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OCEANOLOGICA ACTA 1986- VOL. 9 - W 1
~
-----!~-
Production
Chlorophyll
Light
Mode!
Profiles
Production
Chlorophylle
Lumière
Modèle
Profils
Primary production
profiles in the ocean:
estimation from a
chlorophylljlight model
Alex W. HERMAN", Trevor PLATTb
Department of Fisheries and Oceans, Atlantic Oceanographie Laboratory, Bedford
Institute of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada.
b Department of Fisheries and Oceans, Marine Ecology Laboratory, Bedford Institute
of Oceanography, Dartmouth, Nova Scotia B2Y 4A2, Canada.
a
Received 11/10/84, in revised form 2/7/85, accepted 3/7/85.
ABSTRACT
A chlorophylljlight model has been developed to estimate high resolution vertical
profiles of primary production, and therefore, integrated water column production in
the ocean. The model requires continuous profile measurements of chlorophyll and
light, surface light, and two parameters characterizing the relationship between photosynthesis and light. Primary production profiles measured from in situ incubation
experiments and simulated in situ experiments using a deck incubation box were used
to verify the model by statistically fitting the model equations to profile data from
the Scotian shelf. Generated profiles showed that the subsurface production maximum
occured in the upper chlorophyll gradient, and that much of the subsurface chlorophyll
layer was not photosynthetically active because of the unavailability of light. The
model can also be used as a tool in analyzing the changes in production that result
from small vertical changes in chlorophyll structure and to a lesser extent, from
changes in surface light. The variations in estimates of integrated water column
production indicated the necessity of resolving horizontal variability of profiles.
Oceanol. Acta, 1986, 9, 1, 31-40.
RÉSUMÉ
Profils de la production primaire dans l'océan: évaluation à partir d'un
modèle simulant la teneur en chlorophylle/intensité lumineuse.
On a élaboré un modèle simulant la teneur en chlorophylle/intensité lumineuse en vue
d'évaluer les profils verticaux à haute résolution de la production primaire et, ainsi,
la production intégrée sur la colonne d'eau dans les océans. Le modèle nécessite la
mesure en continu de la teneur en chlorophylle et de l'intensité lumineuse, de l'illumination à la surface et de deux paramètres caractérisant la relation entre la photosynthèse
et l'intensité lumineuse. On a utilisé les profils de production primaire mesurés au
cours d'expériences d'incubation in situ et d'expériences en milieu naturel simulé
réalisées dans des incubateurs à étages, pour vérifier le modèle en adaptant les
équations qui le décrivent aux données sur les profils obtenues sur la plate-forme
Scotian. Selon les profils obtenus, la production sous la surface est maximale dans le
gradient chlorophyllien supérieur, et une bonne partie de la couche de chlorophylle
sous la surface n'est pas active du point de vue photosynthétique en raison de l'absence
de lumière. Le modèle peut aussi servir à analyser les variations de production
provoquées par de faibles variations verticales dans la structure chlorophyllienne et, à
un moindre degré, par des variations d'intensité lumineuse à la surface. Étant données
les variations de la valeur estimée de la production intégrée sur la colonne d'eau, il
est nécessaire de résoudre la variabilité horizontale des profils.
Oceanol. Acta, 1986, 9, 1, 31-40.
0399-1784/86/01 31 10/$ 3.00/ ~ Gauthier-Villars
31
._...,
.. ,"'
l
A. W. HERMAN, T. PLATT
INTRODUCTION
Simulated production profiles
Primary production of the oceanic water column can be
estimated by the 14C technique using in situ incubation
experiments of deck incubations in natural or artificial
light. In a few studies, both methods have been used
simultaneously (Gargas, Nielsen, 1976; Jitts et al., 1976;
Brown, 1982) using samples form approximately 5-6
depths corresponding to predefined light levels, e. g.,
100%, 50%,25%, 10%, 1% of surface light. Comparison of the methods bas generally shown reasonable
agreement although there have often been systematic
differences; deck incubations have generally yielded
higher (approximately 20-40%) production measurements than in situ incubations. Fee ( 1973 a; b) bas
developed a model for estimating integrated column
production using empirical data relating light and photosynthesis. Samples were drawn from only 4m depth
and the assumption that phytoplankton were distributed homogeneously throughout the water column.
The problem in water column production estimates is
the Jack of vertical resolution of either chlorophyll a
or production. Fee ( 1973 b) suggested that continuo us
in situ chlorophyll fluorescence measurements in the
water column would help to refine future production
estimates. We describe here a chlorophylljlight mode)
which can be used to: 1) generate the fine scale profile
of primary production; and 2) to estimate integrated
water column production. In practice, once the
photosynthesis-irradiance (P-1) characteristics of the
sampling area are reasonably weil established, the
method requires only profiles of chlorophyll a and
irradiance thereby permitting quasi-instantaneous estimates of water column production to be made. Application of the mode! is tested against high resolution
(vertical) direct measurements of primary production
on the Scotian Shelf and provides insight into the
importance of the vertical structure of pigment biomass
on water column productivity.
The method of generating simulated production profiles (SIS, simultaned in situ) is based on our standard
incubator procedure (Platt et al., 1980) using an incubation box and artificial lights. This method differs from
the simulated in situ (SIS) profile (e. g., Harrison et
al., 1984) which employs ambient solar light for deck
incubations. Our experimental design afforded higher
vertical resolution ~ 3 to 5 rn, since the samples were
incubated at the same light leve! as that of the collection
depth. Continuous profiles of irradiance and chlorophyll were sampled with the pump sampler system
at about noon. A series of seawater samples was pumped from various irradiance levels as labelled Ll, L2,
... Ln, in Figure 1 a which correspond to an identical
series of irradiance levels in the incubation box (Fig.
