtering several liters of water through a 0.2 pm Nuclepore filter. The chl a was extracted in 90% aqueous acetone overnight at -20°C. The chl a concentrations were determined fluorometrically before and after acidification following the procedures of Holm-Hansen and Riemann (1978). Picoplankton chl a was determined from the difference between the chl a retained on 0.2 and 2.0 pm Nuclepore filters. Area1 production rates were calculated by interpolating the chl a concentrations and visible light quantum flux data to l-m depth intervals. In both cases the dependent variables were converted to logarithms prior to interpolation. The interpolated logarithms were then exponentiated to obtain the interpolated functions (Fig. 2(A)). Productivity indices (g C g- ’chl a day-‘) were interpolated as a function of light intensity in a similar manner using the productivity index (PI) versus light intensity results from the on-deck incubations (Fig. 2(B)). The photosynthetic rates at l-m depth intervals were then calculated from the product of PI(Z(z)) and chl a(z), where PI(Z) is the PI at light intensity Z, Z(z) is the light intensity at depth z, and chl a(z) is the chl a concentration at depth z. The photosynthetic rates so calculated were integrated from the surface to the base of the euphotic zone (0.1% light level) using the trapezoid rule (Hornbeck, 1975). Autotrophic nitrogen assimilation rates were calculated in a similar manner by first interpolating to l-m depth intervals the phytoplankton C:N ratios (Fig. 2(C)) determined from the percentage of 14C incorporated into protein (Ditullio and Laws, 1986). The interpolated photosynthetic rates were then divided by the corresponding C:N ratios and the autotrophic nitrogen assimilation so estimated was integrated using the trapezoid rule. Area1 nitrate assimilation rates were calculated by interpolating the f-ratios (nitrate assimilation divided by autotrophic N assimilation), multiplying the interpolated f-ratios (Fig. 2(D)) by the corresponding autotrophic N assimilation), and then integrating using the trapezoid rule. RESULTS Nitrate

concentrations

The median nitrate concentration above the nitracline at Stas 2 and 6 was only 8 nM, and all values above the nitracline were less than 22 nM (Fig. 3). The nitracline began at a depth

922

C. B. Allen et al.

Pls at station 2 chl a at station 6

m

0 0

50 Light intensity (% surface)

mg chllm3

100

D

t

!,I;:

,.,,i5:, 150

m f-ratio at station 10

I

6

7

8

/

9

,501 -6.7

0.8

0.9

1

Q C/Q N Fig. 2. (A) Chlorophyll a concentrations versus depth at Sta. 6. The smooth curve is the interpolated function obtained from a cubic spline (Hornbeck, 1975) fit as described in the text. The asterisks are the measured values. (B) Productivity indices versus light intensity obtained from the on-deck incubations at Sta. 2. The smooth curve is again the interpolated function, and the asterisks are the experimental data. (C) Phytoplankton C:N ratios as a function of depth at Sta. 2 estimated from the percentage of 14C incorporated into protein (Ditullio and Laws, 1983). The smooth curve is the interpolated function, and the asterisks are the experimental data. (D)f-ratio versus depth at Sta. 10. The piecewise-linear curve is the linear interpolation to the experimental data, which are indicated by asterisks.

of roughly 80m and 45 m at Stas 2 and 6, respectively. At Sta. 10 elevated nitrate concentrations were apparent throughout the euphotic zone, the lowest measured value being 51 nM at a depth of 9 m. Nitrate concentrations below a depth of about 100 m exceeded 100 nM at all three stations. “N recovery Recovery of 15N in particulate nitrogen plus nitrate declined over time in incubation bottles which contained less than 100 nM nitrate prior to addition of the 55 nM K15N03 spike. The total loss of 15N amounted to about 10 nM or 20% of the initial spike over the course of the 24 h incubation (Fig. 4(A)). Most of the loss occurred during the first 12 h, the loss during the final 12 h amounting to only about 2 nM. The remaining incubation bottles contained more than 350 nM nitrate prior to the addition of the K”N03 spike, and in those bottles the loss of “N amounted to only 2-3% after 24 h (Fig. 4(B)).

923

New production and photosynthetic rates of a mesoscale eddy

150-

'=

station 2

x = station 6 0 = station 10

' IO0

200

I

I

I

I

IO'

IO2

lo3

IO'

Log nitrate concentration (nM) Fig. 3.

