Production and respiration in the 1989 North Atlantic spring bloom: an analysis of irradiance-dependent changes

El;&er Saence Ltd Pnnted in Great Emtam 0’)h74617/‘)5 $Y, 50 + (I (0

Pergamon

Production and respiration in the 1989 North Atlantic spring bloom: an analysis of irradiance-dependent changes JOHN KIDDON,*$ MICHAEI. L. BENDER* and JOHN MARRA?

(Received

24 September

1992: in revitedform

25 May

1993; occeppred 26 May

1993)

Abstract-Gross and net OZ production rates at 47”N, 2O”W over 13 days during the 1989 JGOFS North Atlantic (Spring) Bloom Experiment were measured. Gross a_ production was measured by HZ’s0 uptake or calculated from 14C assimilation, and net 0, production was measured by Winkler titration. Production versus irradiance p(I) curves were constructed from gross Q production normalized to chlorophyll a concentration for the five rates (determined with in situ incubations) days of highest total irradiance. Magnitudes of P,:. CLand /3 wcrc high during the bloom. Chlorophyll-normalized gross O2 production, integrated over the euphotic zone, was observed to be linearly related to integrated incident irradiance. This linear trend can be simulated with an 24 h Q respiration rates for each day algorithm using average values of P:!. u and p parameters. appeared to consist of two components: one proportional to the production rate and involving respiration of carbon fixed during the same day’s photoperiod, and the other independent of the production rate and respired carbon fixed prior to the day’s photoperiod. Integrated over time and depth, these respiration components were of comparable magnitude. and together equalled about 60% of gross O2 production. POC turnover times ranged from two days for near-surface waters up to about two weeks at the base of the euphotlc zone.

INTRODUCTION THE general character of a spring bloom in the North Atlantic is well known: activity in the latent phytoplankton community dramatically increases in response to springtime conditions of abundant nutrients, increasing light intensity and developing water column stability. The downward flux of organic carbon produced during the bloom is an important fraction of the annual biological carbon fluxes in an otherwise oligotrophic, temperate sea. Carbon and nutrient fluxes associated with blooms therefore play a particularly important role in fixing the distribution of bioactive tracers in the sea, in providing nourishment to abyssal and benthic organisms, and in forming the sedimentary paleoproductivity record (for instance, DUCKLOW, 1989). The Joint Global Ocean Flux Study (JGOFS) North Atlantic Bloom Experiment (NABE) was conducted to establish the fluxes of carbon, nutrients and other bioactive elements during the spring 1989 bloom occurring at temperate latitudes. The authors’ contribution to the experiment was to measure net and gross O2 production and 14C assimilation rates in the euphotic zone during the first two weeks of the bloom. These data are augmented by determination of irradiance and many related variables measured by

*Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882, U.S.A. ;Lamont-Doherty Geological Observatory of Columbia Unicersity, Palisades, NY 10964, U.S.A. *Present address: Environmental Protection Agency , 27 Tarzwell Drive. Narragansett, RI. 02882, USA.

.I. KIDDON et al

554

other JGOFS participants. A carbon balance for the upper water column was constructed in an earlier contribution (BENDER et al., 1992), using those productivity results along with POC, PON and sediment trap data. Integrated rates of gross and net production, as well as respiration rates, were established. Fluxes to POC, DOC and exported carbon pools were also discussed. In this paper, the production data is presented and used to characterize the processes of photosynthesis and respiration at the community level during the time at the study site. Specifically, a model is described which expresses in situ daily gross production as a function of irradiance and chlorophyll n concentration. Parameters derived from this model characterize physiological aspects of bloom production. Then respiration rates calculated from the 0, and 13C production rates are examined, and two components of respiration based on sources of carbon consumed are distinguished. Finally, turnover times for the phytoplankton biomass in the bloom are determined. The results give some insight into the biological controls of the production and respiration of organic carbon in the euphotic zone.

EXPERIMENTAL

METHODS

AND RESULTS

Measured rates of’net and gross production The measurements described were conducted on Leg 2 of the JGOFS North Atlantic Bloom Experiment, at 47”N, 2O”W, where the R.V. Atlantis II remained at a single location from 25 April to 7 May 1989. This time period corresponded to an important part, but by no means all, of the 1989 spring diatom bloom. The mixed layer lay within the euphotic zone on all but the first three days. Mixed layer temperature remained relatively constant at about 12.6”C, then warmed by O.YC over the final three days. Nutrients were abundant throughout the interval. Nitrate and phosphate concentration decreased by about 40% (NO, from 5.9 to 3.6 pmol 1-l; PO:- from 0.38 to 0.24 pmol 1-r and silica stocks dropped by 85% (2.4-0.4~mo11~1)) during the study (CHIPMAN et al., 1992). Seawater for the incubation experiments was collected from six depths in the upper 60 m of the water column before dawn. bottled for incubation, and redeployed at the depth of collection aboard a drifting buoy. The samples were incubated from dawn to dusk, a 14 h photoperiod. A subset of the samples was further incubated in a darkened, surface watercooled shipboard incubator for the following 10 nighttime hours. Production in the euphotic zone was measured by the following three incubation techniques: (1) 14 h gross and net 0,production. These were measured on six days, employing the tracer method of BENDER and GRANDE (1987). Briefly, four samples from each depth to were drawn into -100 ml quartz bottles. Two samples were processed immediately collect the dissolved gases for later mass-spectrometric determination of the ‘*O/r60 ratio of dissolved O2 and the O& ratio. The remaining samples were spiked with 0.2 ml then incubated and H,‘sO, approximately doubling the seawater “0 concentration, similarly processed. As photosynthesis uses H,O exclusively as an oxygen source, any 0, derived during the incubation period bears the ‘8O/‘6O ratio of the enriched seawater. “0 of the dissolved gross O2 production is then calculated from the measured ‘so enrichment 07: “0

Production

and respiration

“0 gross O7 production

=

changes in N. Atlantic

spring bloom

( lxOl”O)f - ( ‘xO/‘60)i i

(180/16~),,

_

(180~160)f

* [o&3

5.55

(1)

where the subscripts i and f refer respectively to the pre- and post-incubation measurements, and [021i is the initial OZ concentration measured by Winkler titration. The parameter ( ‘XOl’hO),, is the isotopic ratio in the spiked seawater (as well as the photosynthetically generated 02), which in turn is calculated as:

(‘xo/“‘o),, =

V\pike* 0.9X0 i- V\al,,p,c* 0.00204 Vspikc

+

(2)

Vsamp~c

Vrepresents volume, and 0.980 and 0.00204 are the mole fractions of “0 in the spike and natural water, respectively. Net production of 14 h O2 is calculated from the difference between the initial and final Oil/N, measurements: 14 h net O1 production

(02/N& = __

- (O~lNl)i .+,02]i,

(WWi

where (0,/N,), and (Oz/N,)[ denote the pre- and post-incubated values and [O& is the initial dissolved 0, concentration, measured by Winkler titration. The 1xO/‘6O and 0,/N, ratios are both measured by isotope ratio mass spectrometry. Errors are to. 1 and t- 1.7%, for 6’*0 and dOz/Nz, respectively. (2) 24 h net O2 production. This was determined on 10 days as the difference between O2 concentration measured before and after a 24 h incubation: 24 h net 0, production

= [O& - [O,ll.

