PHYTOPLANKTON
IN THE SOUTHWESTERN
SARGASSO SEA AND
NORTH EQUATORIAL
CURRENT, FEBRUARY 1961l
Edward M. Hulburt
Woods
Hole
Oceanographic
Institution
ABSTRACT
In a l,OOO-mile section in the tropical Atlantic,
flagellate
species, with one exception,
tended to be uniformly
distributed,
but diatoms, for the most part, were more numerous
toward the southern end of the section. This change was associated with an intensification
of stratification
of the upper 100 m. The relative ability of flagellates and diatoms to react
to varying growth conditions, in turn dependent on hydrography,
is discussed.
INTRODUCTION
The extensive studies of Lohmann ( 1920)
and Hcntschcl ( 1932, 1936) on the phytoplankton of the tropical Atlantic suggest
that some species are evenly distributed
over large distances because the environment changes very slightly; others undergo
marked fluctuation in response to the occasional physical changes that do occur.
Further, their investigations
suggest that
the two types of species arc distinguished
by the presence or absence of motility. In
the present study, these features of the phytoplankton will bc considered as they arc
related to the hydrography of the southern
Sargasso Sea and northern equatorial current.
IIYDROGRAPHY
The stations occupied are shown in Figure 1 and temperature and salinity profiles
in Figure 2. At the northern end of the section ( station 1001) the greatest thickness
of homogeneous water is observed, extcnding to 140 m. Southward the surface becomes warmer, but this warming penetrates
only to 80 m or even less farther south. In
the southernmost part of the section, the
deeper isotherms rise toward the surface.
The surface salinity decreases from 37.0$
or greater in the north to less than 35.5$& in
the south, and this freshening extends down-
ward to progrcssivcly
shallower depths.
These depths are distinctly less than those
attained by the isothermal layer at the
southern stations, so that the halocline is
above the thermocline
there. Water of
37.0%0 or greater at stations 1000-1014
penetrates southward at mid-depth ( 100140 m ) . This thin layer, called the salinity
maximum because it lies between less salinc
water above and below, becomes diluted in
its southward extension and, at the last two
stations, slopes toward the surface.
The section passes from the relatively
motionless water of the Sargasso Sea southward across the north equatorial current,
which includes contributions not only from
the north but also, from the south Atlantic.
The low salinities between the surface and
60 m at the southern stations are mixtures
of fresher surface water from near the
equator to the southeast and the more
saline water at 100 m to the northeast
700
60’
60”
500
40”
400
30”
200
l Contribution
No. 1263 from the Woods Hole
Oceanographic
Institution.
This investigation
has
been supported in part by a contract with the U. S.
Atomic Energy Commission (AT[3%1]-1918)
and
in part by the National Science Foundation,
grant
G8339.
The author is indebted
to Mr L. V.
Worthington
for use of the salinity and temperature data.
307
70”
80”
FIG.
1.
Positions
60°
of stations
500
occupied.
400
308
EDWARD
M.
( Jacobsen 1929 ) . Although
the effluent
from the Amazon River spreads westward
and northward to the Lesser Antilles in
spring, during winter its effect is slight
(Biihnecke 1936). Somewhat to the westward, within
the Caribbean,
salinities
greater than 36.59/o, occur at the surface
along the South American coast (Bijhneckc
1936; Richards 1960).
The upward slope toward the south of
both isotherms and isohalines is an important Seaturc near the surface of the Atlantic
between latitudes 12”N and 8”N (Iselin
1936). It is present throughout the Caribbean ( Parr 1936)) where the salinity maximum reaches the surface off South America.
Although this slope is observable only at
our southernmost station, it is part of a
general upward inclination
of deeper isotherms between latitudes 25”N and 8”N
( Iselin 1936). Deeper isohalines show the
same inclination and also the intrusion of
low salinity water from the south, with its
main core at 800 m.
Sciwell (1935) also described the southward upward slope of phosphate along the
40th meridian. This is corroborated exactly
by unpublished
data along the 30th meridian from the files of the Woods Hole
Oceanographic Institution.
Thus, there is
southward intensification
of! stratification
not only with regard to temperature and
salinity but also phosphate toward the latitudes of 8” to 12”N.
