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1
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1
Prirnary production
and phytoplankton biornass
in the equatorial region
of the Atlantic at 22° West
Equatorial upwelling
Nutrients
Phytoplankton
Chlorophyll
Primary production
Upwelling équatorial
Sels nutritifs
Phytoplancton
Chlorophylle
Production pnmaire
Eduard BAUERFEIND
Biologische Anstalt Helgoland, Notkestrasse 31, D-2000 Hamburg 52, Federal
Republic of Germany.
Received 12/7/85, in revised form 4/6/86, accepted 10/6/86.
ABSTRACT
Biological and chemical measurements were carried out in the equatorial Atlantic at
22°W, at ten sections between 3°N and 2°S, from February to June 1979. During
this time two periods with low sea-surface temperatures, due to upwelling, were
observed: one in February, the other in June. An increase in chlorophylI
concentrations was measured during these periods, whereas an increase in primary
production was observed only during the upwelling event in June. Small organisms
dominated the phytoplankton composition in numbers as well as in biomass. Primary
production was also highest in the small fraction « 20 f-Lm). Primary production
values as estimated from the input of new nutrients into the productive zone showed
the importance of regenerated production during periods without upwelling.
Oceanol. Acta, 1987. Proceedings International Symposium on Equatorial Vertical
Motion, Paris, 6-10 May 1985, 131-136.
RÉSUMÉ
Production primaire et biomasse du phytoplancton dans l'Atlantique
équatorial à 220 Ouest
De février à juin 1979, des mesures biologiques et chimiques furent effectuées en
Atlantique équatorial au cours de dix trajets entre 3° Nord et 2° Sud le long du
méridien 22° Ouest. Pendant ce temps, on a pu observer deux périodes avec des
basses températures à la surface dues à l'upwelling : l'une au mois de février, l'autre
en juin. Elles étaient accompagnées à chaque fois par une augmentation en
chlorophylle. Par contre, la production primaire ne fut élevée que lors de la
deuxième période d'upwelling, en juin. Le phytoplancton était dominé en nombre
ainsi qu'en biomasse par des petites espèces appartenant au nanoplancton
« 20 f-Lm). C'était aussi dans cette fraction que la production primaire était la plus
élevée. Une estimation de l'apport des sels nutritifs nouveaux dans la couche
productive, par mélange vertical, démontre l'importance de la production régénérée
pendant les périodes sans upwelling.
Oceanol. Acta, 1987. Proceedings International Symposium on Equatorial Vertical
.
Motion, Paris, 6-10 May 1985, 131-136.
131
~~
1
E. BAUERFEIND
!
~----~---------------------------------------------------------------------------~J
grouped in the following size classes: diatoms and
INTRODUCTION
dinoflagellates > 100 /-Lm, 100-30/-Lm, 30-10/-Lm, and
nanoflagellates (flagellates < 20 /-Lm) ; the latter were
In the tropical oceans along the Equator, zones with
grouped
in organisms < 3 /-Lm, 3-6 /-Lm, 6-10 /-Lm and
higher productivity and plankton biomass exist. One
> 10 /-Lm. The high amount of detritus in relation to
of the first observations of this fact was made during
cell numbers and biomass (often more than 95 % of
the " Planktonexpedition " in 1889 (Krümmel, 1892).
the measured particulate organic carbon was detritus),
Since
then,
many
expeditions
have
been
carried
out
in
l'
made it difficult to count the smaller organisms.
this area to explore physical and biological processes :
for a review, see Vinogradov (1981). During the past
25 years, intensive studies have been carried out by
French scientists in the eastern equatorial Atlantic,
RESULTS
e.g. Dufour, Stretta, (1973); Herbland, Voituriez,
(1979) ; Herbland, Le Bouteiller, (1981) ; Voituriez,
(1981) ; Voituriez et al., (1982).