(4)

The O2 concentrations were determined in -100 ml quartz bottles by a modified Winkler titration procedure (WILLIAMSand JENKINSON,1982), employing a Radiometer automated titrator. Four replicate samples were averaged for each pre- and post-incubation measurement. (3) 14 and 24 h 14C assimilations. These were measured daily at the six depths in the euphotic zone (KNUDSONet al., 1989). Four samples were inoculated with 9.0 ,&i sterile NaH’“C03 and incubated in 265 ml polystyrene tissue culture flasks. New sterile flasks were used for each sample, and a teflon liner was placed in the cap of the flask prior to incubation. Samples were deployed for the photoperiod at the same depth as the Oz experiments. Two samples were processed at the end of the 14 h photoperiod, and two samples were further incubated in the darkened, surface water-cooled shipboard incubator for another 10 h. After incubation, the samples were filtered onto Millipore HA filters, and prepared and counted on a shipboard scintillation counter (KNUDSONet al.. 1989). Variation in replicate analyses was less than 15%. Productions in the polystyrene flasks and quartz bottles were compared directly. ‘% productivity measured in the 265 ml polystyrene flasks was 1.08 Ifr 0.22 (n = 7) times the 14Cproductivities measured in 100 ml quartz bottles. Isotope appearing as DO’“C was not measured. Chlorophyll a concentrations were determined by high pressure liquid chromatography (SLAGLEand HEIMERDINGER,1991). POC concentrations were measured from discrete samples by a carbon-hydrogen-nitrogen analyzer (SLAGLEand HEIMERDINGER,1991).

556

J. KIDDON et al

0.98

0

2

4

6

8

14h 14C Assimilation

(flmol C/L/14h) Fig. 1.

Gross 0, production

with “0) versus 14 h ‘“C assimilation techniques were employed.

(measured

for all days both

Daily integrated surface irradiances of photosynthetically available radiation (PAR) were measured with a LICOR LI-190s 27~cosine collector, and subsurface irradiances were measured with a Biospherical4n PAR sensor lowered through the water column at local noon each day. Attenuation coefficients were calculated from an exponential fit of the irradiance versus depth data (KNUDSON et al., 1989), and 1% light levels calculated from the regression. Calculated rates of community production and respiration

We measured “C assimilation rates on all 13 days of the study, but measured gross O2 production on only six days. There is a strong empirical relationship between ‘so gross O2 production and associated 14 h 14C assimilation rates (Fig. 1). The slope of the best fit linear regression relating these two variables is 2.01 (molar units), with a near-zero intercept (-0.13,~mol O2 1-l 14 h-l). Individual points all within +0.5pmoll-’ 14 h-’ of the line (lo). The coefficient of 2.01 in this relationship primarily reflects three factors: (i) the value of the photosynthetic quotient (PQ, i.e. the ratio of gross 0, production to gross carbon production; LAWS, 1991; WILLIAMS and ROBERTSON, 1991), (ii) the amount of gross PO14C production lost to respiration or lost to D014C, and (iii) the dilution of intercellular 14C02 specific activity by recycling of respiratory ‘2C02. We exploit the empirical relationship of Fig. 1 to calculate gross O2 production rates from 14 h 14C assimilation rates for days on which the “0 addition experiments were not performed. That is: gross O2 production

= 2.01 * (14 h 14C assimilation).

(5)

Then the three respiration terms are calculated using the measured production data: 24 h 0, respiration and two operationally defined components of this respiration, called “new carbon” and “old carbon” O2 respiration. The 24 h O2 respiration is calculated as the difference between the gross and net 0, productions (units: pmol O2 I- * 24 h): 24 h O2 respiration

= gross O2 production

- 24 h 0, production.

(6)

557

Production and respiration changes in N. Atlantic spring bloom

It is calculated

from observed

variables

and does not involve

any assumptions

about

the

carbon source associated with the O2 demand. A simple model is used to partition this 24 h OZ respiration into two components. “New attributed to carbon” O2 respiration is defined as the 24 h rate of O2 consumption respiration of carbon which had been synthesized during the photoperiod following 13C additions (i.e. carbon respired during the 24 h experiment period). Operationally, this quantity is the oxygen equivalent of the 14C lost from the PO14C pool value of gross 14C assimilation minus 24 h lJC over a day; it is equal to the calculated assimilation. As gross “C assimilation is not measured directly, its value is calculated as the sum of 14 h 14C assimilation plus the (estimated) quantity of 14C lost to respiration during the photoperiod. It is observed that the 24 h 14C assimilation is 0.80 * 14 h lJC assimilation. This relationship was calculated by BENDER et al. (1992) (see Fig. 2 in their publication) assuming that utilization was first order and all 14C loss was attributed to respiration, and further that the same respiration rate held during the day. They calculated that gross lJC assimilation was 1.17 times the 14 h 14C assimilation. Thus, for every I .17 mole ‘“C fixed during the photoperiod, 32%) i.e. (1.17-0.8)/l .17, is lost to respiration during that photoperiod and the subsequent dark period. This paper assumes that the 32% value holds for OX as well. Accordingly, new carbon O2 respiration is calculated as the difference between gross and 24 h lJC assimilation (units: pmol O2 1-l 24 h-l): “new carbon”

O2 respiration

= 0.32 * (02 gross production).

(7)

There are several assumptions inherent in this calculation leading to equation 7: (i) the nighttime respiration of PO’?Z is first order with respect to PO14C; (ii) the daytime respiration rate of PO’“C is also first order, with the same rate constant as for the night; (iii) none of the PO14C loss is due to DO”C excretion: and (iv) the stoichiometry of remineralization is the same as that of production. It is acknowledged that these assumptions are not all rigorously correct and introduce errors into the calculation of new carbon production, but it is believed that the calculated value of new carbon respiration is a reasonable approximation of the true value. Oxygen consumption associated with new carbon (equation 7) is always less than the measured value of the total, 24 h O2 consumption (equation 6). The difference between the two terms is defined as “old carbon” O2 respiration (units: pmol 0, I-’ 24 h-l): “old carbon”

O2 respiration

= 24 h 0, respiration

- “new carbon”

O2 respiration.