1 b). The irradiance levels within the box were measured
with the same PAR light sensor used on the pump
sampler, thus minimizing any potential systematic
errors arising from improper matching of irradiance
between in situ profile and incubation box. Each irradiance, L1, L2, ... Ln, represents a measured depth,
Dl, D2, ... Dn (Fig. 1 a), and the samples in the
incubation box represent a profile of measured production. With the number of samples N = 20 taken over a
depth range of 0 to 60m, the vertical resolution
between samples was ~ 3 m. The first three positions
of the incubation box were generally in a light saturation region of the photosynthesis - light curve
(1 > 100Wm- 2). The light sources used to irradiate
the incubation box were in most cases tungsten-halogen
lamps; in earlier experiments, incandescent lamps were
used. The dependence of production rates on the
spectral characteristics of these lamps is discussed later.
Following establishment of the initial irradiance profile
with the pumping system at mid-day, the twenty sampies from depths Dl, D2 ... Dn were obtained over a
period of ~ 45 mn during which the stability of the
profile could be monitored from the temperature,
fluorescence, and light measurements. Immediately following the seawater sampling, severa! additional profiles of temperature, chlorophyll, and light were taken
(each requiring only ~ 2 min deployment) with the
pumping system. Incident solar irradiance was recorded
and integrated (h - 1) on the ship's deck. Chlorophyll a
concentrations of each of the 20 samples in the incubation box were determined from acetone extractions.
INSTRUMENTATION AND METHODS
Pumping system
A continuous profiling pumping system (Herman et al.,
1984) was used for seawater sampling and biological
profiling. The system employs a Ramoy progressive
cavity pump. An intercomparison of production rate
measurements from samples taken with the pump and
from Niskin bottle casts showed no statistical difference, implying that no measurable damage to photosynthetic organisms was caused by the pumping system.
Sensors mounted on the pump frame were used to
and pressure
measure temperature ( ± 0.2°C)
(±0.2dbar). A quantum light meter (LICOR) cousisting of a flat plat collector with near eosine response
measuring irradiance over a 4 decade range from a
maximum of~ 500Wm- 2 (integrated over the photosynthetically action radiation (PAR) region of
400-700 nm.) was also mounted on the pump frame. A
small portion ( ~ 4 1 min - 1 ) of the pump outflow was
diverted to a Turner 111 fluorometer which measured
chlorophyll a fluorescence.
In situ production profiles
Experiments were also performed to measure in situ
profiles by inoculating a number of seawater samples
with 14C and returning them to their depth of origin
for incubation. Three botties (2 light and 1 dark) were
attached to a wire at each depth over the full depth
range of 3 to 85 rn at 3 rn intervals, between 20 and
50 rn and 5 rn intervals over the remaining depths. There
were approximately 20 depths sampled overall. The
wire, buoyed at the surface, was deployed at approximately 1100 hrs and recovered after about 4 br of incubation. Chlorophyll and temperature profiles were sampied intermittently near the wire to monitor profile
32
CHLOROPHYLL/LIGHT MODELLING
by integrating equation (3) over time t
(b)
(a)
1\
ln\
o.-------
24 hr
UGHT
SOURCE
Cd (Z) =
L1 ,Dl
201-
.s:r: 40 1-
L2,D2
L3,03
L4,04
x )( x
!--=x x x
X
X X
Z
~60
Ca=
TEMP.
CONTROL
FLOW
1---:.
24 hr
ff
0
Pv(z, t) dtdz,
(4)
0
where Zr, the final depth, is selected to be large enough
( ~ 80 rn) such that increasing it would not increase the
integral significantly.
1---
100
Figure l
A representation of in situ wire incubations (a) in which samp/es,
duplicate /ight botties plus one dark bottle, were suspended at 20 light
depths and (b) the equivalent simulated in situ incubations where
samp/es are po.~itioned in the incubator to receive /ight equivalent to
in situ leve/s.
Non-linear least square analysis
Fitting of the mode! equations ( 1 and 2) to directly
measured production data required two measured input
profiles: light and chlorophyll. Values of a and Pm
from previous production studies on the Scotian Shelf
(Herman et al., 1981; Herman, Platt, 1983) could be
used; however we have no assurance of the constancy
of the se parameters from year to year and therefore
they were considered free parameters (i e., fixed values,
not specified) in the non-linear !east squares analysis.
Consider Aj as representative of the dependent variables a ana Pm, theo the optimum values of Aj are
obtained by minimizing chi-squared, x2 , with respect
to each of the parameters separately:
variations. lntegrated hourly solar radiation levels
required for calculating integrated daily production profiles were recorded on deck using a LICOR integrating
quanta meter. Chlorophyll a concentrations of each of
the bottle samples on the wire were determined from
acetone extractions.
Chlorophyli/Light model
Equations
Our numerical modelling is based on the construction
of production profiles from the relationship between
photosynthesis and light. ln our representation, the
dependence of primary production P (1) per unit mass
of chlorophyll (mgC [mgchl a]- 1 h - 1) on available light
is given by the photosynthesis light saturation curve
(Jassby, Platt, 1976; Platt, Jassby, 1976; Chalker, 1980).