Hydrographical situation
During the First Garp Global Experiment (FGGE) in
1979, the German RV " Meteor" surveyed the equaIn the area studied, a well mixed and nutrient-poor
torial Atlantic at 22°W, .from 3°N to 2°S, from
layer near the surface is separated from the deeper
February to June. At ten sections, biological and
nutrient-rich but light-limited layer by the top of the
chemical examinations were carried out, together
thermocline. Within the thermocline, the equatorial
with hydrographical and meteorological measureundercurrent flows to the east at - 100 cm S-l
ments. This study gives a short description of the
whereas at the surface the south equatorial current
biological situation at the time of investigation, a
flows in the opposite direction at - 25 cm S-l.
period usually described as the "warm" season
Vertical mixing due to the current shear causes
without upwelling and with low productivity. More
spreading of the thermocline and nitracline. During
detailed studies on phytoplankton and zooplankton
the expedition, two periods with increased spreading
were done by Meyerhôfer (1980), Rolke (1981) and
of the thermocline were observed, together with
Bauerfeind (1983).
lower sea-surface temperatures (Fig. 1). This cooling
of the surface layer can only be explained by upwelling
(Fahrbach, Bauerfeind, 1982; Fahrbach, 1983).
MATERIAL AND METHODS
1
Standard methods for the determination of planktological and chemical parameters were applied. Chlorophyll a was determined fluorometrically after acetone
extraction. Whatman GF/C-filters, 025 mm, were
used for filtration.
Primary production measurements were done by the
14C-method according to the simulated in situ technique (incubation from noon to sunset, - 6 h). Solar
radiation at the surface was measured continuously
with a solarimeter (Kipp and Zonen integrator CCI).
After incubation, the water was filtered through
Sartorius membrane filters, pore size 0.45/-Lm,
35 mm. After drying, the filters were stored in a
desiccator. Activity on the filters was counted with a
Geiger-Müller counter by the international 14C_
agency in H6rsholm, Denmark. Production per day
was calculated using the ratio of irradiance per
day/irradiance per incubation time. Inorganic dissolved nutrients were determined with an autoanalyser
using the methods given in Grasshoff (1976). Water
samples were taken with 10 l Niskin bottles mounted
together with the "Bathysonde". Sampling depths
were chosen according to prior determination of the
light levels (Secchi disk readings) in the euphotic zone
(100 %, 50 %, 30 %, 10 %, 1 %). In the deeper parts
down to 250 m, samples were taken at depths of 75 m,
100 m, 125 m, 150 m, 200 m, 250 m. A detailed description of the methods, together with the data, is
given in Bauerfeind et al. (1983).
Phytoplankton was counted in all samples at stations
where primary production measurements were taken.
Due to the low phytoplankton concentration, 100 ml
sub-samples preserved with lugol's solution were
sedimented for 48 h and counted with an inverted
microscope. The organisms were determined and
:~~L~~", ~ !I::V~
Il
1. Oh 15. 1.
15.
1
FEB. 1 MAR.
1.
15.
1.
15.
1.
1
APR.
1
MAY
1
15.
JUN.1979
Figure 1
Continuous temperature record at a depth of 15 m on the Equator
(from Fahrbach, 1983). Dots represent temperatures measured with
the CTD, bars show the time interval of the sections.
o
Low concentrations of nitrate were measured in the
surface layer at stations near the Equator, only during
upwelling (Fig. 2). Highest surface temperatures were
measured in AprillMay at times when the Intertropical
Convergence Zone (ITCZ) had reached its southernmost position. A similar change of the sea-surface
temperatures in 1979 was reported by Molinari et al.
(1983) for other areas of the equatorial Atlantic. In all
areas, a decrease in temperature was observed shortly
after the increase of the trade winds at the end of
May.
Phytoplankton: cell numbers and biomass
During the period of investigation, cell numbers
ranged from 104 to 106 organisms x dm- 3 • Higher
numbers were found at the beginning of the expedition
in February; they decreased in March/April and
showed a slight increase at the end of May and
beginning of June. Nanoflagellates were the dominating organisms, often accounting for more than 90 %
of the cell numbers. Species composition was uniform
during the time of investigation with two exceptions:
r-------------------------------------------------------------------------------------~~
132
r
I~
'.!II
i!l!
PHYTOPLANKTON AND PRODUCTION IN EQUATORIAL WATERS
1.1
___------------------------------------------------.-i~(
1'N
0'
267 269 271 273 275 277
1'S
280
2'S
283
1'S
3'N
St.No.m.292
285 286
312 314
o
m
2'S
i
!
