In January 1991, the Equatorial Pacific enrichment area covered a belt more than 11000 km long, with surface nitrate concentration ... near the Galapagos Islands, decreasing westwards and desappearing beyond 167°E. In this area, the amount of chlorophyll a (Chla) did not change significantly from one end of the upwelling to the other, and was very low everywhere ... Schematically, in spite of the typical longitudinal gradients of temperature and nutrients, all data of chlorophyll, size distribution and cell numbers clearly indicate that an extreme monotony characterizes the distribution of phytoplankton all along the enrichment area due to the Equatorial upwelling, covering 11 million km2 for mean upwelling conditions. (D'après résumé d'auteur

Blanchot, Jean

Le Bouteiller, Aubert

Name not available

Size distribution and abundance of phytoplankton in the Pacific Equatorial upwelling

Horizon / Pleins textes

La mer 29: 175-179, 1991 
Sociktk franco-japonaise d'ockanographie, Tokyo 
Size distribution and abundance of phytoplankton in the 
Pacific equatorial upwelling 
~ 
Aubert Le  B OUTE I LLER* and Jean  B LANGHOT* 
Abstract: In January 1991, the equatorial Pacific enrichment area covered a belt 
more than 11000 km long, with surface nitrate concentration up to 11 ,UM near the 
Galapagos islands, decreasing westwards and desappearing beyond 167"E. In this 
area, the amount of chlorophyll a (Chla) did not change significantly from one end 
of the upwelling to the other, and was very low everywhere. The size structure of 
chlorophyll a was also nearly constant from 95"W to 167" E : on the average, Chla 
>3 ,U m and Chla<l fim represented 2796 and 39% respectively all along the equator, 
with correlative uniform distributions of phycoerythrin-containing cyanobacteria 
(Syrzechococcus sp.) and chlorophyll-fluorescing microalgae. 
Schematically, in spite of the typical longitudinal gradients of temperature and 
nutrients, all data of chlorophyll, size distribution and cell numbers c h r l y  indicate 
that an extreme monotony characterizes the distribution of phytoplankton all along 
the enrichment area due to the equatorial upwelling, covering 11 million lim2 for 
mean upwelling conditions. 
1. Introduction 
Various recent studies have shown t h a t  in 
the tropical open ocean, the size structure of 
phytoplankton seemed t o  be typically distrib- 
uted (HERBLAND et al., 1985, 1987; PENA 
et al., 1990; L E  B O U T E I L L E R  et al., 1992). 
In the nitrate depleted mixed layer of the 
equatorial Atlantic Ocean as well as in the 
one of the western Pacific Ocean, Chl a in 
the <1 p m  fraction was found t o  be always 
pred0minant.h the nutrient-rich waters, 
more than half of the to t a l  Chla was sys- 
tematically contained within the >1 ,U m 
. fraction in which eucaryotic microalgae 
predominated. No major difference distin- 
guished phytoplankton forming ' t he  deep 
chlorophyll maximum from phytoplankton 
observed in surface waters enriched by the 
equatorial upwelling. On the contrary, the 
size structure of phytoplankton usually 
observed in these nutrient-rich tropical wa- 
ters is quite different from the size of algae 
in  coastal upwellings (HERBLAND e t  al., 
*ORSTOM, B.P.A 5, Nouméa, New Caledonia 
I 
1987) or in temperate o r  cold nutrient-rich 
waters (STOCKNER and A N T I A ?  1986; 
R A  I M B A U L T  et al., 1988). Therefore, the 
size distribution of phytoplankton would be 
one df the most significant properties of the 
Typical Tropical' Structure such as defined by 
HERBLAND and V O I T U R I E Z  (1979). 