(5)
where Pv (Z) represents the calculated value (equation
2) of production and Pe (Z), the experimentally measured in situ value. Chi-squared (x 2 ) can be considered a
continuous function of the parameters Aj describing
a two-dimensional surface in three-dimensional space
which can be searched for the corresponding minimum
value of x2 • Problems can be encountered when the
hyperspace contains more than one minimum. Therefore the search must be restricted to a region when
there is only one known minimum of x2 and this is
accomplished by selecting realistic starting values of a
and Pm during the search routine.
(1)
where 1 is the irradiance (PAR-photosynthetically
active
radiation)
in
W rn- 2 ,
a(mgC[Chiar 1 • w- 1 m+ 2 h- 1 ) is the initial slope of
the light saturation curve, Pm(mgC[mgchlar 1 h- 1)
is
the
assimilation
number,
and
R(mgC[mgchtar 1 h- 1) is a measure of dark respiration. The magnitude of R is generally ~ 0.1 in our
shelf waters and small enough to be ignored in equation
(1). The irradiance, 1, is a function of depth and therefore P (1) can also be represented as a vertical production profile.
Absolute
production
profiles
in
units
of
(mg C rn- 3 h - 1 ) are obtained by multiplying the production P (1) of equation (1) by the chlorophyll a profile
( units of mg. rn- 3 ) as measured with the pump profiler
and is given by
Pv(Z) =P(Z). B(Z),
(3)
The areal primary production per day in the water
column (Ca) in units of mgCm- 2 d- 1 can then be
obtained by integrating equation (3) over depth
INCUBATION
BOX
l-
P(l) =Pm tanh(ai/Pm) -R
Pv (z, t) dt.
0
a..
80
f
Dependence of Production on .Spectral Quality
The validity of comparing production measurements
based on solar and artificial light-incubations was examined here. The differences in spectral quality may give
rise to significant differences in production estimates.
Examples of the energy spectra of solar, incandescent
and tungsten-halogen light are shown in Figure 2. The
quantum energy spectra were measured with a quantaspectrometer (QSM-2500, Techtum Instruments). The
artificiallight source spectra were measured at ail positions (two shown in Fig. 2) of the incubator box of
Figure 1 and the solar spectra were measured at various
depths (0-50 rn) in Bedford Basin, Nova Scotia. The
(2)
where B (Z) is the chlorophyll concentration and Z is
the depth in meters. The daily primary production
profile, Cd in units of mgcm- 3 d- 1, can be obtained
33
A. W. HERMAN. T. PLATI
taeniata (Steeman Nielsen, 1975; Fig. 11), a macrophyte
presented to illustrate the magnitude of the effect arising from considerable differences in the action
spectrum at larger wavelengths. The production ratio
of equation (7) was evaluated using the quantum
spectrum, Eart (À.), corresponding to either the incandescent or tungsten-halogen lamps in Figure 2 and the
photosynthetic spectrum of Figure 3 a. The ratios for
both photosynthetic spectra as a function of integrated
PAR quantum energy are plotted in Figure 3 b. High
and low values of light energy in Figure 3 b essentially
correspond to the front and back positions of the
incubator box shown in Figure 1 and from surface to
depth in the case of solar energy. The data of Figure
3 b indicate that production measurements using artifical lights are reduced by approximately 10% relative
to solar energy in the case of Ph 1 ( Ulva taeniata) and
approximately 20-30% in the case of Ph 2 (Pleurochloris).
The overall decrease in production is caused by the
inability of pigments to utilize efficiently the excess
of artifical red light according to its photosynthetic
absorption spectra. There is a small but relatively insignificant slope in Figure 3 b, that is, a dependence of
the ratio on light intensity.
INCANDESCENT
10
0~~~~~--~------~
7
E 5
TUNGSTEN
HA LOG EN
c:
~
'!=
"'
~
!!
Q
SOL AR
Figure 2
Energy spectra of two artificial light sources, incandescent and
tungsten-halogen, and solar radiation. The lower intensity for the
artificial light sources correspond to the rear positions of the incubation
box; the lower intensity of solar light represents the spectrum at 35 m
depth in Bedford Basin.
RESULTS
A compilation of the statistically derived model parameters, et and Pm, and other associated data are presented
in the Table.
artificallight sources of Figure 2 exhibited more energy
in the red region; however, the tungsten-halogen light
had a relatively higher production of blue to red light.
The effect of this red light excess on production will
depend on the efficiency with which it is used by phytoplankton cells relative to blue light. This is defined by
the photosynthetic absorption cross-section or action
spectrum, Ph (À.). Production, therefore, can be defined
in terms of the product of the photosynthetic crosssection, Ph (À.), and incident energy, E (1..):
~
1.o
(a)
l-
u
~~.8
Ul~
'ë
~::J •6
f-
w"'
J:~
~~.4
>-Il:
U>~
1
Pr et
0
2=700
f
b
Ph (À.) E (À.) dl...
.2
I
(6)
a..
~OL0~~~~-5~0-0~~~~~~~~~~70~0
u=400
WAVELENGTH
The fractional change in production induced by artifical
light relative to solar light can be represented by:
t=
~1.0
u
1.2=700
(Pr) ART/(Pr) SOL=
f
t=
w
Ph (À.)
>-
•
lt •
l l l •••• "t • • 1 ~
Ph2
T __
Ph
1 __________________
l
.8
Ul
0
b .7
1.2=700
f
.9
.
I
~
u=400
Eart (À.) dl../
(b)
0
oiNCANDESCENT/SOLAR
x TUNGSTEN/SOLAR
I
a..
Ph (À.) Esol (À.) dÀ (7)
•
•
x••
• • x"
l
•
•x.