316.317
m
50
100
150
150
"0
::. .
o
~22P ·O~
.•
100
•.•
•
200
200
:.
..
300
.
.
250
400 km 500
Figure 2
Vertical distribution of N0 3 - (J.U1Iol dm- 3) in the equatorial
Atlantic at 22°W between 3°N and 2"S measured at a section without
~o
•
rrN'O~-'
<ao
22.5 \
•
100
)
200
300
IJ~Oldm-3
400 km
500
upwelling 11.5-17.5.1979 (on the left) and at a section when
upwelling was observed 3.6.-6.6.1979 (on the right).
1. Rhizosolenia mats were observed during the calm
period in April.
2. Bundles of the nitrogen-fixing cyanophyta Oscillatoria thiebautii and Oscillatoria contortum also appeared for the first time in April, and were present at all
examined stations until the end of the expedition.
Cell volume and carbon content of the organisms
were calculated according to the formulas given by
Edler (1979). Integrated for the euphotic zone, phytoplankton-carbon (PPC) values ranged from 60-441
mgCm- 2 with lower values in April and May. The
average PPC for the whole period investigated was
178 mgCm- 2 (n = 53). The PPC below the euphotic
zone usually accounted for 30-50 % of the total PPC
within the water column.
The quota of the different groups within the euphotic
zone was calculated as follows :
centric dia toms
pennate diatoms
dinoflagellates > 30 fLm
dinoflagellates < 30 fLm
nanoflagellates
cyanophyta
~
be obtained by using epifluorescence microscopy in
addition to the " Utermôhl " technique.
Simultaneously with the phytoplankton, protozooplankton (PZP) was counted in the samples. Cell
numbers of these organisms (mainly naked ciliates
and tintinnids) . ranged from 102-103 organisms x dm- 3• The biomass estimated from these
numbers ranged from 1-45 mgCm- 2 integrated for
the euphotic zone. Tlilis is 1-32 % ofthe phytoplankton
biomass. The average biomass of PZP for the whole
period was calculated to be 10 % of the average
phytoplankton carbon. Although these figures may
underestimate the real PZP-biomass - these organisms being very sensitive to fixation - they indicate
the ecological importance of the protozooplankton in
the equatorial region of the Atlantic.
Chlorophyll concentration and primary production
4%]
2%
18
19
46 ~
6 % diatoms
Chlorophyll a, a measure of the plankton content of
the water column, ranged from 5.6-38.7 mg Chl.a
m- 2 integrated for the euphotic zone. Higher values
were found during periods of lower sea-surface temperatures (section 3 and section 9). The chlorophyll a
content within the euphotic layer was three times
higher in June than at the beginning of the expedition.
During the other upwelling period in February, the
increase was less pronounced (1.7 fold). The effect of
upwelling is also obvious if one considers the seasurface concentrations (Fig. 3). An increase of this
parameter with its maximum near the Equator is
evident at section 3 and section 9.
The vertical distribution of chlorophyll a showed a
subsurface maximum in 50-80 m. This zone with
higher chlorophyll a concentrations has a thickness of
20-30 m, is situated 10-20 m above the nitracline and
parallels the thermoc1ine. The nitrac1ine and the
chlorophyll a maximum are situated at the bottom of.
the euphotic layer and were usually found below the
.10 % light level.
The depth of the chlorophyll maximum did not show
significant changes during the period of investigation
and within the area studied, whereas the changes in
chlorophyll concentrations are significant at the 95 %
%]
'* ] 37 % d'mofiagellates
65 % organisms < 30 fLm
10%
In the area investigated, phytoplankton biomass was
dominated by small organisms. Dinoflagellates
< 30 /-Lm and nanoflagellates account for 65 % of the
biomass in the euphotic zone. But it should be borne
in mind that the figures given are average values for
the entire period of the expedition and that in several
samples larger organisms dominated the plankton
composition. This holds true especially for the Oscillatoria species mentioned above, which were encountered only during the second half of the expedition. The
" Utermôhl " technique used for the evaluation of the
phytoplankton biomass does not work satisfactorily
for the small phytoplankton size fraction, especially
the so-called picoplankton. It has been shown recently
that organisms belonging to this size c1ass form a
considerable part of the phytoplankton in tropical and
equatorial waters (Li et al., 1983 ; Herbland et al.,
1985). Therefore the cell numbers and biomass given
in this study may underestimate the real figures to a
certain, as yet unknown extent. A more valid evaluation of phytoplankton biomass in future studies might
Î'~----------------------------------------------__~______~}
~...