Nitrate concentrations versus depth at the three stations in Fig. 1.

Perturbations caused by addition of jSN spike In unspiked bottles that initially contained less than 22 nM nitrate, there was no significant drawdown of nitrate during the course of the 24 h incubation (t-test, p > 0.05). In the bottles which contained less than 100 nM nitrate prior to the addition of the K15N03 spike, the addition of the 55 nM spike stimulated a drawdown of nitrate which exceeded the decrease of nitrate in the corresponding unspiked bottles by a median value of 24 nM (95% confidence interval; 13.1-39.4 nM) after 24 h. In the bottles that initially contained greater than 350 nM nitrate, there was no comparable statistically significant stimulation of nitrate uptake by addition of the nitrate spikes (t-test, p > 0.1). Isotope dilution and nitrification Nitrification of reduced forms of nitrogen with the background APE of 0.365 would reduce the APE of the nitrate in our bottles during the course of the incubations. Calculation of nitrate assimilation would require a correction for this effect (Laws, 1984). Furthermore, the rate of phytoplankton nitrate assimilation would then overestimate new production to the extent that some of the nitrate was derived from nitrogen recycled within the euphotic zone. Nitrification rates in oligotrophic open-ocean waters have been reported as high as lOrates of this magnitude 70nM day-’ (Kaplan, 1983; Eppley et al., 1990). Nitrification

924

C. B. Allen et al. 100 /

loo\-

B 99.5

99 = ,m ‘ .-; 96.5 ‘;i

Initial nitrate

greater than 350 nM

I

in unspiked bottles

c 96 5 z z 97.5

/

!! 2 ”

97

5 j 96.5 5 96

95.5

75’ 0

Fig. 4.

95L 0

5 10 15 20 Hours since sunrise

5 10 15 20 Hours since sunrise

Recovery of lSN in particulate nitrogen plus nitrate in bottles which contained less than (A) or more than (B) 100 nM nitrate prior to the addition of the K”N03 spike.

would have little effect on the APE of the nitrate in bottles containing in excess of 1 PM nitrate, but in one day a nitrification rate of even 10 nM day-’ could reduce by a factor of 2 or more the APE of the nitrate in water initially containing only 10 nM nitrate. Table 1 shows the results of comparing the APE of the nitrate after both 12 and 24 h to the initial APE for samples initially containing greater than 350 nM nitrate and less than 80 nM nitrate, There was no significant evidence of a reduction in the APE for either group of samples after either 12 or 24 h. As explained below, we used the incorporation of 15N into

Table 1. Median +95% confidence interval (Cl) of ratios of nitrate APE after 12 h and 24 h to rhe initial nitrate APE in eight incubation series initially containing greater than 350 nM nitrate and nine initially containing less than 80 nM nitrate* Initial nitrate (nM) >350 t80

APE( 12)/APE(O) Median 95% CI 0.995

1.03

0.98-1.00 0.8c1.13

APE(24)/APE(O) 95% CI Median 0.985 0.97

*One series with an initial nitrate concentration of 88 nM was excluded analysis because of analytical problems with the APE measurements.

0.95-I .Ol 0.86-1.33 from the

New production and photosynthetic rates of a mesoscale eddy

925

particulate nitrogen to calculate phytoplankton nitrate assimilation only for samples that initially contained greater than 1100 nM nitrate. For those samples we could clearly ignore isotope dilution effects in our calculations. Calculation