•tl. tl
•• x
x"
JL
••
10
100
1000
LIGHT QUANTA (J.LE m-2 s·l)
u=400
The photosynthetic cross-section, Ph (À.), changes with
species and is dependent on the light absorbing pigment
complex. Two examples are shown in Figure 3 a:
Pleurochloris (from Prézelin, 1981; Fig. 3) and Ulva
Figure 3
a) The photosynthetic action spectra of two a/gal species with different
bluefred energy absorption characteristics; and b) the ratio of the
expected photosynthetic rates of samples irradiated with artificial a.~
compared to solar light sources.
34
CHLOROPHYLL/LIGHT MODELLING
PRODUCTION (mgC·m· 3 (5hrr 1)
4.0
8.0
12.0
0
1
1
1
CHLOROPHYLL (mg
10
20
0
1
1
0
0
approxima tel y 35 m. The measured total production,
Pv (mgcm- 3 ) (equation 2) for the 5hr incubation
period indicated a distinct subsurface layer with a maximum at approximately 25 m. Here we refer to the
subsurface layer as that which has a distinct chlorophyll
maximum at depth in contrast to the much lower
concentration but more uniform chlorophyll in the
mixed layer (i.e., 0 to 15 m).
The mode! (best-fit) production curve Pv (Z) of
equation (2) was obtained by varying the parameters,
ex and Pm, in equation (1). Ambient irradiance 1 was
not constant over the 5 hr incubation period and therefore P (Z) in equation (2) was evaluated from equation
( 1) by integra ting over the 5 hr interval. This was accomplished by taking the 1130 h irradiance profile as representative of the period between 1100-1200h and subsequently scaling the profile proportionately with fractional changes in the hourly integrated surface
irradiance as measured on deck. Hence production in
Figure 4 is expressed as mg C rn- 3 /5 hr interval.
Curve fitting the model to the measured in situ production data set resulted in best fit values ex= 0.08
(cr~ ±0.006) and Pm= 1.35(cr ~ ±0.16)(Tab. ). Starldard deviations were extracted from the twOdimensional x2 hyperspace plotted as a function of ix
and Pm in Figure 5. The x2 contours describe both
the minimum, and first and second standard deviation.
Daily areal production Ca was evaluated from equation
(4), using measured houri y rates of solar irradiance.
For this station, we measured Ca= 327 mg C rn- 2 d - 1 •
This was slightly lower than production rates measured
in our past work which averaged approximately
400mgcm- 2 d- 1• The slight reduction is due partly
to a lower than average Pm, which in previous years
was in the range 2.0-2.5 (see Herman et al., 1981; Irwin
et al., 1977) and partly due to a deeper than average
chlorophyll layer and therefore generally Jess light
available.
14.0
1
1
m
3)
1
TEMPERATURE ("C)
2
4
6
3;0
1()
12
STATION No. 67
Ca•327mg C·m" 2 ·d" 1
Figure 4
Station 67 showing chlorophyll, temperature, and production profiles
samp/ed from the Scotian Shelf The points represents in situ production measurements. The production curve represents a best-fit to the
data set. Ch/orophyll concentrations were estimated at the same depths
as production indicated by the data points. The 1 % /ight depth is
indicated.
In situ production profiles (wire experiments)
Station 67
Figure 4 illustrates a vertical profile sampled at the
outer edge of the Scotian Shelf including measured
chlorophyll a ( determined from acetone extractions),
temperature, primary production (mgCm- 3 ), and the
mode! generated production curve. The chlorophyll
maximum was at approxima tel y 30 rn depth; its vertical
distribution was skewed at greater depths such that the
centre of gravity of the chlorophyll distribution was
Table
Compilation of data from both in situ wire experiments and simulated in situ deck experiments including best-fit parameters IX, Pm. Mean light
represents an average over 24hrs and are meant to provide relative indications on/y. The standard deviations quoted for ex, Pm are given as means
of negative and positive errors which actually differas seen in Figure 5. Positive errors were actually ~ 10% higher and negative errors ~ 10%
lower than the mean.
Station No.
Day
April-May
Incubation
Time (hrs)
27
50
58
72
105
126
28
30
1
2
4
4.0
4.0
4.0
4.0
4.0
4.0
5
Mean light
W.m- 2
Ca
mgcm- 2 d- 1
IX
SIMULATED IN SITU INCUBA TI ONS
109
49.5
196
127.5
282
112.9
125
87.6
59.2
52
59.5
123
(±cr)
0.022
0.036
0.035
0.021
0.036
0.033
Pm (±cr)
(0.0014)
(0.0047)
(0.007)
(0.003)
(0.005)
(0.007)
1.60 (0.13)
2.57 (0.45)
1.67 (0.20)
1.11 (0.09)
1.32 (0.13)
1.47 (0.11)
Mean=0.032 (0.0055)
1.62 (0.51)
Mean=
55
67
98
119
1
2
4
5
5.0
5.0
4.5
5.5
IN SITU WIRE INCUBATIONS
112.9
235
327
87.6
59.2
214
183
59.5
35
(0.007)
(0.006)
(0.016)
(0.010)
0.98 (0.11)
1.35 (0.16)
1.59 (0.36)
3.09 (0.31)
Mean=0.086 (0.010)
1.75 (0.24)
0.044
0.081
0.14
0.079
A. W. HERMAN, T. PLATT
x2- CONTOUR
at STN 67 was situated again on the upper chlorophyll
gradient with the production maximum at approximately 35 to 40 m. A real daily production was low with
Ca=214mgcm- 2 d- 1 as a result of two factors: 1) a
deeper chlorophyll maximum; and 2) a sharp decrease
in incident light caused by fog in the latter 2 hr of the
wire experiment.