(8)

This term reflects respiration of carbon stock not labeled with “C. i.e. DOC or POC present before the beginning of the incubation. Finally, gross O2 and 24 h net O7 productivities are integrated over the euphotic zone (taken to be the region where light is greater than 1% of the surface irradiance). Production rates are integrated by the trapezoidal rule (linear interpolation between measured production values). When the I % light level is deeper than the sampled interval (e.g. the first and last few days), production values are assumed to decrease linearly from the value of the deepest measurement to zero at the 1% light level. All of these production and respiration data and values of auxiliary parameters are presented in Table 1 and Figs 2-4. In particular, note the close agreement in the integrated O2 production measured directly by the “O-incubation method and the production estimated as 2.01 * 14 h “C assimilation (open squares and filled circles in Fig. 4). The two measures agree closely,

558

‘fable surface

J.

I.

Rate;, of production irradiance

parentheses

and respiration,

lo, attenuation

indicate

integrated

coefJicient

KIDDON

POC and chlorophyll

k. and depth of

gross 02 production

Values in bold denote culculated

rather thm

et al.

a concentrations.

1% light

level) for

and 24 h net 0, production.

measured

results.

Units, definitions

and irradiance

12 days on station.

calculated

data (14 h Values

over the euphotic

and calculation

forrnulue

are listed

as footnotes

2.2

5.4

I 6 It 0 0

2.2

5.8

I.1

+ 0 Y

1.X

4.7

(I.0

i- 0.8

II

3.5

(I Y i

(1.X

2.0

0 4

-m2.4

‘-

0.x k 0.x

-3.2

1.1 (212

14)*

(--52

i

34)’

27

7.5

4.3

+ 0.X

2 3

6.6

3.‘)

+ I) 7

I7

4.5

1 0 + 0 8

I I

2.4

I 4 -c 0.6

(I.5

1.2

-1). I k 0.x

0.3

0.6 (197

+

2.3

6.2

22

5.9

I2

3.6

0 7

1.6

0.2

0.8

fj.2

-0.6 14p

(Y4

+

Ih).

0.5 (155 + 12j*

32

7 Y i

3.1)

6.Y

f I) 2

I.5

2.2

I’)

2.7

i

I 0.0 I)2

0 Y

0.2 0.2

0.7

I Y + 0.2

0 3

0 7 i

0.0

0 ? i

(I.2

0 I

(1%

(1.F I

2.1

ll.(l I) 2 0 I

t 1)

3X

8.6

27

3.X

9.2

?.Y

0 093

2.7

7.2

2.3

50

15

2.9

0.‘)

0.6

1.2

0.4

0 3

+

4i.Y

0 2

0.5 (250

Is)*

3.Y

2 6

I3 Y

2.6

l.Y

Il. III0 42

I.1

0 ‘J

0 5

0.3

11.2

0 0

0. I

0.0

in

zone.

Production

and respiration

changes

Table 1.

III

PI

asim

awm

131

Depth 14h “C 24 h ‘?(m)

559

spring bloom

Continued

ISI

PI

in N. Atlantic

Gross 0,

I4 h net

24 h net

PO<

prod

01 prod

o2 prod

GImol I-‘)

Chl o (,ug I-‘)

WI

[71

PI

24 h O2 “new” C “old” C V?sp

resp

I,> PI atten k (l/ml

resp

1% light(m)

4.5

11.3+0.2

X.3

34

s.5

3.6

I4

53.0

4 6

I I.6 i

X.8

10.7

7.3

3.7

3.6

0. IOh

31

6.7 i

0.1

51

II 5

4.6

2.1

2.5

43

1.2

3 4 2 0.1

I.3

IO Y

x.7

I.1

4 6

1.7

0.3

3’)

0.2

0 1

1.0 i

0.c

-1.0

X.6

0.2

0.4 + 0.0

-1.X

7 (I

(2X1 f

4) j

4.7

I I .h f

37

Y 4 r 0.1

2..?

4.5 * 0 2

0.2

X.5 I.2 3.0

I .o

2 n i

0.2

0.3 i

0.2

-0.4

I).1 f

0.1

-(I I i

0.3

10.2

5.4

3.7

1.7

5 h + 0.X

10.6

3.8

3.0

0.X

0.1 + 0.4

0. I

4.4

I.4

3.0

6.2 i

62+

0.2 0. I

0. I

I .2 + 0.‘)

-0.7

i- 0.7

.i Y

2.7

0.0

2.1

0.5

4.4

0.1

0.1

0 I

0. I i I.1

II)”

47

11.7

IO 2

i!

8.2

II 2

1.s

5.0

II.9

I3 7

37 5.6

2.6

3 (1

I.1

2.6

IT.4

I7

(I 8

I).‘)

0.7

I!.3

2.4

0.2

2.2

0 I

0.3

I? h

I

(I I

14)*

? (1

13.3

Y h

54

4.2

12

1.7

12.7

‘1.6

0.7

4.0

27

4.11

11.0

I.\.4

33

a.2

10.6

I .o

2.4

46

(1 X

0.8

X4

0 .3

0 -I

0.1 I II

I .h

0.1

(179 f

0.106 43

0 0

x7

-2.5 (5n+

(173 + 4)~

24.8

0 12x 36

3.5 I.6

2.6

50.3

2 0

(304 + 22j* 4. I

II

7 3 k (1.4

If1.Y

7.1

3.5

3 h

3.5

8 I io.1

2.6 f

0.8

II 7

5.6

2.h

3.0

24

6.4 k 0.5

3.5 + 2.X

10 I

51

20

3.1

2.0

4.Y + 0.2

1.ni

16.5

4X

I.5

33

I.3

3 2 * Il.3

-I

II.1

51

I 0

11

0.Y

2 i

0 4

2.2

1.1

1 +- 1.3 -2.1

Ii.6

27 I 0. IO4 14

0 7

(2.11 k 4)’ 5.7

1s I * 0.0

1.7

13.1 i

3.4

8.0 f

2.0

6.0 * 0.1)

4.2

IS

3.1 in2

4.X

0 6

IO.4 0.X k 2.5

0.2

’l4h-’ ‘1-tII ’ O1 I ~’ 14h ‘: hold O2 I -’I4 h-‘. O2 I ’24 h ‘; bold

II

6.8

1.4 (276 i

[I]

0 I

-0

7x

-I

IO 8

1)

(00 + 70)

/~molCI

[2] jcmol C I

(31{rmol 141 jrmol [S] /Imol

[h] umol O2 I-’ 24 hF’;

~pc = 2.01

(I4 h “C assim)

type: gross O1 prod

gross 0;

24 h O? resp

prod - 24 h net 0:

[7] ~rmo1021~‘24h~‘;(l

-0.8011.17)‘gross0~pmd.

(81 /u~~ol 02 I-’ 24 h

‘: 74 h 02 resp - “new C” rap.