of nitrate uptake

For 11 samples that initially contained less than 360 nM nitrate, we used the change in nitrate concentration after 24 h to calculate nitrate uptake. Ten of those samples initially contained less than 100 nM nitrate. The rationale for ignoring the “N results for these 10 samples was the perturbation caused by the addition of the 55 nM nitrate spike. The initial nitrate concentration in the 11th sample, 356 nM, was large enough that the spike should not have been a problem. The concentration changes associated with nitrate uptake, however, were expected to be roughly lo-20 nM after 24 h, or in this case about 5% of the initial nitrate value. Such a change is well within the detection limits of the chemiluminescent method. The initial nitrate concentrations in the other seven samples exceeded 1100 nM, and in these cases nitrate assimilation was calculated from the i5N results. The rationale for using the i5N results was two-fold. First, there was no perturbation effect from the nitrate spike. Second, to measure a concentration change of 10 nM to two significant figures with the chemiluminescent method when the nitrate concentrations are - 1000 nM requires that one be able to inject into the nitrogen oxides analyzer a sample of water whose volume or weight is known to four significant figures. By carefully weighing our syringe before and after drawing a sample, we were able to obtain an analytical precision (standard deviation) of + 5 nM in the concentration range 1000-2000 nM. In the concentration range 20005000 nM the precision was f 10 nM. Since the nitrate concentrations in six of the seven high-nitrate samples exceeded 2000nM, we did not feel that changes in nitrate concentration could be expected to give accurate nitrate uptake rates in these cases. In interpreting our results we have assumed that nitrate uptake and assimilation were identical in bottles which received no nitrate spike and in bottles spiked with Ki5N0s when the initial nitrate concentration exceeded 1100 nM. Areal biomass andproduction

Table 2 shows the area1 chl a biomass, photosynthetic rates, autotrophic nitrogen assimilation, and nitrate uptake at the three stations. There was only about a 25% difference in the chl a biomass outside and at the center of the eddy, but the vertical distribution of the biomass was quite different. At Sta. 2 the upper 75 m of the water column accounted for only 38% of the chl a. The corresponding figures at Stas 6 and 10 were 48% and 70%. Thus there had been a significant upward movement of phytoplankton in the water column at the center of the eddy and to a lesser extent near the edge of the eddy. The area1 photosynthetic rate was about 66% higher at Sta. 10 than at Sta. 2, with Sta. 6 being intermediate. At Stas 6 and 10 about 60% of the photosynthesis occurred below 75 m, while at Sta. 2 only 28% of photosynthesis occurred below the same depth. Autotrophic nitrogen assimilation was about twice as high inside the eddy as outside. About 213 of the autotrophic nitrogen assimilation occurred below 75 m at Stas 6 and 10, while at Sta. 2 about 2/3 occurred above 75 m. Nitrate uptake was about nine times higher at Sta. 10 than at Sta. 2, with Sta. 6 again being intermediate. At all three stations most of the nitrate uptake occurred below 75 m, but at Stas 6 and 10 about 30--40% occurred above 75 m. At Sta. 2 only about 7% of nitrate

926

C. B. Allen et al.

Table 2. Biomass and production characteristics integrated to the depth of the 0.1% light level and the percentage contribution from the water column above 75 m. The depth of the 0.1% light level was 136 m, 125 m, and 113m at Stas 2, 6, and 10, respectiveI) Station Chl a (mg m-‘) % above 75 m Photosynthesis (mg C me2 day-‘) % above 75 m Autotrophic nitrogen assimilation (mg N mm2 day-‘) % above 75 m Nitrate uptake (mg N rnp2 day- ‘) % above 75 m ,f-ratio

uptake occurred was about 0.2.

Phytoplankton

above 75 m. Thef-ratio

2

15.5 38 399 72 51 72 10 7.3 0.20

Station

6

16.3 48 523 38 98 36 19 27 0.19

Station

10

20.4 70 662 39 110 32 89 38 0.81

at Sta. 10 was about 0.8. At Stas 2 and 6 thef-ratio

C:N ratios and relative growth rates

Table 3 shows phytoplankton C:N ratios and relative growth rates (Goldman, 1980; Ditullio and Laws, 1986) calculated for the entire euphotic zone and for the euphotic zone above and below 75 m. The C:N ratios were consistently smaller in the lower part of the euphotic zone (see also Fig. 2(C)). There was no evidence of severe nutrient limitation even outside the eddy. The lowest relative growth rate was 64% in the upper 75 m of the euphotic zone at Sta. 2. Growth rates appeared to be nutrient saturated throughout the euphotic zone at Sta. 6 and below 75 m at Sta. 10.