Best-fit estimates of a= 0.14 and Pm= 1.59 were
obtained. Note that the model was unable to fit satisfactorily production at 5 m, tending to underestimate it.
This suggests that Pm is probably not constant with
depth but tend to be higher (approximately 2.0-2.5)
near the surface ( < 10 rn).
The dashed line curve of Figure 6 indicates the potential
production if ambient light were doubled. Under these
conditions the vertical structure of production layer
remains essentially unchanged; however, the curve area
bas increased by approximately 50% such that
Ca=334mgCm- 1 d- 1 • Doubling ambient light did
not proportionately increase production since the production in the surface water < 15 rn was already saturated
and did not benefit from increases in ambient light.
a • .081 +- .006
•007
.:: .10
''.c
+'.17
Pm•1.35
-.l 5
~
ct
..J
5·09
Ë
':'E
3: .08
u
E
"'
;.o1
1.2
1.3
1.4
1.5
Pm (mgC·[mgCHLg)' 1 ·h' 1 )
1.1
1.6
Figure 5
contours for STN61 corresponding the first (X 1 =3.11) and second
(X 1 = 3.65) standard deviations of r:x., Pm.
x1
The temperature profile (Fig. 4) showed; a) a deep
layer of nutrient-rich slope water at depths > 50 m
intruding over the shelf edge; b) an intermediate cold
water layer between approximately 20 and SOm; and
c) a surface mixed layer down to 15 m. Both the surface
and intermediate layers were approximately 3ec and
cooler than temperatures measured in previous years
(e.g., Herman et al., 1981), a result of a tate discharge
of cooler, fresher Gulf of St. Lawrence water flooding
the shelf in March. Generally, we find subsurface chlorophyll layers in the intermediate water layer where the
nutricline is situated, above high-nutrient slope water.
The temperature structure shown in Figure 4 was typical of the entire cruise.
Stations 55 and 119
Figure 7 illustrates stations with deep chlorophyll
maxima at 40 m and 50 m depth where production was
low. The profile of STN 55 showed surface chlorophyll
nearly double that of STN 119 at depths < 30m which
resulted in higher production near surface. As a result
of significant light absorption in surface waters, there
was no distinct production layer formed within the
chlorophylllayer at approxima tel y 30 to 40 rn depth.
The wire experiment of STN 119 showed low production Ca= 183 mg Cm - 1 d - 1 as a result of low ambient
light 1Mean=59.5W.m- 2 • The mean light lmean represents an average over a 24-hr period and is only meant
to be used as a relative indicator of average light
measured from station to station. Production was
restricted to depths (15 rn; the best-fit value of
Pm = 3.1 (±0.31).
Station 98
The chlorophyll maximum layer in the profile of Figure
6 was at approximately 45 m as a result of a deeper
intermediate cold water layer. The production layer as
PRODUCTION (mg C m· 3 ,(4,5 hr)'1)
4D1
0
1
ao
CHLOROPH~LL
LO
0
1
œo
1
PRODUCTION (mgC·m·3 , (5hr)' 1)
(mg m' 3 )
2,0
1
0
3.0
1
ep
4;0
1
TEMPERATURE (•c)
LO
o.--.-.--------------To--~zr-.-4r-~sr-
o,________:4'T,O'-------'B';',O'----
CHLOROPHYLL (mg m' 3 )
O'l---,------';"-'r--------'2"f'-,O
20
20
.5
I40
l-
a.
w
0
(
eo
STATION No. 98
1 Ca= 214
mg C· m-zd- 1
STATION No. 55
Ca= 235mg C·nf2 ,d'1
Figure 6
Station 98 wire experiments showing temperature, chlorophyll, and
fitted production profiles. The dashed line production curve corresponds
to a doubling of solar light intensity.
STATION No. 119
Ca= 183mg C·m2 ·d-1
Figure 7
Stations 55 and 119 wire experiments showing chlorophyll and fitted
production curves.
36
CHLOROPHYLL/LIGHT MODELLING
~ 28 rn
depth, and low surface concentrations
( ~ O. 2 mg rn- 3) at depths < 15 m. Light penetrations was
high at this station resulting in a production layer
nearly coïncident in depth with the chlorophyll layer.
The mean depth of the production layer was located
slighly shallower at approximately 25 rn depth. The
best-fit value of Pm=2.5 was the highest obtained for
the six stations sampled.
PRODUCTION (mg C·m· 3- (4hr)"1)
o.__ _ _4.:.r.o~- o
4.0
s.o
CHLOROPHYLL (mg m·3 )
0 or-....---=-r--4.:;;.o::..... o
2.0
1.0
1
(
•
·._FITTED
)•
PRODUCTION
Station 72
It had low areal productivity, Ca=125mgcm- 2 d- 1,
as a result of low ambient light, Imean=87.6W.m- 2 • A
relatively small production peak was observed at 32 rn
depth.
)
60
Î
STATION No. 27
Co=I09mgC·m" 2 ·d-1
STATION No. 58
Stations 105 and 126
Co= 282mgC·m" 2• d"1
These stations exhibited low areal production due to
low ambient light. There were no subsurface production
layers formed (see Fig. 10) as a result of low light
penetration into deep chlorophyll maxima between
~ 40-50m depth.
Figure 8
Stations 27 ans 58 showing chlorophyll and fitted production profiles
corresponding to the simulated in situ (SIS) experiments. Each profile
shows a distinct production maximum at the upper chlorophyll gradient.