DTSTRIBUTION
OF THE
PHYTOPLANKTON
There was a general increase in total phytoplankton toward the south (Table 1))
with a minimum at 20 m at station 1061 and
a maximum at station 1025. But, in spite of
this general trend southward, the actual increase was slight and was chiefly confined
to the last 6 stations.
the most abundant
Coccolithophores,
algae in most samples, were more numerous
southward.
This was particularly
so of
Coccolithus hu~leyi, the dominant species
at 12 of the 19 stations ( at 20 m). When it
is omitted from the counts, the remaining
coccolithophores showed no southward incxcrease. Similarly, the dinoflagellates
HULBURT
FIG. 2. Distribution
of temperature
Depths where phytoplankton
samples
are shown by crosses (surface samples
though not so indicated,
at stations
1024, and 1025 ) .
and salinity.
were taken
were taken,
1004, 1020,
hibited no systematic change throughout
the l,OOO-mile section, the range of values,
5 to 29, at 20 m being remarkably small.
From stations 994 to 1018, diatom counts
at 20 m showed no significant differences
from their mean value when tested by the
chi-square method. But at the southern
stations, particularly
1025, diatoms were
more numerous and contributed considerably to the total phytoplankton,
So also did
the blue-green alga, Trichodesmium
thiebautii, observed principally between 16.5”N
and 11.5”N.
The counts at the upper bound‘ary of the
thermocline were similar to those at 20 m
along the northern and middle portions of
the section, but at the 6 stations in the
south, where the halocline was shallower
than the thermocline, lower values were ohtamed at deeper levels than at 20 m (Table
Vertical
distributions
at 4 stations
0
(Table 2) indicate the importance of the
stratification
of the water in limiting the
phytoplankton
populations to a superficial
layer above the thermocline and above the
layer of maximal salinities. Only traces of
the abundant populations at the southern
stations 1020, 1024, and 1025 extended
The distribution
21
14
20
19
5"
11
31
6
10
6
4
Coccolithophoridaceae
minus C. huxleyi
Dinophyceae
Bacillariophyceae
40
5
10
13
16
7
0
8
4
12
9
1
15
31
2
6
14
10
32
14
10
21
41
72
8
4
14
18
31
29
16
18
47
47
84
84
7
8
21
13
6
11
7
5
12
32
30
32
23
3
55
33
82
65
21
12
17
43
38
86
67
6
1
26
6
49
5
40
0
89
5
133
16
10
17
21
22
29
29
50
51
75
74
10
8
13
8
35
12
26
14
61
26
63
95
11
7
20
17
33
87
33
40
66
127
8
3
12
15
46
38
25
2
71
40
1021
13”33
74
9
63
51
21
10
28
14
70
33
98
47
31
1
30
-
44
16
-12
112
8
21
8
6
7
52 1,342
51
29
4
57
2
42
69
210
-
F
%
g
8
2
il
8
z
g
$
z
226
48
76
39 165
2.5 54
26
11
46
19
102 196
51 35
148 254
70 37
~
M
?
3
ll”29’
162 1,710
158
-
12017’
1022 -1023; --1024; 102%
14”23’
270 354 510
68 117 98
15”22’
156
11
10
7
20
29
32
9
52
38
130 110 288
159 63 63
* The plankton was preserved in 3% formaldehyde and counted after settling in a chamber shallow enough for use of a high dry objective.
f Only 33 cc actually counted, but correction made so that figures are comparable with those from other stations.
$Only 13.3 cc actually counted, but correction made so that figures are comparable with those for other stations.
0 Counts refer to the number of filaments, not the number of cells.
Cyanophyceae§
3
14
6
19
24
15
12
41
16
18
1
huxleyi
9
Coccolithus
36
56
37
32
26
40
Coccolithophoridaceae
78
82
61
59
40
58
Nos. in all groups
1020
16”28’
1018
19O28’ 18”23’
1017
of major groups of plankton and the dominant species at 20 m (upper ualum) and in the upper luyer of the thermocline
(lower z;alues). Values are number of cells counted in 50 cc of sample*
-_._______
--_ 994
Stations _..___..___...__...______
993
999
1000 1001 1002 1003 1004 1014 1015 1016
----_____---Latitude ________________________________________
29"43' 28"15' 26"43' 26"ol' 25'12' 24"15' 22'58' 22~57’ 22027’ 21°25’ 20”25’
TABLE 1.