[Y]

I4 h PAR

irrad~ance: mol photons mm’.

7

prod.

6.6

1.X

1.X

6.2

31

21

2 0

1.Y

0.

3.5

I.9

I6

35

I.0 04

37.Y 0. 10s

I

14

560

J. KIDDON et al.

Production

0-4 -2

0

2

0-2

4

10

10

20

20

40

40

50

sta10 4125189

30 T 60 -

-20

0

(ymollL/time)

2

50

2

4

6

6

StalI 4126

30 v 60 1%

4

light level

8

-2

0

2

4

6

-20

2 4 6 810

0 10 20 30 40 50 60'

60

net 0, production and 14 h “C assimilation versus depth for each Fig. 2. Gross O2 production, day. Depth of 1% light level on each plot is indicated by the heavy horizontal line. Errors (one standard deviation) typical for all 14 h “C assimilation and net O2 productions arc included in the graph for Sta. 10. Errors for gross 0, productions are smaller then the plot symbols.

highlighting the close relationship between tions expressed in equation (5) and Fig. 1. DISCUSSION

14 h ‘“C assimilations

AND

and ‘“0 gross produc-

CONCLUSIONS

Bloom productivity BENDER

inventories

et al. (1992) described the general changes in production rates and nutrient during the period of our observations. They examined inventory changes in the

Production

and respiration

changes

in N. Atlantic

Production

-4 0

0

4

8

-4 0

12

(pmol/L/time)

0

4

8

12

-4 0

10

10

10

20

20

20

30

30

30

40

40

50

; 50

40 sta 17 512

50P

0

0

2

4

6

-4 0

8

0

561

spring bloom

4 M

8

12

0

0

-

0

4

-

4

8

St819 514

8

12 16

10 20 30 Pi

Ad

40

sta 21 516

50

+ + --o--

Gross 02 (pmol021L114h) Cl4 Assim. (ymol CIL114h) Net 02 (ymol021Ll24h)

Fig. 2.

Conrincwd

upper 50 m of the water column, which roughly corresponds to the 1% light level. Between 25 April and 5 May, the nitrate inventory in the top 50 m dropped from about 290 to 200 mm01 rnp2. The POC inventory roughly doubled, rising from 250 to 480 mmol m-‘, as did the chlorophyll a inventory (Table 1). However, as a plot of integrated gross and net O2 production rates versus time (Fig. 4) indicates, primary production, integrated to the 1% light level, rose by less than a factor of two, from 180 to 250 mmol0, m-l day-‘. Not surprisingly, production rates are high for the bloom period. Daily depth-integrated gross O2 productivity averaged about 0.2 mol O2 n-’ day-’ (about 1.6 gC me2 day-‘, assuming a PQ of 1.5). As shown below, gross production rates, and especially production rates per unit biomass, were tightly linked to light availability. Net O2 production rates were not as clearly related to any measured parameter, no doubt reflecting complex

562

J. KIDDON et al.

Respiration

0123456

012

0

0

10

10

20

20

30

30

40

40

50

sta10 4125189

50 c':

I

60 -

60 -

(pmol02U24h)

3

4

12

3

stall 4126

1% light level

-3 -2 -1 0 1 2 3

-1

012345 0

0

10

10

20

20

30

30

40

40 sta 15

50

4130

60'

60

*

24h 02 Respiration

d

‘New Carbon’ Respiration

d

‘Old Carbon’ Respiration

0

12

3

sta 16 I,

511

J

Fig. 3. 24 h 0, respiration. old carbon respiration and new carbon respiration versus depth for each day. Depth of 1% light lcvel is indicated by a heavy horizontal line. Errors (one standard deviation) typical for all respiration rates are included in the graph for Sta. 10.

trophic level interactions and varying physical conditions such as mesoscale variability. As a result, daily integrated net/gross ratios were variable with an average of 0.40 + 0.23, ranging from zero to 0.75. That is, on average, about 40% of gross production escaped immediate remineralization and contributed to bloom biomass and DOC or was exported beneath the euphotic zone (see BENDER et al., 1992). PB(I) curves and controls on the rate of gross primary production We investigate versus irradiance

how the community phytoplankton utilize light by examining production PB(Z) curves, where PB represents production per unit biomass. Such

563

Production and respiration changes in N. Atlantic spring bloom

Respiration (pmol 02/L/24h) 0123456

0123456 (I

0

0

2

4

6

8

0 2 4 0 _LL_II

6

8

0

0

2

4

6

8

10

20 30 bf 40

-

50

Fig. 3.

sta21 516

Continued

curves are instructive because environmental changes are reflected in the parameters which describe the curves (PLATT and JASSBY, 1976). We recognize that a variety of conditions may confound interpretations and may limit the usefulness of PB(I) curves (see e.g., MARRA er al., 1992). However, our data occur along well-described gradients of nutrient availability and mixing depth (as well as irradiance) over time scales appropriate to algal growth. Thus the results of this analysis should prove useful over a broader set of conditions. Generally, P’(Z) curves are constructed from “instantaneous” data generated in a simulated in situ experiment, where the light flux is constant over the course of the experiment. In contrast, in our in situ experiments, irradiance is not constant with time, but varies in magnitude over a photoperiod due to normal diurnal variations in the light field. We therefore next explain how we adapt an “instantaneous” PB(I) model (PI-ATT et

564

J. KIDDONet al

-100’ April 24

I May 8

May 1

Fig. 4. Three measures of integrated O2 production versus day: (i) gross 9 production measured and (iii) net Q using “0 , (ii) gross O2 production calculated as 2.01 * 14 h 14C assimilation, Error bars of the ‘*O-derived

production.

productions

are within the symbol.

al., 1980) to reflect the integrated

nature of our data. We then use this adapted model to establish the value of three constants that relate instantaneous production (normalized to chlorophyll concentration) to irradiance. In the following discussion, we denote the usual instantaneous, biomass-normalized production as PB. In addition, we designate biomassnormalized production which is integrated over a photoperiod as PINTT. To ease comparison with existing literature, we report our results primarily in carbon weight units rather than oxygen molar units. We employ a PQ of 1.5 for the conversion, therefore 1 mm01 O2 me3 day-’ is equivalent to 8 mgC mm3 day-‘. A PQ of 1.5 is reasonable as it lies within the range of values, 1.5-1.8, typically associated with rapidly dividing populations using nitrate as a nitrogen source (WILLIAMS et al., 1979). PLATT et al. (1980) related instantaneous production to instantaneous irradiance by the following equation:

where PA is instantaneous production per unit chlorophyll a, and Pf is a constant equal to the asymptotic value of PR at large irradiances in the absence of photoinhibition. al/P: characterizes the curve’s approach to the asymptote, where Q is the slope of the linear region of the curve at low irradiances and is related to the photosynthetic efficiency, and I is the instantaneous irradiance. PI/P: characterizes photoinhibition, if any, at high irradiance. The carbon based units of PB and P! are (mg C)(mg Chl a)-’ h-’ ;a and /3 are (mg C)(mg Chl a)-’ h-’ @mol photons m-2 s~‘)~‘; and Z are @mol photons me2 s-l). The following describes how to extract these instantaneous parameters (Pf, G(and/3) from our in situ data. Time-integrated, chlorophyll-normalized production at any depth is expressed as: PINTT =