Simulated production profiles (Incubation exp.)
PRODUCTION (mg G- m· 3 • (4hr)' 1 )
Stations 27 and 58
Samples used in these simulated profiles were taken at
1300 hr and incubated for 4 hr. The chlorophyll maxima
were located at approximately 30m (Fig. 8) and the
production maxima were found approximately 5 rn shallower and coïncident with the upper chlorophyll gradient. Although the production curve was fitted for the
data derived from incubations at a constant irradiance,
daily areal production was obtained by integrating the
measured hourly production over day and over depth.
Areal production at STN 27 was extremely low
(Ca= 109mgm- 2 d- 1) due to low ambient light. Chlorophyll concentrations, however, were extremely high
(~ 6mgm- 3 at the maximum).
j
0
4.0
4.o
8.0
FITTED
0~-----~1.0~---
----,_.
-PROOU~----
60
STATION No. 105
Co=52mgC·m·2 -d·l
STATION No. 126
Co•l23 mg C·m·2 -d" 1
Figure 10
Stations 105 and 126 showing chlorophy/1 and production profiles.
PRODUCTION (mg C·m3·(4hr)' 1 )
1
o
~r---~~-~0_ _ _~2.0~
The chlorophyll layer of STN 50 (shown in Fig. 9)
showed a low maximum concentration ( < 1 mg rn- 3 ), at
4.0
e.o
CHLOROPHYLL (mg m·3)
Stations 50 and 72
0
4.o
0
DISCUSSION
8.0
CHLOROPHYLL (mg m'3 )
h-r--'ii.O,___
0
1.0
One objective of this investigation was to examine
the vertical structure of production relative to that
of chlorophyll. One important vertical feature became
apparent in our data set, i. e., carbon production did
not parallel chlorophyll in the vertical scale. In general
only the upper chlorophyll gradient supported high
primary production, i.e., STN 67, 68 (wire experiments
and STN 27, 58 (SIS experiments). The lower half of
the subsurface chlorophylllayer, below the chlorophyll
maximum, contributed little to the areal production in
nearly ali cases. There are severa! conditions necessary
for this to occur. First, the near-surface chlorophyll
must be low in concentration ( < 0.5mgm- 3) such that
light can penetrate to the subsurface chlorophyll layer.
Second, the subsurface chlorophylllayer must be large
enough in biomass to absorb essentially ail available
light. If we take the example of STN 98 (Fig. 6) it can
be seen that light was able to penetrate through low
2.0
--------
?
:J:
....
Q.
"'0
STATION No. 50
Ca•l96mgC·ITi 2·d'1
STATION No. 72
Ca= 125 mg C· m' 2·d' 1
Figure 9
Stations 50 and 72 showing chlorophyll and fitted production profiles.
37
A. W. HERMAN, T. PLATT
surface concentrations (0.25mgm- 3 ) to the subsurface
chlorophyll layer and drive maximum production at
35 m. The subsurface chlorophyll layer was situated
between approxima tel y 32 and 55 rn depth with à maximum at 43 rn; however, only the chlorophyll subpeak
at 35 rn depth was active in carbon production. Similar
observations were made for the remaining STNS 67, 27
and 58.
·
At STN 55 (Fig. 7) slightly higher surface concentrations (> 0.5mgm- 3 ) of chlorophyll resulted in high
surface absorption of light and active production there.
Despite a high irradiance Omean = 113 W.m - 2 ), there
was little penetration to the subsurface chlorophyll
layer and as a result there was no observable production peak within the upper vertical chlorophyll gradient
at approximately 35 m.
At STN 50 (Fig. 9) the production peak was nearly
coïncident with the chlorophyll maximum. This was a
consequence of 1) low surface chlorophyll concentrations permitting light penetration into the subsurface
chlorophyll layer; and 2) a relatively shallow and low
concentration subsurface chlorophyll layer. In the
remaining stations (105, 119, 126) carbon production
was generally low and these profiles had no subsurface
production peaks. This most likely resulted from 1) low
surface irradiance and 2) deeper subsurface chlorophyll
layers.
Changes in production were most sensitive to vertical
changes in the subsurface chlorophyll layer. This was
due to the rapid exponential decay of light with depth
which is dependent on principally two components
roughly equal in magnitude: a) attenuation due to inorganic material in water column; and b) that due to
photosynthetic plant material. Light dependence may
be represented by
I = Io exp [- (CX;n + rxp 1)Z],
where rx;n represents attenuation due to inorganic material and a.P1 represents attenuation due to photosynthetic plant material. If we were to shift, for example, a
chlorophylllayer with a maximum at 40m depth to a
depth of 20 rn, the chlorophyll layer would gain a
considerable increase (exponential) in light intensity
which was previously attenuated between 20 to 40m
depth by inorganic material. The more intensive light
at 20 rn results in greater penetration throughout the
chlorophylllayer rather than just the upper chlorophyll
gradient. Hence a large increase in production results.
Mode! estimates indicate that a 20 rn deepening results
in a decrease of approximately 2.5 in areal production.
The second objective of this investigation was to test
the utility of the chlorophylljlight model in estimating
water column carbon production as determined from
equation (4). Each measured production data set was
curvefitted to the modelled production equation (2) by
adjusting the parameters ex and Pm. The mode!
equation (4) (not the experimental data set), using the
bestfit values of ex and Pm, was used to estimate areal
water column production. The most striking feature of
these estimates was the wide range of areal production
estimates, which we attributed primarily to vertical
changes in the subsurface chlorophyll. For example, the
wire experiment of STN 67 (Fig. 4) showed a subsurface
chlorophyll maximum at approximately 25 rn which
supported
areal
carbon
production
of
Ca= 327 mg Cm - 2 d-t.