Table 3. Phytoplankton C:Nratios and relativegrowth rates* estimated ,from thepercentage ofassimilated ‘? allocated to protein carbon. Values have been calculatedfor the entire euphotic :one andfor the ecrphotic zone above and below 75 m Station C:N(gCg-‘N) above 75 m below 75 m Relative growth above 75 m below 75 m

rate

7.7 7.9 7.2 67% 64% 75%

2

Station

6

5.3 5.6 5.2 100%

100% 100%

Station

10

6.0 7.3 5.4 100% 73% 100%

*Relative growth rates (p,.) were calculated using the equation C:N = S/(0.5 +p,.), where C:N is the C:N ratio in g C g-’ N (Ditullio and Laws, 1986). A C:N ratio less than 6 implies !tr = 1.

New production and photosynthetic rates of a mesoscale eddy

927

DISCUSSION Photosynthetic

rates

The photosynthetic rates in Table 2 are all within the range of values reported from the time-series monitoring work at Station ALOHA, 100 km north of Oahu. During the period 1989-1990 the photosynthetic rates at that station averaged 380f 92 mg C m-’ day-’ (mean + standard deviation, n= 17) (Karl et al., 1995). Since the monitoring work at Station ALOHA began in December 1988 photosynthetic rates have averaged 477 f 163 mg C mm2 day-’ (n=43) with a range of 219-1055 mg C m-’ day-’ (L. Tupas, personal communication). The differences in production between our three stations seem largely attributable to changes in phytoplankton biomass and relative growth rate. The ratio of chl a concentrations at Stas 10 and 6, for example, is 1.24. The ratio of the corresponding photosynthetic rates is 1.27. Relative growth rates at the two stations were similar, particularly below 75 m, where about 60% of the photosynthesis occurred. The ratio of relative growth rates at Stas 2 and 6 is 0.67. The ratio of the corresponding photosynthetic rates is 0.76. Area1 chl a concentrations at the two stations were very similar. Falkowski et al. (1991) estimated photosynthetic rates at Stas 6 and 10 using a pump-andprobe fluorometer. Their estimate for Sta. 10, 657 mg C mm2 day-‘, is virtually identical to our value of 662 mg C me2 day-‘. The agreement is satisfying but probably fortuitous, since the two estimates are not strictly comparable. The pump-and-probe method estimates the instantaneous rate of gross photosynthesis, and Falkowski et al. (1991) integrated to the 1% rather than the 0.1% light level. At Sta. 6, Falkowski et al.‘s (1991) fluorescence-based area1 production estimate was only 173 mg C me2 day- ‘. Such a low rate seems hard to reconcile with our estimate of 523 mg C mm2 day-’ or with the Station ALOHA results, none of which lies below 219 mg C m-’ day-’ . The explanation for the discrepancy appears to be two-fold. First, a nontrivial amount of photosynthetic production occurs below the 1% light level. Venrick et af. (1973) have estimated that photosynthetic production below the 1% light level in the central North Pacific averages about 12% of total integral production. Had we integrated production rates at Sta. 6 to the 1% light level, the area1 production rate would have been reduced by 8%. Second, and more importantly, Falkowski et a/.‘s profile was taken later in the day. Their integrated chl a value at Sta. 6 is only 8.5 mg mp2. Clearly they did not sample the same Sta. 6 that we sampled. Their area1 chl a estimate at Sta. 10, 20.5 mg m -2, is virtually identical to ours. Nitrate

uptake and new production

The nitrate concentrations we encountered at Sta. 2 appear to be very typical of the oligotrophic ocean. Eppley et al. (1990) for example, found surface water nitrate concentrations in the tropical Pacific and subtropical Atlantic that averaged 8.4 and 69.5 nM, respectively, and noted that there was roughly a linear relationship between nitrate uptake and nitrate concentration at nitrate concentrations less than 50 nM. The implication of Eppley et al.‘s (1990) analysis and the results presented here indicate that addition of 50 nM nitrate spikes to typical oligotrophic ocean surface waters constitutes a major perturbation and that uptake rates based on such additions will not be indicative of in situ nitrate uptake. As a check on this conclusion, we calculated nitrate assimilation rates from the incorporation of 15N into particulate nitrogen for the six samples that had initial nitrate