However, such a generalization is always 
risky as far as the size structure of phyto- 
plankton is not determined everywhere in the 
tropical ocean, and especially in the poorly 
known Pacific equatorial upwelling. So, a 
wide study of the distribution of phyto- 
plankton associated with physical and chemi- 
cal properties, was performed during the 
ALIZE cruise, stretching all along the equa- 
to r  from 95" W t o  165" E. 
2. Methods 
All the methods uséd were described by 
R E V E R D I N  et aL(1991). Total  Chla (100ml) 
was collected on GF/F fil ters at 9 or 10 
depths. Chla in the fractions was estimated 
on subsamples (260 ml) filtered on Nuclepore 
polycarbonate filters ( 1 and 3 ,U m), always 
at<30 mm of Hg vacuum pressure. Samples 
- ,  
Fsnds ~ ~ ~ ~ ~ ~ f l ~ ~ i ~ ~  
176 La mer29, 1991 
for  cell counts (60 ml) were collected accord- 
ing t o  B LANCHOT et aL(1992) on black 0.2 
,um Nuclepore filters. All f i l ters were frozen 
for  a later analysis at the laboratory: Chla 
with the methanol method and cell counts by 
epifluorescence microscopy. 
3. Results and discussion 
Position of the stations is shown on Fig. 
1. The equatorial upwelling was well devel- 
oped in January 1991. Nitrate was present at 
surface (NOa> 0.1 ,u M) at every station 
f rom 95" W (maximum 11 ,UM) t o  169" E and 
on all the meridional transects, except south 
of 11"s at 150"W and westwards beyond 
168" E (Fig.2). Surface temperature rised 
f rom 22"8 C at 95" W t o  29"4 at 173"E. 
The amount of surface Chla (mean for  the 
0-20m layer) did not change significantly 
f rom one end of the upwelling t o  the other, 
1ON 
EQ 
10 s 
20 s 
ALEE 2 
, ' . , , . : , , , , i , ,   , :'. , : f : , . . . .  * .  , , , i ,  , . , i  
t .  . 
L . .  
o" . *  . .  . .  
30S:', I I I I 1 I I I I I t  I I I I ,  I I I 
160 E 180E . 160 W 140 W 120 w 100 w 80 W 
Fig. 1. ALIZE II cruise. January-Fabruary 1991. Location of stations from 95" W to 165"E 
O 
50 
1 O0 
150 
200 
o 
. Longitude 
Longitude 
Fig. 2. Nitrate distribution (,u M) along the equator. 
Size distribution and abundance of phytoplankton in the Pacific equatorial upwelling 177 
Fig. 3. Depth integrated values of chlorophyll a along the equator (mgm-'1. 
Squares: 0-20m layer. Triangles: euphotic layer. 
ao 80 
70 70 
60 60 
50 50 
40 40 
30 30 
20 20 
10 10 
O 0 %  
-1702 , I B O  170 160 160 i40 130 IZO n o  ioo0w 
LONGITUDE 
Fig. 4. Relative size distribution of Chla within the euphotic layer along the equator. 
9 
178 La  mer29, 1991 
and was very low everywhere: Chla=0.215 mg 
m-3 (mean of 55 stations) from 95" W t o  
145 W, and Chla=0.218 m g  m-3 (n=35 sta- 
tions) from 150" W t o  169"E (Fig.3). Surface 
Chla maximum was only 0.402 mgm-3. These 
values are quite similar t o  those obtained by 
the same method at 165" E during the 6 
transects of the ORSTOM programmes (i.e. 
PROPPAC and SURTROPAC) which have 
crossed the equatorial upwelling since 1988: 
surface Chla (0-20m layer) =0.231 mg m-3 
(mean of 57 stations). 
Daring the ALIZE I cruise in 1965, cover- 
ing the same area as ALIZE II, the equato- 
rial upwelling presented nearly the same 
spreading (G U E R E D R A T ,  1971). Chla was 
collected on Millipore HA 0.45 y m  filters 
and analysed on a spectrophotometer af ter  
acetone extraction. The mean surface Chla 
(0-20m layer) was 0.192 mgm-3 (n=17) 
from 94" W t o  140" W and 0.194 mgm-3 
(n=13) from 146" W to 174" E, without any 
high Chla value in nutrient-rich waters, even 
when surface NO3 was up t o  13 or 14 y 
M at 95" W. 
The Chla content of the euphotic layer was 
