Journal of Plankton Research Vol.18 no.6 pp.941-951,19%
Surface hydrography and phytoplankton of the Brazil-Malvinas
currents confluence
Ana Maria Gayoso and Guillermo P.Podestd
Centra Nacional Patagdnico, Boulevard G. Brown s/n, 9120 Puerto Madryn,
Chubut, Argentina and 'Rosenstiel School of Marine & Atmospheric Science,
University of Miami, 4600 Rickenbacker Cswy, Miami, FL 33149-1098, USA
Abstract. Results are presented on the phytoplankton species composition and abundance from bottle
samples collected in September 1989 near the confluence of the Brazil and Malvinas currents off Argentina. The phytoplankton assemblages were dominated by diatoms and dinoflagellates. A surface diatom bloom was found along the west side of the Brazil Current, and was dominated by Thalassiosira
delicatula Ostenfeld emend. Hasle (cell numbers up to 5.5 x 105 cells 1')- The bloom was associated with
strong temperature gradients separating Brazil and Malvinas waters, and with the presence of a cyclonic eddy near the confluence of the currents. These features were detected in satellite imagery coincident with the in situ sampling dates.
Introduction
The surface circulation of the Western South Atlantic is dominated by the opposingflowsof the Brazil and Malvinas currents. The warm, subtropical Brazil Current flows south along the shelf break until it separates and turns towards the
interior at a latitude that varies between 33 and 38°S (Olson et al., 1988). The
cooler, nutrient-rich waters of the Malvinas Current flow north along the shelf
break until they meet the Brazil Current, in an area referred to as the Brazil/
Malvinas Confluence (or simply the Confluence), where the twoflowsturn offshore. The Confluence is the western edge of the subtropical front in the Atlantic
and represents the transition between subtropical and subantarctic waters. The
region shows a complex mesoscale circulation associated with intense mixing of
dissimilar water masses (Olson et al., 1988; Garzoli and Garraffo, 1989; Gordon,
1989; Peterson and Stramma, 1991; Bianchi et al., 1993). Satellite imagery of the
Confluence has revealed warm and cold core eddies, filaments and meanders
(Gordon, 1981; Legeckis and Gordon, 1982; Olson et al., 1988).
The biology of the Confluence area can be described as a transition zone characterized by an admixture of subtropical and subantarctic organisms (Deacon, 1982;
Boltovskoy, 1986). Phytoplankton from the Confluence area have been studied by
Hentschel (1932), Frengiielli and Orlando (1959), Balech (1971a, 1976,1978) and
Lange and Mostajo (1985). Plankton and hydrographic surveys in two different
seasons have been reported by Hubold (1980a,b). Based only on the study of net
samples, Balech (1949,1965,1971b, 1976, summarized in 1986) has described with
reasonable accuracy the circulation of the southwestern Atlantic, including the
Confluence region.
The presence of strong contrasts in various ocean properties (e.g. sea surface
temperature, ocean color, sea level), together with complex mesoscale patterns,
make the Confluence region particularly suitable for study using remote sensing.
In this paper, we present results on phytoplankton composition and abundance
© Oxford University Press
941
A.M.Gayoso and G.P.Podesta
V.
t 5 ] 4 13
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36° S-
M
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12' --.• H
17 16
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.
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•
,
2
•
•
a1»1£1
•*
A.
^
•
23
/•
,*
•'
40° S-
56° W
52° W
48° W
Fig. 1. Satellite-derived sea surface temperature (SST) for the study area, 10-11 September 1989. Colder SSTs are indicated by darker gray tones, whereas lighter tones represent warmer temperatures.
Circles indicate the location of the stations and their numbers are shown. The line on the left half of the
image indicates the 200 m isobath.
from the Confluence region combined with the oceanographic context provided by
contemporaneous satellite imagery. The main objective of this study was to understand the association between phytoplankton abundance, species distribution and
the complex circulation of the Brazil-Malvinas Confluence.
Method
During September 1989, afieldsampling was carried out in the area encompassed
by 35-40°S and 46-55°W, aboard the R V 'Oca Balda', as part of a joint ArgentineUS-French program to study the Brazil-Malvinas Confluence (Confluence Principal Investigators, 1990). Water samples were obtained with a rosette of Niskin
bottles; 11 subsamples werefixedby a few drops of neutral Lugol's iodine and later
processed ashore. Phytoplankton composition and abundance were studied for 18
of the preserved samples (see Figure 1 and Table I for station locations). Surface
temperature and salinity values at each station were taken from Charo etal. (1991).
Phytoplankton cells were identified and enumerated in a Wild M40 inverted
microscope using bright-field optics. Cells were settled from a 50 ml cylinder and
an area of 100 mm2 was always examined at 400x magnification. The organisms
were identified, when it was possible, to species, but in the case of smaller ones only
the genus or algal group were recorded. Thalassiosira species were identified using
phase contrast. Frustules were cleaned by the method of Simonsen (1974).
Principal component analysis (PCA) (Margalef, 1980) was used to reveal patterns of spatial distribution relating phytoplankton abundance and species composition. The application of this technique to plankton studies is discussed by Matta
942
Hydrography and phytoplankton of Brazil-Malvinas Confluence
Table I. Station dates, locations, surface temperature and salinity, and phytoplankton concentrations.
All dates correspond to 1989. The hydrographic data were taken from Charo el al. (1991)
Station
Date
Latitude
(N)
Longitude SST (°C)
(W)
Salinity
(p.p.t.)