C. B. Allen et al.

928

concentrations less than 22 nM. A time series of the results is shown in Fig. 5(A). Based on the incorporation of 15N into particulate nitrogen, little nitrate assimilation was apparent during the first 12 h of the incubations. During the subsequent 12 h, however, nitrate assimilation increased to about 20 nM. This calculated assimilation rate is comparable to the stimulated drawdown of about 24 nM due to the addition of the 55 nM K”NOs spike. In addition, the temporal pattern of assimilation is very improbable. According to Fig. 5(A), over 90% of the nitrate assimilation occurred during the night. In the samples that initially contained more than 350nM nitrate, most nitrate assimilation occurred during the photoperiod (Fig. 5(B)). The explanation, we believe, for the results in Fig. 5(A) is that the phytoplankton in these samples were not assimilating nitrate and perhaps lacked the ability to reduce nitrate to ammonium. To check this hypothesis, we have plotted in Fig. 5(A) the uptake of nitrate based on the drawdown of nitrate in the incubation bottles. Nitrate was taken up during both the day and night in these samples, and at a somewhat higher rate during the day than during the night. The fact that the nitrate did not appear as particulate nitrogen implies that it was excreted in a form other than nitrate, possibly nitrite or dissolved organic nitrogen (DON). Laws and Wong (1978) for example, observed that rapid uptake of nitrate in response to a nitrate spike was accompanied by nitrite excretion

100 B

A Samples

/

containing

3

/

2 2

/

< 22 nM nitrate initially

90

/

60.

c

/ /

z %

70-

ae ‘E 60 .e I s = E 50 ‘5 : t Samples containing

> 350 nM nitrate

initially

i

16 6 12 Time since sunrlse (h)

24

~0

6 12 16 Time since sunrise (h)

24

Fig. 5. (A) Time series ofmedian nitrate uptake (dashdot) and assimilation (solid lines) in six cases in which initial nitrate concentrations were less than 22 nM. In each case the bottles were spiked with 55 nM of K’5N03. Median assimilation after 24 h was 20 nM (range: 4-96 nM). (B) Time series of median nitrate assimilation in eight cases in which initial nitrate concentrations were greater than 350 nM. Median assimilation after 24 h was 5 nM (range: 2-32 nM).

New production

and photosynthetic

rates of a mesoscale

eddy

929

when cells were grown in nitrate-limited continuous cultures. Nitrite excretion reduced the net assimilation of nitrate spikes in the dark by Pavlova lutheri and Dunaliella tertiolecta by 79% and 23%, respectively. Bronk and Glibert (1994) and Bronk et al. (1994) have demonstrated that uptake of a nitrate spike may also be accompanied by substantial excretion of D0i5N (see below). In their seminal paper on new production, Eppley and Peterson (1979) state that thefratio for the central North Pacific is 0.05. The basis for this statement is the data set in Eppley et al. (1977) covering five cruises in which the assimilation rates of “N-labeled ammonium, urea, and nitrate were measured in the upper euphotic zone of the North Pacific Ocean. The “N-labeled substrates were added to incubation bottles at a concentration of 100 nM and incubations carried out for 24 h. The depth of the top of the nutricline was in all cases greater than the depth of the 1% light level, and at the upper four of the six depths sampled by Eppley et al. (1977) the nitrate concentrations were only about 10 nM in most cases. The reported ammonium and urea concentrations averaged 340 nM and 260 nM, respectively, over the range of depths studied. Based on the data presented here and elsewhere (Eppley and Koeve, 1990), it seems doubtful whether the nitrate assimilation rates reported by Eppley et al. (1977) are to be considered representative of nitrate uptake in the euphotic zone of the North Pacific Ocean. First, the 15N-labeled nitrate spikes almost certainly constituted a perturbation that altered ambient uptake rates. Recognizing this fact, Eppley et al. (1977) reported their nitrate assimilation rates as “potential” rates. Second, it seems probable from our work and that of Eppley and Koeve (1990) that most nitrate assimilation in the oligotrophic ocean occurs within a depth range of perhaps a few tens of meters near the top of the nitracline between the 0.1 and 1% light levels. This region was apparently not sampled by Eppley et al. (1977). Knauer et al. (1984) reached a similar conclusion concerning the locus of new production based on an analysis of sediment trap data at VERTEX stations in mesotrophic North Pacific waters off Mexico (VERTEX 2) and in oligotrophic Northeast Pacific waters (VERTEX 5a). They “could not resolve carbon flux at either VERTEX 2 or 5a at the 50 m trap depth” and concluded “that new production in this layer must be relatively low”. They postulated that “in terms of particle flux the North Pacific oligotrophic euphotic zone may function as two reasonably distinct systems; an upper layer whose new production characteristics are representative of classical values (i.e. in the area of lo%, Eppley and Peterson, 1979) and a lower layer with new production characteristics more in line with neritic waters (i.e. in the area of 40%)“. If many of the new production estimates based on the use of i5N-labeled nitrate are misleading due to perturbations caused by the addition of nontracer amounts of substrates, what reliable information is there to estimate new production and f-ratios in the oligotrophic open ocean? Eppley and Koeve (1990) estimated nitrate utilization at a total of 16 stations in the eastern subtropical Atlantic Ocean based on changes in nitrate concentration measured using Garside’s (1982) chemiluminescent technique. Mean ( f sd) utilization rates at stations with low euphotic zone nitrate concentrations were 6.6+6.0mgNm-* during the day (n= 8) and 5.2k3.3 mg Nm-* at night (n= 3). When their mean rates for the day and night at these stations are summed, the result, 11.8mgNm -*, is comparable to our daily nitrate uptake rate of 10 mg N rnp2 day- ’ at Sta. 2. Laws et al. (1989) measured the fallout of particulate nitrogen in cylindrical sediment traps at a depth of 120 m at 26”N, 155”W in the North Pacific subtropical gyre. Their mean