133
1.1,"
E. BAUERFEIND
4·'r-T--rr-..,......-,---,--,.--,--"TT'",.--;----,--,,-;----,--n-,---,----,
3'
2'
d
:
. .
..
..
..
\~
/.
• >0.15
,(/
,/ \.2~
qK
>QI ••'
C»0'1•
..::0.05
.
.
.
.
'.
..
..
l'
N
'<D,' :
.
.
.. " ,
. ,
\j'\
,
,
,,
1
1
1
O'
5
l'
2'
10.
20.
1.
Figure 3
Surface chlorophyll a concentrations (JLg dm- 3) in
the area of investigation.from February-June 1979.
10.
JAN 1979 - r - - FEB --1-1- -
10
20
30 }J mol 1-1
01-1----I0~-2---0+.14----i016 mg m-3
2
4
5
6mg Cm-3d- 1
0
o
level (H-test, Kruskal and Wallis). Herbland and
Voituriez (1979) reported a similar vertical distribution with a nitrate impoverished mixed layer and a
subsurface chlorophyll maximum in the Eastern equatorial Atlantic at 4°W during the warm season. This
situation is known as "typical tropical structure".
Primary production was always highest in the top
30 m of the water column in the central equatorial
Atlantic at 22°W, separated from the chiorophyll
maximum and from the nitracline. Figure 4 shows the
typical vertical distribution of chiorophyll a, N03and primary production obtained at a long-term
station at the Equator. Integrated primary production
ranged from 78-741 mgC m- 2 d- 1 in the period of
investigation, with highest values in June when upwelling was observed. At the other upwelling period in
February, however, no increase for this parameter
was observed. The lowest primary production was
measured in May along with low chiorophyll values.
For the whole period of investigation, an average of
247 mgC m- 2 d- 1 (n = 53) was calculated. The
results of size fractionated production measurements
showed that the major part of the primary production
(- 90 %) was due to organisms < 20 flm. Larger organisms were of minor importance which is supported
by the results of the cell counts. These results of size
fractionated primary production are similar to those
reported by Herbland and Le Bouteiller (1981), and
Herbland et al. (1985) for the region of the equatorial
Atlantic.
As shown in Figure 4, the vertical distribùtion of
primary production always showed a maximum in the
top 30 m of the water-column, a zone where, during
periods without upwelling, nitrate concentrations were
below the detection limit of the method used. During
this time vertical mixing was the most important
source of input of nitrate into the layer near the
surface. Using the vertical turbulent diffusion coefficient for dissolved substances Kb = 2.7 cm2 S-1 for
the warm period, given in Fahrbach and Bauerfeind
0
m
\ , , _______
"
~?
·;Xl
j,
100
.. 0 ....
t. . .
51.197
150 '';
200
- - NO)
--·0--- Chl.a
- a - Prim.Prod.
250
Figure 4
Vertical distribution of chlorophyll a, N0 3 - and primary production. Average values obtained at a long term station at the Equator
(30.3.-2.4.1979).
(1982), 1 estimated the flux of nitrate into the mixed
layer within a zone 30' north and 30' south of the
Equator. The input of new nutrients calculated by this
method ranged from 0.6-2.0 mmol N0 3m- 2 ct- l •
Using these values, a primary production ranging
from 43-137 mgC m- 2 d- 1 was calculated using a
C/N ratio of 6. This result shows that on an average,
- 42 % of the measured primary production can be
explained,by the input of nitrate from the deeper
134
r
li::
1':1
...
;Ii:
PHYTOPLANKTON AND PRODUCTION IN EQUATORIAL WATERS
Iii
,1 __- - - - - - - - - - - - - - - , \
'r
1
layers in the period without upwelling. Production
based on the input of nitrate can be termed new
production (Dugdale, Goering, 1967), whereas the
remaining 60 % of the total production may be based
on recycled nutrients.