calculated according t o  M O R  E L (1988). The 
mean values were respectively 13.1 and 13.8 
mg m-' in the eastern and western parts of 
the Pacific in 1991 (Fig.3), and 14.3 mgm-' 
at 165" E 
These depth distributions of Chla are quite 
similar t o  those reported by CHAVEZ et al. 
(19901, showing low Chla values along the 
equator from 110" W t o  140" W. C U L L E N  et 
al. (1992) also observed similar Chla profiles 
at O", 150"W. CHAVEZ and B R U S C A  (1992) 
presented means of surface mitrate and 
surface Chla along the equator calculated on 
da ta  collected from 1980 t o  1988: surface 
Chla concentrations are regularly low from 
west of the Galapagos t o  the end of the 
upwelling, near 165" E. 
The size structure of chlorophyll a ap- 
peared nearly constant from 95" W t o  167" E: 
on the average, Chla > 3 ,u m represented 
27% and 28% of total  Chla in the,eastern 
and western equatorial Pacific respeciively, 
and never exceeded 39 . Chla <1 ,um-39 
(mean of 42 profiles) all along the equator 
(Fig.4). These size distrbutions are not 
different from those observed at 165" E in 
the western part  of the equatorial upwelling 
by L E  B O U T E I L L E R  et al. (1992): Chla 
also found tha t  Chla < l p m  = 40% on the 
average (n=6 profiles) in the upwelling at 
135" W, but C HAVEZ (1989) reported a mean 
Chla content of 62.4% in the < 1 ,um frac- 
tion at 110" W(n=6), which seems atypically 
high for equatorial nutrientrich waters. 
Besides, counts of cells by epifluorescence 
microscopy at 48 hydrocasts distributed 
between 95" W and 169"E revealed t h a t  the 
euphotic zone contained on the average 4.2 
10" cyanobacteriaper m2 and 2.1 10" 
eucaryotic microalgae per m2, without any 
bloom anywhere. 
Consequently, in spite of the typical longi- 
tudinal gradients of temperature and nutri- 
ents showed by the equatorial transects of 
both ALIZE I and ALIZE II cruises and 
by the analysis of historical data ,  all the 
biomass indexes available today clearly 
indicate tha t  an extreme monotony charac- 
terizes the distribution of phytoplankton all 
along the enrichment area due t o  the equato- 
rial upwelling. Exactly the same experimen- 
tal procedures were used t o  perform the Chla 
fractionations and the cell enumerations 
during the ALIZE II cruise, so t h a t  the 
phytoplankton properties are described as 
reliably as  possible from 95" W t o  165" E. 
The relative abundance of large algal cells 
such as diatoms observed in the eastern 
equatorial Pacific by D E S R O S  I ERES (1969) 
and CHAVEZ et al. (1990) or suspected by 
BENDER and MCPHADEN (1990) seem t o  be 
locally and temporally restricted, and do not 
induce significant changes of Chla (<1 ,u m, 
>3 y m and total)  o r  cell numbers 
(cyanobacteria and eucaryotic microalgae) 
from the west of the Galapagos t o  the end 
of the upwelling. 
As a conclusion, when mean conditions 
prevail, the typical monotonous distribution 
of phyto-plankton strongly suggests t h a t  
primary production, new production and 
<1 ,um=38% (n=26). P E N A  et d(1990)  
Size distribution and abundance of phytoplankton in the Pacific equatorial upwelling 179 
particles exportation are probably also uni- 
formly distributed through the pacific equa- 
torial  system.This result is important t o  be 
considered in the near future, especially for  
global scale studies such as the JGOFS 
programme. 
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Size distribution and abundance of phytoplankton in the Pacific Equatorial upwelling

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