I

(1P! *{1 - exp(-[alPf]l’(t))}

* exp( -[/3IPi]Z’(t))

dt

(10)

PINTT is integrated over a photoperiod (t = O-l) and the instantaneous irradiance Z’(t) is time dependent. See also PLAIT et al. (1990) for an alternative approach. Note that this

Productionand respirationchangesin N. Atlanticspringbloom

565

calculated value PINTT is equivalent to the measured value of chlorophyll-normalized production determined at any depth by experiment. Since the calculation of P’NTT involves gross O2 production, no respiration term is required in equations (9) or (10). The photosynthetic parameters a, /? and Pf are here considered to remain constant throughout the day, despite the demonstration by CULLEN et al. (1992) that CLand Pf show die1 variations of about a factor of two. Thus, the a, p and Pf parameters are average values for a 14 h day. We use the sin’ approximation (IKUSIMA,1967) to describe Z’(t) as a function of time from dawn to dusk: Z’(t) = I;,,

* sin”(n * t).

(11)

The parameter I,!,,,, is itself calculated for each depth on each day from the expression obtained by integrating equation (11) over a photoperiod (t = @l): 1 Zk,, * sin”(n * t).

1; =

!0

(12)

where IZ is calculated as Ii = IOephZ

(13)

from the depths z and the measured values of integrated surface irradiance I,, and attenuation coefficient k listed in Table 1. A calculation is initiated by choosing values of the photosynthetic parameters P,“, a and p which are assumed to hold for all depths on a given day. Equation (10) is then numerically integrated for each depth using Z’(t) appropriate for the depth (equation ll), and the calculated values of PINTTare compared with the corresponding measured values of chlorophyll-normalized production determined that day. The calculation is then repeated for slightly different values of Pf, C( or /3. The values of photosynthetic parameters are thus optimized by simultaneously minimizing the least-squares relative difference between calculated and observed values of PINTT at all six depths. The process of optimizing the parameters was insensitive to the initial choice of parameter values or order of optimizing parameters. The model determines Pf (the value of the PB at saturating irradiances), but a closely related parameter will be discussed: PE, the maximum value of PB evident when the community is photoinhibited. It is calculated as: Pt = P.! * {(xI(a + /3)} * {/3l(a + /?)}^/3Ia (P LA-IT et al., 1980) and is equal to e when p is zero. To summarize, given the daily incident irradiance I,, and attenuation coefficient k, the irradiance, I’(t) is calculated as a function of time for any depth. Z’(t) is then used to numerically integrate equation (10) simultaneously for all six depths. The instantaneous parameters P.f, a and p are varied until a best fit is achieved with observed pNTT data for all six depths. Consequently, we deduce the values of the instantaneous photosynthetic parameters from integrated field data. P,!, (Yand /3 during the North Atlantic Bloom Experiments Before examining results derived from the modeling exercises, consider a production versus irradiance plot constructed from all of the field data collected over the two week period (Fig. 5). Note that this figure displays integrated production and irradiance

566

J. KIDDONet al

160

f

40

all data

z.

0 0

10

20

30

40

50

Irradiance (mole photons/m2) Fig. 5. Plot of PINrT (chlorophyll normalized, 14 h gross production) versus I(14 h irradiance) for all experiments. Lines show simulated P lNTT ( I) relationship, calculated using best-fit photosynthetic parameters P”, a and /3 for cases where 13 > 0 and p = 0. Values arc reported in carbon units. converted from oxygen units by PQ factor of I .S.

parameters rather than instantaneous parameters typically employed in the classic PB vs I curves; here PINTT values reflect day-long gross production accumulations and the irradiance axis represents radiation accumulated over a photoperiod. The classic features of a production versus irradiance curve are evident in Fig. 5 (light-limited production at low irradiance and light-independent response or perhaps photoinhibition at higher irradiance), but the data are too scattered to reliably extract photosynthetic parameters which are characteristic of the bloom ecosystem for the two-week period. For instance, best-fit curves for cases where /3 > 0 and/I = 0 are not significantly different (parameters in Table 2)) and the slopes of the linear region of the curve do not fit the low irradiance data well. However, no other choice of parameters provides a better fit at both low and intermediate irradiances when we attempt to fit all data together. Much better fits between observation and simulation are achieved when the data are treated separately for the five days when irradiance was high enough to meaningfully constrain the simulation (Fig. 6a-e). The photosynthetic parameters obtained by modeling for these days are reported in Table 2 and are used to construct the PINTT versus irradiance curves (curved lines in Fig. 6a-e). Independent estimates of a are provided by the slopes of the linear regression of P ‘NTTversus I data at low irradiances (straight lines in Figs 6a-e). These estimates of a are also listed in Table 2. Good agreement between the observed data and the simulations, and between the two estimates of a, provide confidence in the values of the photosynthetic parameters generated by the modeling algorithm. On the remaining seven days when total irradiance intensities were less than saturating, the simulations were too poorly constrained at high irradiances to provide meaningful estimates of Pz andp. For these days, only a was estimated, calculated as the slope P INTT versus I for I < 5 mol photons mP2 (Fig. 6f-1 and listed in Table 2). Summarized in Table 3 are the photosynthetic parameters evaluated from field studies analysed by either the exponential model of PLATT et al. (1980) or a similar hyperbolic Bloom tangent model of JASSBY and PLAT-I (1976). Pi values at the North Atlantic Experiment (NABE) site range from 5.5 to 12.7 (mg C)(mg Chl a)-’ h-‘. The average Pf, value we observed, 9.5 ? 2.6, is about twice other values observed at mid-latitude coastal

Production

Table 2.

Instantaneous

and respiration

changes

in N. Atlantic

photosyntheticparametersPz, a andfi.

irrudiance,

and (I parameters

parameters

calculatedfrom

calculated

as the slope

derived

of PNTT

versus

567

spring bloom

by model simulationforfive

1for low

irradiance

data.

days ofhigh Al.so.

all data. for the case where /3 > 0 and p = 0. Allphotosyntheticparameters

best-fit

are reported

in carbon weight units, converted from oxygen molar units using a PC2 of 1.5

sta.