310
TAULlc
EDWARD
2.
The
oertical
--.__-
Depth
(m)
M.
HULBURT
distribution
of the phytoplankton
and the physical
Counts reported as number of cells per 50 cc
_._ _ ---- -__
- ____
------____..___.______________________
------.. .__....__
--..- _-
0
-~
.--
Station
76
Total ccl1 number”
-_..-______
------ ___.________._...
Coccolithophoridaceae
___-_____________.
__________
61
Dinophyceae
__..-___.-..__.-__________________
--__--___- 4
Bacillariophyccae
_____
--- _____._.
---______
-- ._______.__7
Tempcraturc,
“C __.._______._______._______
-_---_____
- 24.42
Salinity, go __.___.________
--_____._________
-----_ _-__.__. 37.05
Density, sigma t -_..__..._.._______.._
____.____
__...__ 25.09
20
70
34
20
7
24.39
37.18
25.19
1020
Total cell number ___-_.____.__
-_____
-_-____
--__._
-__ 214
288
Coccolithophoridaceae
______
--____
---_____________
71
52
Dinophyceae
___._________________________
---___-______
- 25
10
Bacillariophyceae
__.________________._
- _____
--_..--_--_ 62
156
Temperature,
‘C __..___.__.________________
-___._
---. 25.87
25.79
Salinity, go --_____-__._-._._.._____________________..---.-35.82
35.83
-____________._
-____
--______
-__ 23.70
Density, sigma t __..___
23.75
-
..-
of the
--
- -
water.
T_zzzT
100
140
65
33
12
7
24.52
37.26
25.12
49
26
12
8
24.10
36.93
25.10
-
63
38
7
15
4
2
60
1004
82
55
5
11
24.41
37.05
25.09
properties
180
Station
69
12
20
37
25.56
36.34
24.22
Station 1024
Total ccl1 number _______-_______.___.__________
___._200
162
Coccolithophoridaceac
__________-.___._...________
77
48
Dinophyceac
______.._____.-_______________________
.__._ 12
8
Bacillariophyceae
_._-_.-_..____.._-_._...__
.._-___._ 76
52
Temperature,
“C -_______________.__.____________
-_____ 26.35
26.35
Salinity, gd __...____.._____._____.-____._.___.______..__
_ 35.27
35.31
Density, sigma t ____..
-___.________.___._._-....-___._ 23.14
23.18
* Includes
species
not
in
the
three
groups
downward to depths with
density changes.
SPECLES
listed
and
unidentified
large vertical
COMPOSITION
The coccolithophores, with the exception
of Coccolithus huxleyi, showed a marked
tendency toward uniformity
in cell numbers and a rather small accretion of species
(Table 3) as more samples arc included
from north to south. Dinoflagellates
also
tended strongly toward uniformity but the
accumulation of species southward is more
rapid than among the coccolithophores.
The first 6 species of diatoms listed arc
represented by a few occurrences in small
numbers througho,ut the section. The remaining 23 forms were present only at the
25.52
37.01
25.29
z
2
23.40
37.08
25.36
158
76
18
9
1
19
11
0
5”:
6
6
25.92
36.12
23.93
Station 1025
Total cell number __.-___._______.______.___________
1,215
1,710
Coccolithophoridaceae
_____-____.___.--_________
323
226
Dinophyceae
__...______
-___._..___.______..___________
8
12
Bacillariophyceac
-._._______.__-___.___
._---_--_.-- 593
1,342
“C ____
-_.____
-__-._.-__.-.-__-.-.
Temperature,
26.10
26.05
Salinity,
go ____
---__--._____________________________
-__
35.77
35.67
Density, sigma t .--.______
--_.__
-_____
-____
-______
-23.62
23.54
11
-
20.37
36.56
25.87
2
20.59
36.83
26.00
27
24
0
36.44
25.63
36.36
26.10
3
17.31
36.23
26.42
36
14
12
7
13
7
21
17.25
36.26
26.44
5
14.84
35.89
26.72
12.73
35.52
26.88
20.91
.- -
18.97
6
cells.
southern end of the section and the abundance of many of these makes a sharp contrast to the evenly distributed coccolithophores and dinoflagellates.