La ter that afternoon an SIS ex periment STN 72 (Fig.
9) showed a depth increase of chlorophyll maximum
now at approxima tel y 35 rn which resulted in a significant decrease in a production peak at that depth and
a similar decrease in areal production where
Ca= 125mgcm- 2 d- 1 • A similar situation occurred in
the wire ex periment of STN 98 (Fig. 6) where a subsurface chlorophyll layer was found 10m deeper in the
afternoon profile of STN 105 (Fig. 10). The resulting
increase in chlorophyll maximum depth accompanied
by a decrease in surface irradiance resulted in a fourfold
decrease in areal production. In comparing the wire
experiment of STN 55 (Fig. 7) with the SIS experiment
of STN 58 (Fig. 8) performed that afternoon, we
encountered a decrease of approximately 10 rn in the
subsurface chlorophyll layer which resulted in an
increase for the SIS estimate of areal production.
It was also clear that production is most sensitive to
vertical changes and to a lesser extent to absolute
surface irradiance. The latter instance was seen in the
example of STN 98 (Fig. 6) where the ambient light
was arbitrarily doubled in the calculation and resulted
in an integrated production (short dashed line) increase
of on! y approximately 50%. This comparative!y smaller
increase was caused by light saturation in the surface
layer, < 20 rn as seen in Figure 6 where rouch of the
incident light was not utilized.
The curve-fitting analysis of each of the wire and SIS
experiments yielded independent estimates of ex and
Pm. The mean values of the assimilation number, Pm,
for either wire (Pm= 1.62) or SIS (Pm= 1.75) experiments were in close agreement, within approximately
10%. An independent check of the assimilation number
was made on the same cruise from an experiment
reported in Herman et al. (1984). Fifteen samples were
taken from various depths and incubated (triplicate)
for 4 hr at light saturation (I ~ 175 W.m- 2 ). The mean
assimilation number for 15 samples was Pm=1.3±0.2
indicating agreement with the curve-fitted data. The
absolute values of Pm were somewhat lower than previous years' measurements (Herman et al., 1981) where
Pm= 2.4. There were also indications of sorne dependence of Pm on depth in STNS 98 (Fig. 6) and 126
(Fig. 10). The mode! equations underestimated Pm in
the 5 rn surface layer. More realistic values of Pm= 2-4
would have improved the fit in this depth region.
We have evidence from our previous data (Herman et
al., 1981) which favors the view that photosynthetic
parameters are constant with depth. This depends,
however, on the degree of stratification in the water
column. The tendency of phytoplankton cells to modulate their photosynthetic apparatus according to
ambient light levels (photo-adaptation) is opposed by
vertical mixing which redistributes the cells to different
light levels. The resulting vertical structure of photosynthetic properties will then depend on the ratio of the
characteristic time scale for vertical mixing to that for
photoadaption (Lewis et al., 1984a). It is estimated
that, in the sea, vertical mixing caused by winds
38
CHLOROPHYLL/LIGHT MODELLING
~
5 rn s- 1 is strong enough to erode vertical stratification (upper 30m) in the photosynthetic properties of
phytoplankton (Lewis et al., 1984b). This situation
would prevail on the Scotian Shelf much of the time.
We judge, therefore, that the assumption of uniform
photosynthesis parameters throughout most of the photic zone as used in our analysis is a reasonable one for
most of the stations sampled. We have observed sorne
deviations from uniformity of Pm in the upper few
metres of surface water. Similar discrepancies were
observed by Harrison et al. (1984) in both surface layer
(Om depth) estimates and the 10% light depth. White
recognizing these problems, it must be also noted that
this surface layer strata and the 10% light depth represent relatively small fractions of the overall water
column production resulting in changes of only 10 to
15% in the areal production estima tes.
The mean values of IX for either wire (1X=0.086) or SIS
(1X=0.032) experiments differed by nearly a factor of
3. However, the mean standard deviation for the wire
experiments was nearly an order of magnitude greater
than that of the SIS experiments, the latter clearly
yielding more consistent measurements. We offer no
clear explanation for these deviations; spectral differences in the wire and SIS experiments make little difference to photosynthetic rates according to our estimates
in Figure 3. One common difference is the constancy
of light used in the SIS experiments as opposed to the
more variable ambient source experienced in the wire
ex periment.
Harrison et al. (1984) compared data from a number
of deck incubations experiments using five light depths
with production estimates made from our model
equations and experimentally determined light saturation parameters IX and Pm. He found good agreement
in the comparison with the exception of small systematic differences where the mode! overestimated production in the surface layer and underestimated near the
bottom of the euphotic zone at (1% light level, 30-40m
depth). Estimates of areal production using either
method, however, differed by only 10 to 15% and
were not statistically different. We have no basis of
comparison since neither of our experiments were deck
incubations; however, our model in sorne cases slightly
underestimates production in the surface layer. Harrison et al. (1984) also found better agreement between
direct measurements and model estimates when derived
from the hyperbolic tan equation than when derived
from the more detailed exponential functions (Platt et
al., 1980) which allow for photo-inhibition at high
irradiances. This indicates that the hyperbolic tan function, simpler in form, is suitable for computing primary
production.
necessity for detailed information on the vertical structure of chlorophyll and light.