Date

IO II

25 Apr 1989 26 Apr 27 Apr 20 Apr 30 Apr I May 2 May 3 May 3 May 5 May 6 May 7 May

12 I4 I5 I6 17 IX

19 20 31 2’

a\ eragc all data: best-fit all data: zero-beta

Photosynthetic P(R)m’

parameters alpha 0.066

9.3

(from model) beta+.’ 0.014

(by regression) alpha“ 0.064 f 0.01

I

0. IO5t 0.008 s.5 11.0

0.046 0.051

0.000 0,005

9.4

0.058

0.001

12.5

0.071

0.020

9.5 k 2.6

0.058 .t 0.010

0.008 f 0.009

10.4 Y.6

O.{J77 0.081

0.006 o.ootJ

units: (gC gChl ’hh’). .“units: (gC gChl_’ hh’)/&mol photons rn~-‘s ‘). Note: values are converted from oxygen units using PQ =

0.080 i 0.002 0.05$ i- O.(JOY

0.060+ 0.007 0.076 0.077 0.035 0.101 0.073 0.060 0.012

IL 0.004 i 0.002 IL O.OOY -t 0.006 t 0.005 t 0.018 i 0.017

0.070 t 0.020

1.j

of comparable magnitude were sites or laboratory algal-culture studies. Pz values reported for the summer months in Nova Scotian coastal waters despite low values measured there at other times of the year (PLAIT and JASSBY. 1976). Our Pi values were about 50% greater than those evident in the bloom occurring concomitantly in the Sargasso Sea (PI-ATT et rd., 1992). Variability in PE values at the NABE site were not related to variations in any obvious environmental or hydrographic parameters, e.g. incident light intensity, depths of mixed layer or euphotic zones. NABE photosynthetic efficiencies (u parameters) range from 0.046 to 0.071 (mgC)(mg Th e average alpha is 0.058 _+ 0.010, slightly Chl a))’ hh’ bmol photons at rnp2 s-l)-‘. greater than is observed at other natural sites. NABE efficiencies are comparable to those in mid-latitude blooms (PLAIT and JASSBY, 1976; MALONE and NEALE, 1981), but are 1.8 times those evident in the Sargasso Sea bloom (PLATT et al., 1992). The photoinhibition parameter, B, ranges from 0.001 to 0.020 (mgC)(mg Chl))’ hh’ “‘(umol photons m-’ s-I))‘. PLAIT et al. (1980) define a parameter I,, = Pf/p, having dimensions of light flux, as an index of relative photoinhibition. A small value of I,, is indicative of strong inhibition. Values of I,, on the five bright days range from 1200 to 7700 prnol photons m -2 s-‘, indicating moderate to strong photoinhibition. But even on these days, photoinhibition was most evident only in samples incubated at depths shallower than 10 m. There are both physiological and procedural reasons for the difference between the values of a and Pz and those found in the literature (Table 3). First, the production values are reported as gross carbon rather than the 14C assimilation values used by others. As was

20

10

0

40

40

h-radiance

30

30

0

10

(mole photons/m*)

50

50

20

Fig. 6. (a-e): Plots of f”vrT (chlorophyll normalized. greater than 25 mole photons m-‘. Arrous denote total best-fit photosynthetic parameters. (f-l): P’vT T(I) plots constrained to pass

20

10

0 40

50

0

60

120

180

i

0

0

10

h-radiance

20

sta 19 May 4

ci)

20

(mole photons/m*)

10

Sta 16 May 1

0

(h)

I i

0a 0

60

120

180

0 102030

1-l h gross producrion) versus 1 (14 h irradiancc) for days when total surface irradiance 1,) was daily surface irradiance. Curved line\ show simulated P”” r(l) relationship. calculated using for days when I < 25 mol photons no-‘. On all plots. straight lines show linear regressions. through the origin. for data when 1 < 5 mol photons m-‘.

30

lj,/y$ l,b,

Production

and respiration

changes

in N. Atlantic

569

spring bloom

Tuhle 3. Literature values of PE and a parameters determined by investigations employing exponential or hyperbolic tangent P’(l) models. Experimental field studies are described by location, season and number of experiments. All values are reported in carbon weight units. converted from oxygen molar units using a PQ of 1.5

Location

Time interval

II

*P(R)m (gC/gChl/h) average range

**Alpha (gClgChl/h)l@mol photonsim’is) average range

Field studies High latitude Arctic coast Arctic coast Mid latitude Nova Scotia coast Nova Scotia coast Nova Scotia coast Nova Scotia coast N. Pacific gyr-e S. California coast NW York Bight Sargasso non-bloom Sargasso bloom N. Atlantic

bloom

1.0~ latitude Peru coast Equatorial Pacific Laboratory

Summer 1978 1977 to 1980

50 279

I .O i 0.4 1.2 + 0.6

0.4-2.7 0.2-10.0

0.010 f 0.004 0.014 t 0.005

0.004-0.01x 0.001-0.057

Ill 121

1973 to 1975 Spring 1975 1975 to 1077 1977 to 1980 Spring 1984 Summer 1080 1977 to 1979 1983 to 1988 Spring 1988

188 70 146

4.9 t 0.4 4.9 1 1.4 5.5 t 2.6

o.F7-2s 2.0-8.4 -

0.051 + 0.031 0.034 i 0.010 -

0.007-0.152 O.OlY-0.063 -

[31 141 [51

424 3 33 79 80 72

2.4 3.1 6.0 9.7 2.1 6.2

+ 1.1 + 1.7 f 6.2 I6.4 & 0.2 i 0.6

0.5-13.7 1.2-4.6 0.5-24.5 0.3-30.X -

0.01 I 0.060 0.078 0.056 0.013 0.033

0.002~~.043 0.014-0.083 0.010-0.294 0.002-0.154 -

? f * f ? f

0.005 0.040 0.061 0.036 0.001 0.003

198Y

12

9.5 + 2.6

5.5-12.7

0.058 * 0.006

Fall 1977 Spring 1988

7 12

-

O.Y--15.2 1.1-5.2

-

0.015~).230 0.011-0.035

1.6-20.8

0.024 ?!I 0.005 0.020 * 0.002

-

Spring

0.04w.071

1;; I71 [Xl [91 191 this stud!, [lOI [Ill

studies

1 I marine algal sp. litct-aturc survey; 65 sp. Skeletonema costatum

5.3 t 5.2 6.2 i 0.3

--

1121

[131 [Ill

References: (11 PLATTetal. (1982); [2] HARRISON and PI-ATT(1986); [3] PLATTand JASSBY (1976): [4] Colt’ and PLATT (1983); [S] HARRISON and PLATI. (1980); [6] GRANDE etal. (1991); [7] HARDING et al. (1982); [8] MALONE and NEALE (1981): [9] PLATT et al. (1992); [lo] PLAY et al. (1980); [I 11 CULL.EN et al. (19YO): [12] CLOVER (1980); [14] LANCDON (1988).