Most oE the coccolithophores in Table 3
were recorded by Lohmann (1920) and
Hcntschcl (1932) from the tropical Atlantic, and 4 of the frequently
occurring
species, Coccolithus huxleyi, Discosphaera
tubifer, Syracosphaera pulchra, and Cyclococcolithus leptoporus, they found to be
frequent and abundant. The dinoflagellate
forms recorded here are also characteristic
of the tropical, open ocean ( Schiller 1933))
with the exception of Prorocentrum redfieZdi, described by Brusa ( 1959) from the
New England coast, and Prorocentrum
-~-~-
TABLE 3.
Stations:
of the phytoplunkton
:i
4
10
48
1
1
3
1
1
2
1
2
1
3
4
4
Oxytoxum z;ariabiZe ___----____...___..______.____
_._____
--___---____
-_ 2
8 10
Oxytoxum sphaeroideum
-____..___._______...___...____
-_____
---____ 1
1
3
CZudopyxis setifera _____.._____-..__._.--..-.---...---....____..____
--__- 1
Katodinium
rotundatum ____..-____-__
._.-_____._____________________
6
3
Oxytoxum scolopax _--_______
-___..___________...__
-_-__--- __________
-_
1
Ceratocorys horridurn _.__________.._____._________________
- ____
--__-1
Gymnodinium
punctatum __..___._..____..______________________
--__
1
Ceratium pentagonum __-______..___...___...____________
-_____
--___-1
Ceratium teres -____..____._____.._____________________--..-----..----___Oxytoxum gladiolus ____...___-..-___..__^__________________--------___
PodoZumpas palmipes _________________..__________________
-_._________
-__-_____
Ceratium fums ..___._.____....__--____________________-.---.__
Oxytoxum tesselatum ___-__._______..___
- ..____
-___-----____________
Pwrocentrum
redfield; - ____--__.._____._______________
- _____
--___PTOrOCmdTUm
minimum __._---___.-____._____________________
--___Goniuulax unicornis ( ?) __.____.______________
-________
-__-__..___
PTCNOmntTUm
rostratum - __--___________________________
-__-___-Dinophysis punctatum _____________.._
- __.________
---____________
Amphidinium
carteri ( ?) __---._._____..____._____________
- ______
-_
Ornithocercas magnificus
( 3) _-.____.-____________________
--___
Ceatium
mussiliense _....__._-._______._________________
- ____
-_______
Ceratium trichoceros
_._____-----___-________________________.--_____
No. of species, accumulated
_____.__
-._____
--.___-______
-_______._3
7
8
So. of cells, accumulated
_--___.___..________
--.____
-___.__
-___._.. 4 22 39
Undetermined
dinoflagellates
___....___..__
--______
-.___-----~--- 2
2
2-
. ..___________..___
---____
- ____
-__ 2
7
coccolithophores
1
2
1
65
11
1
1
2
86
11
1
2
10
1
90
11
2
114
12
1
12
8
3
13
326
4
1
4
1
55
2
12 12
191 241
1
1
40
8
5
21
1
1
4
3
128
16
13
375
3
29
2
1
7
1014 1015
Dinophyceae
4
42
4
4
7
11
153
11
113
11
102
7
Undetermined
2
1
3
1
Coccolithophoridacea
2
16
12 23
2
8
8 10
4
6
4
3
1
4
6
8
1
6
2
1
1
1
6
3
2
1004
1
140
17
1
3
4
3
13
436
6
26
8
1
6
12
2
1
159
18
8
6
4
1
13
517
4
1
1
33
12
6
15
8
1
171
18
7
3
2
7
13
581
1
2
1
25
13
3
12
7
181
18
1
7
2
1
13
632
1
32
9
2
7
1016 1017 1018 1020
2
1
1
201
18
12
7
1
3
1
1
1
1
9
197
17
8
8
1
42
2
1024
2:;
10
13
2
4
4
2%*
1
1
12
8
1
1
2
2%
1
2
3
3
3
15
16
16
871 1,033*1,063”
5
102
26
4
2
2
1
14
727
-_
1022 1023
1
1
1
70
16
2
2
1
1021
species at 20 m. Values are number of cells per 50 cc
994 993 999 1000 1001 1002 1003
The distribution
Coccolithus huxleyi ______.___._.______....----.._________.______________
1 16 12
Discosphaera tubifer _._____.______..._______________________-------.---15 12
6
Syracosphaera pulchra .._--.._____..__. . ..___--..__..___....______._
2
1
4
CJmbeZZosphaero irregularis __._.____..____.._________
--___--_--_-__-_1
4
Cyclococcolithus
leptoporus _...____.._____....__..___
-_____
----____ 1
1
Coccolithus pezagicus __._____.._____.________________________...-----3
Rhubdosphaera stylifer ---______.____._______.__________
-______
---___-- 2
2
Braarudosphuera
bigelowii ___...-___.._____._.________________
--___- 1
CycZococcoZithus fragilis __-_._-___-.-.___..________________________.-- 1
Syracosphaera heimii __----__.....___..____.._.____.____
---_____
-____
-_
3
Acanthoica acanthifera -_- ______..______-____.__________
_._____
-___Thoracosphaera mediterranea ___._._________
--___----____
---___.-_.