DISCUSSION
From February to June 1979, chlorophyll concentrations and to a lesser extent primary production in the
equatorial region of the Atlantic at 22°W increased
during the observed upwelling periods. This is different from the situation in the eastern part of the
equatorial region at 4°W reported by Voituriez et al.
(1982). Primary p~oduction in this region was higher
(- 1 gC m- 2d- 1 ln 1978/79) than at 22°W; only
small variations were observed and chlorophyll
concentrations were the same in August and April
(Voituriez, 1981). Another difference is obvious if
one considers the vertical distribution of primary
production. At 22°W,· this is highest in the layer near
the surface at a depth with low nutrient concentrations, whereas at 4°W the productivity maximum is
present in the depth of the chlorophyll maximum
close to the nitra cline (Herbland, Voituriez, 1979;
Voituriez et al., 1982). This difference between the
two areas is certainly due to the discrepancy between
the depths of the nitracline and the chlorophyll
maximum in the two regions ; this is caused by the tilt
of the thermocline, which shallows in the eastern
Atlantic (Merle, 1980 ; Henin, Hisard, 1987 ; Oudot,
Morin, 1987). At 4°W the nitracline and the chlorophyll maximum are located in 30-40 m, whereas at
22°W these layers were observed in 50-80m at a
depth where light limitation may occur. This fact is
supported by the results of experiments obtained on
board during the FGGE-cruise. Samples from the
depth of the chlorophyll maximum showed higher
productivity when they were incubated at the 100, 50
and 30 % light level than parallel samples incubated at
light levels of the chlorophyll maximum (Bauerfeind,
unpublished data). In the central equatorial Atlantic
at 22°W, - 40 % of the primary production during
the warm season could be due to the input of new
nitrogen into the productive zone, while about 60 %
may be regenerated production. A further hint
towards the importance of regenerated production in
the area investigated is given by the vertical distribution of zooplankton biomass. Rolke (1981) reported
that higher zooplankton biomass was present at
depths less than 100 m with a maximum in the depth
range of 0-25 m. Numbers and biomass of bacteria
derived from direct counts of samples stained with
acridine orange also had a maximum in the layer near
the surface (Bauerfeind unpublished data).
In conclusion, the following is assumed for the
foodchain in the area of investigation. Phytoplankton
composition is dominated by small organisms (nanoplankton organisms and dinoflagellates < 30 f.Lm).
Primary production is to a great extent regenerated
production, and - 90 % are found in the size fraction
< 20 f.Lm. The importance of small phytoplankton
organisms in equatorial waters is stressed also by
Herbland and Le Bouteiller (1981) and Herbland et
al. (1985). Due to the relatively high growth rate of
the phytoplankton (an average doubling time of 22 h,
range: 6-34 h, was calculated from the PIB values for
the water column) a rapid turnover of the nutrients
excreted by the micro- and mesozooplankton and
remineralized by bacteria must exist. Primary pro ducers are balanced by the grazing of micro- and
mesozooplankton resulting in a more or less constant
phytoplankton standing stock. Protozooplankton
organisms are an important link between the small
primary producers and the mesozooplankton. In times
with upwelling and/or a shallow position of the
nitracline high amounts of new nutrients are transported into the productive zone. This causes an increase
in primary production and results in a surplus of
phytoplankton biomass (Fig. 3) which, at least at the
beginning of the upwelling season, is no longer
controlled by the grazers and thus can be exported
into other regions of the equatorial Atlantic.
:i,:
l',
,II
Acknowledgements
This study formed part of the author's dissertation.
The work was funded by the" Deutsche Forschungsgemeinschaft" and was conducted at the Institut fur
Meereskunde in Kiel, FRG. 1 thank E. Fahrbach,
R. Boje and M. Rolke for valuable comments on the
manuscript. Gratitude is further expressed to Mrs.
G. Kredel and Mrs. C. Berger for correcting the
English manuscript and to Mrs. C. Schuster for typing
it.·
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--------------------------:-------------------------l
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136