noted above, the gross carbon production rates are 17-32X greater than the “C assimilation rates in this study. On the other hand, most other at-sea estimates of photosynthetic parameters are based on short-term (15 min to 1 h) incubations where it is often assumed that 14C assimilation will estimate gross productions (see citations of Table 3). Second, the measurement of PB(Z) parameters is strongly dependent on methodology (SAKSHAUG and SLAGSTAD, 1991), which further limits the ability to compare these results with other data. Length of incubation, light source, time of day, and other procedural details vary considerably. Perhaps the most important is light source. The parameter a is wavelength-dependent, such that blue light often yields higher values of a than white light (typically tungsten) sources (SCHOFIELD etnl., 1991). Diurnal variations in the parameters have also been shown to be significant (HARDING et nl., 1982). In contrast to most earlier studies, our a values were derived under perfectly natural irradiance conditions (the water column itself) and should be unbiased with respect to irradiance quality. Also, our measurements reflect the averaging of irradiance conditions over the day and are thus largely immune to diurnal effects. The NABE photosynthetic parameters show little overall variation in magnitude (Fig.

J. KIDDONet al.

570

24

Apr

0.0

4

Apr Fig. 7.

(a)

Alpha

May 8

May 1

!O May 1

24

and (11) Pfi: parameters

simulations for five days of high irradiancc.

May 8

vcrsub date. Open Error bars for e,

symbols arc derived

are ?I 10% of parameter

rcflccting sensitivity test of model. Solid symbols dcnotc alpha parameters linear rcgrcssions (with one standard photon mm2. Values arc reported

deviation

calculated

from model magnitude. as slopes of

in slopes) for p’ versus I data when I < 5 mole

in both oxygen and carbon units. rclatcd to a PQ of

1.5.

7) although chlorophyll concentrations are sampled from 0.5 to 1.8pg Chl ll’, mixed layer depths from 130 to 10 m, and euphotic depths from about 65 to 35 m over the course of our experiments. Nutrients were plentiful throughout the period. If increasing stratification of the water column were a factor, it might be expected that a would decrease and PfZ increase with the increase in average photon flux experienced by the populations with the shallowing mixed layer depth. Unfortunately, the NABE data are too few and the trends too uncertain to support the scenario. Time- and depth-integrated

production

versus irradiance

PLAIT (1986) introduced a parameter W which is useful in calculating productivity from remotely sensed ocean color data. The parameter W is calculated as the slope of a linear relationship between chlorophyll-normalized production and irradiance, where both measures are integrated over a long time period as well as integrated over depth in the euphotic zone. In essence, \v is the depth- and time-integrated analog of the a parameter, characterizing the light-dependence of production in the field (FALKOWSKI, 1981). The parameter Y for the NABE site is determined as follows. First, values of P’N’T,Z are calculated as primary production integrated over a photoperiod and over the euphotic zone, divided by the chlorophyll a concentration, integrated over the euphotic zone (units: gC gChl-’ day-‘). Then PINr T,Z versus daily incident irradiance

Production and respiration

changes

In N. Atlantic

spring bloom

571

60 50 40 30 20 10

o-Fig. 8. P’“rT,Z surface irradiance parameter used in the relationship,

’

3

7

40

50

60

0

10

Total

Irradiance I, (mole photon/mz)

20

30

(depth-integrated. chlorophyll normalized, 14 h gross production) versus total I,,. Solid line is the linear regression of observed data; the slope is equal to Y (the calculating productivity from remotely sensed ocean color). Dashed curve shows derived by simulation, calculated from the best-fit (/3 > 0) photosynthetic parameter\ listed in Iablc I!.

(mol photons mP2) are plotted and 1I’taken to be the slope of the line fitting these data (straight line in Fig. 8). The average value of 9 for bloom conditions during two weeks at the NABE is site 0.55 gC gChl-’ me2 mol photons-‘. This value is consistent with the range of 0.32 to 0.66 gC gChl-’ m-I, mol photons-’ found for a wide variety of environments (PLAIT, 1986). The trend of P'NTT,Zversus irradiance can be simulated by integrating equation (10) over time and depth (where the irradiance over an assumed 50 m euphotic zone is exponentially distributed as in equation (13)), and using the best-fit photosynthetic PINTT,Zversus parameters derived from all data (Table 2 and Fig. 5). The simulated irradiance curve (curved line in Fig. 8) is nearly linear for incident irradiances greater than about 10 mol photons m--l, and the slope of this linear region is very similar to the empirical linear fit of the observed data. As was noted by PLATT et al. (1988) and CULLEN (1990), the finite intercept of depth-integrated productivity at zero irradiance is an artifact resulting from the fact that the linear regression contains few data in the non-linear region (irradiances < 10 mole photons m-“). The good agreement between the best-fit line and the simulated curve at intermediate irradiance levels reflects the physiological basis of the empirical I! parameter, and thereby supports the rationale of estimating oceanic productivity from remotely sensed properties (e.g. PLATT, 19M).

Community respiration processes The rates of respiratory OZ consumption in 24 h is calculated as gross OZ production minus 24 h net O2 production (Table 1). There are two observations regarding the relationship between this 24 h O2 respiration and gross O2 production. First, the respiration rate shows a highly-scattered, positive linear dependence on gross production rates (Fig. 9). The regression line in Fig. 9 implies there are two components to the respiration. One component in linearly related to production, amounting to about 35% of gross production, i.e., slope = 0.35 _+ 0.06. The other component, as indicated by the intercept, utilizes 1.5 f 0.42 pmol 0, 1-l day-‘. regardless of the production rate. This

572

J.

KIDDON erul.

+0 6

0” 00

4

35x

R2 =

*

0.16

s

El

3-10m + II-25m 0 30-50m ??

+

??+ ??

+ -2 0

2

1

6

8

10

12

1-I

16

Gross 02 Production (pmol O?/L/l4h) Fig. 9.

24 h 0, respiration plotted versus gross Oz production rates. Symbols by depth. Solid line is linear regression of all data.