Rhubdosphaera hispida _--___...____..___..__
--_.____
--____
---__.-._
CaZciosoZeniu granii __.....___-____._-._____________________...---_____
Michaekarsiu
elegans ___---_________.._.___.._____r____
--_____
--_._._..
CaZciosoZeniu murrayi ._.___
-____...____.
___....___..___
---.___--__.-__
Ophiaster hydroideus ___--___..___
-_.___
-_____
---___-____.
--________
_
No. of species, accumulated
___....__..____
--____
--____
--_____._._8
8 10
No. of cells, accumulated
____..___...
__....____.__---____
--____.
-_ 26 59 88
-__--__
2&*
12
4
17
1,121**
4
210
1025
18
15
1
18
2
2
3
1
2
3
1
1
1
1
;
2
5
4
5
10
2
4
1
1
1
19
18
17
16
12
occurs
rence
312
EDWARD
M.
1
c
HULBURT
e,
9
it
----_----
__-_-^_- 1N
---
YHY’I’Ol’.LANK’I‘UN
THE
SARGASSO
313
SEA
minimum, also recorded from coastal water
(Lillick
1937; Ryther, et a2. 1958). The
first 6 species of diatoms listed are tropical
or oceanic forms, but of the rest 11 are
neritic and 13 are temperate ( Cupp 1943).
FREQUENCY
WHICH
RARE
SPECIES
ARE
ENCOUNTERED
The accumulation of rarer forms as more
samples are examined from the same population is predicted by a theory (Fisher,
Corbett, and Williams 1943) in which the
cumulative number of species, S, is related
to the cumulative number of individuals, N,
by the equation
s= nln(N+l).
a
The value of (Y should remain constant
for progressively larger values of S and N,
and will bc large when rarities accumulate
rapidly and common forms occur in low
concentrations, and will be small when rare
species accumulate slo,wly with high concentrations of dominant forms. For coccolithophores, observed values are fitted moderately well by a line of constant a. This
indicates that the addition of “new” species
with continued examination of the l,OOOmile stretch of ocean is at a tempo expected
from examination of the same population,
i.e., the population at a given depth at a
single station, The dinoflagellates
were
somewhat more diverse ( a = 4) and there
was a more rapid accumulation of species
towards the southern end of the section,
The diatoms from the first 13 samples,
which are tropical and oceanic in character,
fit a line of constant a and thus typify a
homogeneous population in the northern
and central parts of the section, The last 6
observations, from water near the Lesser
Antilles, show in their marked departure an
overly rapid appearance of species and indicate a consequent heterogeneity of the
diatom assemblage for the section taken as
a whole. This heterogeneity is also suggested by the fact that many of the species
are characteristic of temperate coasts.
DISCUSSION
The low total cell number in the northern
and middle portions of the section indicates
i
CUMUL AT/I/E
NUMBER
OF
SPECIES
FIG. 3. The relation
20-m samples.
of species to individuals
in
the poor growth conditions there; higher
values at 20 m in the southern portion are
This
evidence of improved
conditions.
change is associated with an increase in the
abundance and diversity of diatoms, with
the appearance of Trichoclesmium,
and
with an increase in the numbers of CoccoZithus huxleyi. All of the remaining coccolithophores, all of the dinoflagellates, and 6
of the 29 diatoms show no marked fluctuations. The manner in which the rarer
coccolithophores are encountered indicates
homogeneity.