The spectral quality of the light source did not affect
the photosynthetic rates appreciably. The effect of using
artifical lights was to reduce photosynthetic rates by
approximately 10 to 30% as shown in Figure 3. This
means that the areal production estimates from the SIS
experiments in Table 1 are probably underestimated
and should be increased by approxima tel y 20%- Such
an increase, however, is insufficient to alter any of our
previous conclusions.
In ali this data analysis we have used chlorophyll a
concentrations derived from acetone extractions and
not relied solely on fluorescence measurements. The
implication for future sampling strategies is that fluorescence profiles accompanied by calibrations are adequate while overall accuracy will clearly depend on
calibration quality. In most cases we find that fluorescence measurements are preferable to use since they
represent a continuous systematic measurement. By
taking the mean calibration we may then reference the
fluorescence profile to absolute chlorophyll concentrations.
CONCLUSIONS
The chlorophyllflight model provides: 1) high resolution vertical profiles of primary production; and 2)
integrated water column production. In practice, model
production estimates require only measurement of:
(PAR) profile; 3) surface solar irradiance; and 4) the
photosynthetic parameters IX and Pm. Production profiles may then be generated from equation (2) and areal
production from equation (4). The photosynthetic parameters IX and Pm must be determined from field measurements of P-I curves as described by (Gallegos, Platt,
1981). Deck incubation using artificial lights yielded
the least variable estimates of IX and Pm in this work
(Tab. 1). The extent of these measurements in the field
and their adequacy in characterizing the environment
remain indeterminate. As little as four to five deck
incubation experiments (20 samples per incubation box)
were sufficient for sorne cruises on the Scotian Shelf;
however, three to four times as many experiments were
required for Peruvian shelf waters due to high variability of vertical chlorophyll structure (Herman, 1981;
Herman, 1984).
Considering the wide range of production measurements, it is clear that we must account for spatial and
temporal variability of areal pr_oduction estimates in
any survey. For example, we have obtained data at the
outer edge of the Scotian Shelf using a Batfish vehicle
(Herman, Denman, 1979; Herman et al., 1981) where
profiles were sampled continuously 0.5 km apart over
transect lengths of 40 to 50 km. Analyses of these data
(to be published) indicated spatial variability of ± 25%
for production over the transect length. However, while
moving from the Scotian Shelf to the shelf/slope where
a shelf break front was encountered, we found as much
as a factor of 2 in production variability along with as
much a variability in biomass and depth of chlorophyll
maxima. For example, in sorne areas we found that
Earlier modelling in the Great Lakes by Fee (1973 a)
assumed vertical homogeneity while recognizing the
problem of vertical variability in phytoplankton distribution. It is clear from our work that integrated water
column production is most sensitive to vertical structure in the biomass profile; even small changes of
approximately 5 to 10m in the vertical displacement
of the subsurface chlorophyll layers can result in large
changes in the areal production. This points to the
39
A. W. HERMAN, T. PLATT
subsurface chlorophyll layer in Scotian shelf waters
(Herman et al., 1981), Baffin Bay (Herman, 1983) and,
to sorne extent, in Peruvian Shelf waters (Herman,
1984). The mode! can be used as a diagnostic tool in
analyzing vertical production characteristics and trends.
For example, we were able to compare the large contribution of a surface spring bloom on the Scotian Shelf as
opposed to the Iesser contribution of summer stratified
subsurface layer. These differences may be qualitatively
self-evident but are often difficult to quantify. In the
Eastern Canadian Arctic, we were able to quantify Iow
areal production as a result of low irradiance at these
high latitudes; however, we also characterized the persistence of surface production as a result of extended
daylight.
the integrated chlorophyll was reduced by a factor of
2 and yet we encountered a peak in production (30%
increase) there simply because the subsurface chlorophyll maximum was shallower there than anywhere
else in the tow. By contrast, data we have anlyzed for
the Eastern tropical Pacifie showed variability of less
than 10% for integrated chlorophyll and production.
Here we found a reproducible series of profiles.
We have also determined a simple method (to be
published) which may be used as a rapid indicator
of spatial and temporal variability. Since integrated
production is directly dependent on integrated light the
1 % Iight depth can be used as this indicator. ln certain
areas of the Scotian shelf we found a inverse Iinear
relationship between integrated production and the 1%
Iight depth such that we were able to use the 1 % light
depth as a direct measure of areal production. A similar
pattern was found in the Eastern tropical Pacifie where
the 1 % light depth varied Iess than ± 5% about the
mean. In data from the Peruvian coast, we found the
relationship to be non-Iinear but consistent from tow
to tow.
There is considerable information derived from vertical
profiles of primary production. We have found a strong
coïncidence between the subsurface production layer
and copepod layers which were situated above the
l
Acknowledgements
We wish to thank Dr. Robert Fournier and Dr. Glen
Harrison for their critical review of the manuscript.
We also wish to thank the staff of the Metrology
(AOL) and Biological Oceanography Divisions (MEL)
and the crew and officers of the Lady Hammond for
their able assistance during field experiments and the
collection of data.
REFERENCES
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horizontal distribution patterns of copepods near the shelf break
south of Nova Scotia, Can. J. Fish. Aquat. Sei., 38, 1065-1076.
Herman A.W., Mitchell M.R., Young S.W., 1984. A continuous
pump sampler system for profiling copepods and chlorophyll in the
upper oceanic Iayers, Deep-Sea Res., 31, 439-450.
Irwin B., Harrison W.G., Denman K.L., Platt T., 1977. Phytoplankton and nutrient measurements at the edge of the continental shelf
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Jassby A.D., Platt T., 1976. Mathematical formulation of the relationship between photosynthesis and light for phytoplankton, Limnol.
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40
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~
1
1
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