930

C. B. Allen et al.

and median nitrogen fluxes were 12 + 6 and 8 mg N mm2 day- ‘, again very comparable to our result at Sta. 2. Lewis et al. (1986) estimated nitrate assimilation with the use of 15N techniques at a station in the oligotrophic eastern Atlantic where nutrient concentrations measured by traditional calorimetric techniques were below the limit of detection along near-surface isopycnals. Their near-surface rates are therefore suspect because of perturbation effects. However, between 80 and 180 m, the upper portion of the nitracline, integral nitrate assimilation estimated from their Fig. 2 was about 1.0mgNmw2 hh’. Assuming that assimilation continued for 12 h each day, this rate becomes 12mgNm-*day-’ , very similar to our value of nitrate uptake at Sta. 2. The upward diffusion of nitrate into the euphotic zone at Sta. 2 can be estimated from the nitrate concentration profile. There was a linear relationship between the logarithm of nitrate concentration and depth between 62 and 132 m at this station (Fig. 3) i.e. = A + BZ

ln(N0;) where Z is the depth.

Differentiating

equation

(4

(2) with respect to Z gives

-p-1 d(NO;)_ NO; dZ

B

(3)

A least squares fit of our nitrate concentration data at Sta. 2 between 62 and 132 m gives A = - 4.4 and B = 0.092 m- ’ At 136 m, the 0.1% light level at Sta. 2, these values of A and B give

NO;

= 3261 nM

= 3261 pmol m-’

and

d(Tg7)

= (3261)(0.092)

= 300 pmol m-‘.

Multiplying this gradient by a vertical eddy diffusion coefficient of 3.2 m* day- ’ (Lewis et 1986) gives an upward nitrate flux of 13.4 mg m-* day- I, again similar to our estimated nitrate uptake of 10 mg N m-’ day-‘. The agreement between these various estimates of nitrate uptake and new production in the oligotrophic ocean is undoubtedly somewhat fortuitous, since there is considerable scatter in the data, and the temporal variability is undoubtedly real (Platt and Harrison, 1985). Nevertheless, the numbers seem to be converging on an average figure of 1O-l 5 mg N m-’ day- ’ for new production. The f-ratio associated with new production appears to be at least as variable as new production itself. Our estimate of 0.20 at Sta. 2 is almost identical to the mean f-ratio calculated by Laws et al. (1989) from sediment trap data, but the range of Laws et al.‘s (1989)f-ratios is 0.10-0.32. Platt and Harrison (1985) calculated monthlyf-ratios ranging from 0.03 to 0.53 at Sta. S, the annual mean being 0.31. One obvious problem in estimating f-ratios from nitrate uptake at oligotrophic stations is the fact that most of the nitrate uptake occurs within a region of a few tens of meters at the top of the nitracline. This is not a region where more than two samples are likely to be taken for purposes of measuring area1 photosynthetic rates. Traditionally water samples are taken from 5 to 6 depths within the euphotic zone for purposes of estimating area1 production, and at least three of those samples are likely to be taken from above the 1% light level. This is a reasonable sampling design for estimating area1 production, but it is marginally adequate for estimating nitrate uptake. Our sampling design at Sta. 2 suffers from this problem. We detected nitrate uptake in only the two deepest samples. Our estimate of nitrate uptake at Sta. 2 would obviously be more precise if we had more samples from below the top of the nitracline. Our nitrate uptake and.f-ratio estimates at Stas 6 and 10 do not suffer from the same problem, due to the shoaling of the nitracline. Nitrate uptake was detectable at the four deepest samples at Sta. 6 and at all depths at Sta. 10. The f-ratio of 0.81 at Sta. 10 is al.,