Thus, under changing conditions, the
fraction of the phytoplankton which exhibits
a tendency toward homogeneity is predominantly motile and that capable of reacting
to improved growth conditions is primarily
non-motile.
In spite of the facts that the
flagellate Coccolithus huxleyi reacts nearly
as well as do diatoms to a more favorable
environment and that 6 of the diatoms remain uniform throughout the section, this
distinction is borne out by tho great majority of species.
The flagellates are tropical and oceanic
and the non-motile forms (except the 6
diatoms just mentioned) are temperate or
thrive in coastal waters. Therefore it would
seem that the success of the flagellate in
barren, tropical seas is due to its ability to
stay within the euphotic zone and that the
diatom is at a disadvantage because of its
tendency to settle out. But in productive
314
EDWARD
M.
regions -found
typically
near temperate
coasts although, as our observations show,
occasionally
elsewhere-an
apparently
greater reproductive capacity of the diatom,
as compared to the flagellate, may far outweigh loss due to settling.
The extensive study of equatorial and
southern Atlantic phytoplankton
by Hcntschel ( 1932, 1936) corroborates the validity
of distinguishing
accentuated growth and
dominance of non-motile organisms from
poor growth and dominance of motile organisms. Hentschel (1936) found that at
latitude 50’S, where high concentrations of
phosphate reach the surface, a large quantity of phytoplankton was present and was
dominated by diatoms. Just to the north,
where a moderate amount of plankton occurred, Coccolithus huxkeyi was the most
numerous species. Other rich areas, with
abundance of diatoms, existed close to the
African coast in regions of upwelling ( Wiist
and Defant 1936; Fuglister 1960). Well offshore, where the stratification was extreme
in the upper 100 m, collections from three
stations ( 263, 266, and 146) showed high
concentrations of diatoms. In most of the
equatorial and southern Atlantic, however,
there wcrc very low concentrations of total
phytoplankton,
dominated by coccolithophores and dinoflagellates,
To IIentschel’s three exceptional records
of enhanced growth with extreme stratification may be added our observations. In our
section, the hydrographic
changes toward
the south should do three things: bring nutrients closer to the surface, lessen the rate
of vertical exchange, and make possible a
much thinner but more densely populated
plant layer in the superficial, homogeneous
water ( Seiwell 1935). Along the 40th mcridian, where the hydrographic structure is
closely like that described here, Seiwell
( 1935) found that the first two exactly balance each other. Thus, the southward intensification of stratification should increase
the plankton in the near-surface water, but
reduce its downward extent. Our vertical
counts are in accord with such a conception.
Whether or not alterations in the density
HULBURT
structure have anything to do with the appearance of Trichodesmium near the Lesser
Antilles is difficult to say. Its appearance
there in February has, however, a simpler
aspect, for its distribution changes scasonally. It is confined to more southern waters
in winter, only extending northward in summer (Dugdale, Menzel, and Ryther 1961).
TAXONOMIC
NOTES
Because this study is in a region close to
that investigated by Lohmann (1920) and
Hcntschel ( 1932, 1936), a few comments
should be made in regard to apparent disparities in the names of several important species. The coccolithophore Umbellosphnera irregularis, a frequently occurring
form in our counts, was thoroughly
described by Markali and Paasche ( 1955).
It resembles the incompletely
described
Heineckia
bnrkowi
(Gemeinhardt
and
Schiller 1930). Lohmann and Hentschel
found Heineckin frequently, and we feel
that the same organism is being referred to
despite the two names used.
Oxytoxum variabile was an abundant
dinoflagellate in the section under study. It
was first described by Schiller ( 1933). It
is very similar in size and shape to Amphidinium acutum ( Lohmann 1920) and is only
distinguished by a dclicatc shell that is quite
difficult to see. Lohmann and Hentschel
both record A. acutum as the most abundant dinoflagellate, and again we feel that
the names 0. variabile and A. acutum refer
to the same organism.
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