‘New Carbon’ Respiration (pm01 021w24h)

S 5+

-m (ID 20 aIaxrD I 0 30

data

‘Old Carbon’ Respiration (ymol02ILD4h)

10~

10

E

differentiate

0

0 aBocD0 0

0

20”

OOaxIO

30~

CD a0

0 0

n

40

40~

50

50’

0 0 0 0

Fig. 10. (a) New carbon respiration rates, calculated as 0.32’ gross 0, production, plotted versus depth. (b) Old carbon production. calculated as 24 h net O2 rcapiration minus new carbon respiration, plotted versus depth.

two-component behavior is as expected for a community in which autotrophic respiration (and perhaps heterotrophic respiration tightly linked to autotrophy) is an important fraction of total respiration. The second observation is that the rates of both respiration and production are highest in the surface waters and decrease with depth (Fig. 9), i.e. one of the components of the respiration rate is proportional to the production rate. A simple model is proposed in which the two components of total O2 respiration are computed as described earlier to be distinguished. The first is “new carbon respiration”, 32% of gross production. New carbon respiration is the respiration recorded by loss of “C, and is therefore associated with the highly labile, short-lived pool of carbon fixed in the recent photoperiod. New carbon respiration decreases smoothly with depth (Fig. lOa), a reflection of the fact that new carbon respiration is taken to be a constant percentage of gross O2 production, which itself decreases with depth. The second component of respiration, “old carbon respiration”, is equal to total O2 respiration minus new carbon respiration. Old carbon respiration is therefore associated with the loss of unlabeled POC

Production

and respiration changes in N. Atlantic

Biomass Turn-over

Fig. 11.

Biomass

turn-over

spring bloom

573

Rate (days)

time, calculated as POC concentration production rate. plotted versus depth.

divided

by the gross carbon

pool present at the beginning of an incubation. From Fig lob, we calculate that the average value of old carbon respiration over all depths is 1.8 ? 1.4 pmol O2 I-’ 24 hh’, in close agreement with the estimate of the constant component of O2 respiration indicated by the intercept in Fig. 9, i.e. 1.5 t- 0.4 pmol O2 I-’ day-‘. There is no trend in old carbon respiration versus depth (Fig. lob) and, by inference, no trend with production. It is believed that the absence of a trend reflects the largely heterotrophic source of the respiration, and the fact that POC is uniformly distributed throughout the mixed layer. Both new and old respiratory components presumably have autotrophic and heterotrophic contributions, though autotrophic activity probably accounts for most of the new carbon respiration. The results of both Figs 9 and 10 are therefore consistent with a model which apportions respiration into two components: a “new carbon” component which consumes about 32-35% of the carbon produced during the photoperiod, and an “old carbon” component, which consumes POC present at the beginning of the incubation at a constant rate of about 1S-1.8 pmol O2 1-l 24 hh’. Integration of the total respiration throughout the euphotic zone and over the two week period yields the result that respiration consumes about 60% of gross production, 142 mmol O2 m-* day- ‘. Integration of new and old components of respiration separately shows that each component contributes equally to O2 consumption, 70 and 72 mol 0, I-’ 24 hh’, respectively.

POC turnover rates Finally, the results can be used to estimate POC turnover times. The turnover times (units: day) were calculated simply by dividing the measured POC concentration (mmol C m-‘) by the gross production rate (mmol C m-s day-‘). This simple calculation yields a maximum value for biomass turnover time. Figure 11 plots these values versus depth. The turnover times increase with depth, as would be expected for the case where a uniform

574

J. KIDDON et al

POC distribution is maintained by circulation in the mixed layer, while light-dependent production decreases with depth. Turnover times are remarkably short in the upper 10-15 m; the entire POC supply is regenerated in a day or two. From 20 to SO m, the POC turnover rate is from 10 to 15 days. On average, the production in the euphotic zone replaces the standing stock of POC in four to five days.

SUMMARY

The production experiments yield high estimates for gross production in the 1989 North Atlantic spring bloom, an average of 1.6 gC rn-’ day-‘. About 40% of gross production survives immediate remineralization and contributes to bloom biomass and DOC or is exported beneath the euphotic zone. The empirical relationship between 14 h 14C assimilation and ‘so gross 0,production (Fig. 1) is used to infer gross 0, production rates on the days it was not measured. Using an empirical relationship between 24 and 14 h ‘“C assimilation rates, two conclusions can be drawn. First, it is found that the 14C respiration rate is proportional to the rate of production. Second. it is concluded that 14 h and 24 h “C assimilation rates respectively underestimate gross carbon assimilation by 17 and 32%~~ at the site and time of work. A production-irradiance mode1 is presented, which simulates gross production as a function of time-varying irradiance, chlorophyll a concentration, and three model-derived photosynthetic parameters. The model does a good job of simulating the in situ incubation data obtained on days of high irradiance during the bloom study. The photosynthetic parameters a, /? and P:Z obtained from fitting the bloom data are therefore representative of a natural and diverse marine community. Generally, the P,: and u parameters, respectively reflect production capacity per unit chlorophyll and photosynthetic efficiency of the phytoplankton community, are about twice the magnitudes typical of other midlatitude sites. The moderately large /? values on days of high irradiance indicate production was photoinhibited throughout the two week period. Production per unit biomass, integrated over the euphotic zone. is found to vary linearly with total daily irradiance. The linearity is an expected feature of the P& relationship in 1990) and can be simulated with the the euphotic zone (PLATT et al., 1988; CULLEN, production-irradiance model used in this paper to describe daily production. At the NABE site, the slope of the linear relationship between production and irradiance (the q\Ir parameter) is within the range observed for other marine environments. Community respiration consumes an amount of carbon equal to about 60% of the carbon fixed daily. About half of this O2 demand is associated with respiration of “new carbon” (carbon fixed in the more recent photoperiod), and half with consumption of “old carbon” (carbon present prior to the most recent photoperiod). New carbon respiration is on a given day is proportional to the production rate, while old carbon respiration independent of production and depth. Despite the high POC accumulation evident at the NABE site (typically 5-15 ,umol C I- ‘), high production rates permit high turn-over rates: one to two days in surface waters. Deeper euphotic waters can turnover their POC supply in 10-15 days. Acknowledgements-Wc express OUTgratitude to Chris Brown, W. S. Chamberlin. Carol Knudson and Jot Orchard0 for assistance in measuring production rates. We are also most appreciative of the helpful manuscript reviews provided by John Cullen. Pctra Stcgmann, Jim Yoder and John Milliman. This research was supported

Production

by grants

from the National

and respiration

Science

Foundation

changes

OCE

in N. Atlantic

spring bloom

X6-09923 and OCE

88-17514

575 (JK and MB) and OCE

88-17.515 (JM).

REFERENCES BFNDER

M. L., H. Ducn~ow, J. KIDDON, J. MARRA and J. MARTIN (1992) The carbon balance during the 1989 spring bloom in the North Atlantic Ocean. 47N , 20W. Deep-Sea Research I, 39, 1707-l 726. BENDER M. L. and K. D. GRANGE (iY87) Production, respiration, and the isotope geochemistry of 0, in the upper water column. Glohul Biogeochemical Cycles. 1, 49-N. D. W.. J. MARRA and 1‘. TAKAHASHI (1992) Primary production at 47”N/20”W in the North Atlantic Ocean: A comparison between lJC incubation methods and the mixed layer carbon budget. Deep-Seu

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