New production and photosynthetic rates of a mesoscale eddy

931

noteworthy, since it is virtually identical to the asymptotic&ratio of 0.83 for high nitrate concentrations predicted by the empirical model of Platt and Harrison (1985). Loss qf “N

The fact that almost all the disappearance of i5N in Fig. 4(A) occurred during the first 12 h of the incubations, when nitrate uptake was most rapid (Fig. 5(A)) is consistent with the conclusions of Bronk and Glibert (1994) and Bronk et al. (1994) that the mechanism responsible for the loss is uptake by phytoplankton and subsequent excretion in some form other than nitrate. The fact that no comparable loss was observed in samples that contained greater than 350 nM nitrate (Fig. 4(B)) does not prove that uptake and excretion of “Nlabeled nitrate did not occur. In these high-nitrate samples, uptake after 24 h accounted for only a small percentage of the ambient nitrate. Hence only a small percentage of the K”N03 was taken up. Even if a substantial percentage of the “N taken up had subsequently been excreted, the overall 15N balance would have been good, because most of the added 15N remained as nitrate. Loss of 15N will therefore be detectable only when uptake removes a substantial percentage of the nitrate from the water. Where the concentration of phytoplankton is low, this can happen only when the ambient nitrate concentration is also low, and in such cases a 50 nM spike of nitrate will be a significant perturbation to the system. It is possible, therefore, that the excretion of “N during the first 12 h is merely a response to this perturbation. Nitrification

and heterotrophic

uptake of nitrate

The interpretation of nitrate assimilation as new production sensu Dugdale and Goering (1967) requires that other allochthonous sources of nitrogen are negligible and that heterotrophic bacteria account for a negligible percentage of nitrate assimilation. Although heterotrophic bacteria are capable of utilizing nitrate as a nitrogen source (Horrigan et al., 1988) several studies (Probyn, 1985; Probyn and Painting, 1985; Wheeler and Kirchman, 1986) have indicated that heterotrophic bacteria do not account for a significant fraction of nitrate uptake in the open ocean. These conclusions have been based on the use of metabolic inhibitors and the low percentage of nitrate assimilation accounted for by organisms that pass a 1 pm filter. The fact that assimilation of “N-labeled nitrate was apparent only during the daylight hours in unperturbed samples (Fig. 5(B)) suggests that most of the nitrate assimilation was due to autotrophic organisms. At Sta. 10 we carried out size fractionation studies after both 12 and 24 h. The < 1 pm fraction accounted for a median of 51% and 50% of the 15N assimilation after 12 and 24 h, respectively. If heterotrophic bacteria had been responsible for a significant part of this assimilation, it is likely that the percentage accounted for by the < 1 pm fraction would have increased during the night. In fact there was virtually no change in this percentage. Size fractionation of the chl a at Sta. 10 revealed that 69% (range: 57-79%) of the chl a fell in the picoplankton (0.2-2.0 mm) category. If heterotrophic bacteria were responsible for a significant fraction of the nitrate assimilation in the < 1.Opm fraction, one would expect the ratio of nitrate assimilation to chl a to be higher in this fraction than in the > 1.Opm fraction. This appears not to have been the case, assuming that the l-2 pm fraction accounted for no more than 18-19% of the chl a. Nitrification in the euphotic zone has been inferred in several studies based on “N-nitrite oxidation, isotope dilution of added “N-nitrate, or increases in nitrate concentrations

932

C. B. Allen et al.

during incubations (Ward et al., 1989; Eppley et al., 1990). Because of the photoinhibition of nitrite oxidation (Olson, 1981) nitrification is likely to be important only at night or in the lower part of the euphotic zone. Since, in our studies, there appeared to be little assimilation of nitrate during the night (Fig. 5(B)), nitrification during the night should have been easily detectable from an examination of our time series of nitrate concentrations. In fact, there was no evidence of a significant increase in nitrate during the night in our experiments. The median increase from all 18 time series was 1 nM (95% confidence interval = -21.5 to 8.5 nM). In the six time series from the 0.1 and 1% light levels, three showed an increase in nitrate concentration over the course of the incubations and three showed a decrease. Thus there is no evidence from this study of significant nitrification or bacterial utilization of nitrate within the euphotic zone. Release of dissolved organic carbon Barber et al. (1971) and Barber (1973) have discussed the impact of recently upwelled water on phytoplankton growth. They noted that rapid growth of phytoplankton in recently upwelled water sometimes requires the addition of dissolved organic compounds analogous to EDTA. These compounds, presumably excreted by the phytoplankton themselves, appeared to condition the water by increasing the mobility of essential metals or possibly complexing toxic metals. The need for such conditioning was most apparent when the initial inoculum of phytoplankton was low. Since phytoplankton biomass in the waters surrounding the Hawaiian Islands is low, it occurred to us that a similar conditioning might occur in the eddy we studied. To examine this question, we compared photosynthetic rates based on the accumulation of 14C in particulate matter after 12 and 24 h. It is common for particulate 14C activity to decline in the dark due to respiration and excretion. Dark loss rates reported from studies in the North Pacific gyre range from 14% to 45% (Eppley and i4C activity did Sharp, 1975; Laws et al., 1987, 1989, 1990). In our studies, particulate decline during the night in the six samples taken from above the nitracline (initial nitrate < 22 nM). The median dark loss was 9%. However, in five samples that initially contained 46-356 nM nitrate, there was actually a 25% increase in particulate i4C activity during the night, and in the seven samples that initially contained more than 1.1 PM nitrate, there was a 35% increase in particulate 14C activity during the night. Observations of 14C assimilation during the night are not uncommon in low latitude open ocean waters and towards the base of the euphotic zone (Saijo and Takesue, 1965; Morris et al., 1971; Taguchi, 1983; Taguchi et al., 1988). This characteristic is especially common in tropical and subtropical plankton less than 1.O pm in size (Li et al., 1983; Herbland et al., 1985). Although nonphotosynthetic assimilation of inorganic 14C could account for some of 14C activity during the night, we are sceptical that the increase of particulate nonphotosynthetic assimilation would increase particulate i4C activity by 25-35%. Anaplerotic COZ uptake is believed to account for only about 6% of carbon metabolism (Romanenko et al., 1972) and it seems doubtful that chemosynthesis would be a significant source of fixed carbon in the oceans’ surface waters. A more likely result, we believe, is that labeled dissolved organic carbon excreted by the phytoplankton or released as a result of the photoperiod is assimilated by grazing activities (Jumars et al., 1989) during microorganisms at night. The excretion hypothesis is consistent with the requirement for conditioning of recently upwelled water lacking a sufficient concentration of metal chelators. If phytoplankton react to such water by excreting dissolved organic matter, it

Newproductionand photosyntheticratesof a mesoscaleeddy

933

would not be surprising to observe microbial uptake of some or perhaps most of the excreted compounds. The implication is that phytoplankton exposed to recently upwelled water may be stressed and that they respond by excreting dissolved organic matter, only a fraction of which may be effective in conditioning the water. This may explain why observations of dark 14C assimilation are most common near the base of the euphotic zone, i.e. below the top of the nutricline. Acknoa,ledgemenrs-This work was supported by National Science Foundation Biological Oceanography Program grant 0CE88-000033. We gratefully acknowledge the technical assistance of Mr David Hashimoto and the help of Dr P. Bienfang, Dr D. Ziemann, L. Conquest, and Dr M. Olaizola of the Oceanic Institute and Dr Z. Kolber of Brookhaven National Laboratory, who also participated in this cruise, and the captain and crew of the R.V. Kilo. We are also indebted to two anonymous referees for their constructive comments. School of Ocean and Earth Science and Technology Contribution No. 3973.

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