Total no.
cells 1 •
T.delicatula
cells I"1
1
2
3
4
5
6
8
10
12
13
14
15
16
17
21
23
24
25
September 5
September 5
September 5
September 6
September 6
September 6
September 7
September 8
September 9
September 9
September 9
September 9
September 10
September 10
September 11
September 12
September 12
September 13
37°37.6'
37°47.0'
37°49.7'
37°59.5'
38°O0.7'
38°08.9'
35°19.8'
35°18.5'
35=72.9'
35°23.9'
53°34.2'
52O47.6'
52°13.7'
51°53.3'
51°10.9'
50°24.1'
46°30.0'
48°0U'
49°30.4'
51°00.0'
51°43.6'
52°16.8'
5O°50.2'
51°23.2'
53°O4.O'
53°01.0'
53°44.0'
54-14.9'
35.12
34.04
34.73
35.89
36.05
36.00
35.65
35.75
36.05
36.28
36.26
35.00
35.88
35.81
35.54
34.33
35.27
34.04
364 105
620 756
345 109
6629
60 137
22 857
956
4927
11051
896
2822
4652
257 082
8136
4870
141 549
169 455
7885
305 083
551 559
282 604
686
17 267
10117
0
2254
4631
0
0
0
209 500
3420
0
109 748
104 125
0
35°21.r
35°27 4'
36°29.9'
36°30.6'
36°27.4'
39°30.9'
39°18.8'
39°06.9'
15.12
9.70
13.16
16.09
17.02
16.98
15.62
15.77
17.61
18.33
18.54
16.85
16.93
17.70
17.50
13.16
15.25
7.03
and Marshall (1984). Only taxa that were present in at least four of the 18 stations
were considered. The exclusion of rare species was intended to reduce overall
variability and noise in the correlation matrix (Matta and Marshall, 1984; Gould et
al., 1986). Sixteen taxa (indicated in Table II) were included in the analysis. The
PCA was performed on the correlation matrix, which assigns equal weight to all
variables (Gould et al., 1986).
Sea surface temperature (SST) fields for the first 2 weeks of September 1989
were derived from data collected by the Advanced Very High Resolution
Radiometer (AVHRR), an IR radiometer flying aboard the NOAA-11 polarorbiting spacecraft. SST was computed using a multichannel algorithm (McClain
et al., 1985). The SST fields were re-mapped to a fixed geographic projection. A
composite SST image was assembled using a 'warmest pixel' approach (Podesta et
al., 1991) with data for September 10 and 11,1989, as the imagery for these days
was relatively cloud free (Figure 1).
Results
Species composition and abundance
Phytoplankton cell concentrations are presented in Table I; the frequency and
maximum cell densities of the taxa and algal groups are shown in Table II. Diatoms
were dominant in number of species and abundance. Diatom genera identified
included Thalassiosira, Lauderia, Chaetoceros, Corethron, Rhizosolenia,
943
A.M.Gayoso and G.P.Podesta
TaWe II. List of the taxa and algal groups, frequency of occurrence in 18 bottle samples (Fq) and
maximum concentration observed (in cells I 1 ) For each major group, taxa are sorted by decreasing
frequency of occurrence: in the case of ties, the taxa are sorted by the maximum observed concentration. The last column indicates codes of taxa in Figure 2; taxa with codes were used in principal
component analysis
Taxa
Fq
Maximum
concentration
(cells 1-')
Code
Diatoms
Thalassiosira dehcaiula Ostenfeld
Lauderia annulata Gran
Chaeioceros sp. 1
Rhizosolenia seligera Brightwell
Hemiaulus sinensis Greville
Corethron criophilum Castracane
Thalassiosira sp.
Nitzschia closlerium (Ehrenberg) Smith
Coscmodiscus sp
Nitzschia sp.
Chaeioceros sp. 2
Dilylum brighlwellii (West) Grunow
Pseudosolenia calcar-avis (Schultze) Sundstrom
Achnanthes brtvipes Agardh
Chaeloceros sp. 3
Thalassionema mtzschioides Grunow
Cyclotella sp.
Rhizosolenia stolliforthi H.Peragallo
Thalassiosira simonsenii Hasle et Fryxell
Unidentified pennate diatoms
Unidentified centric diatoms
12
10
6
6
5
4
4
4
4
4
3
3
3
2
2
2
1
1
NA
17
11
551 559
39 359
15 827
747
830
6240
6240
790
417
250
5623
208
208
1495
833
310
1495
25
NA
10 621
42 593
Td
La
Ca
Rs
Hs
Cc
Ts
Nc
Cd
Ns
9
8
5
4
4
Gr
Pk
Cl
Gs
Cf
Df
Dmoflagellates
Gyrodinium sp.
Polykrikos sp.
Ceranum lineatum (Ehrenberg) Cleve
Ceranum fusus (Ehrenberg) Dujardin
Katodinium sp.
Ceranum candelabrum (Ehrenberg) Stein
Coryihodinium reticulalum (Stein) Balech
Prowperidinium sp
Unidentified dinoflagellates
1
1
1
14
2210
2707
600
14 198
208
61
30
29
25
18681
Silicoflagellates
Dictyocha fibula Ehrenberg
Diciyocha speculum Ehrenberg
13
3
797
747
Unidentified flagellates
18
118813
8
19 637
Gymnodiniwn sp.
Unidentified cells (<10 u.m)
3
Hemiaulus, Nitzschia, Achnanthes, Coscinodiscus and Cyclotella. The major dinoflagellate genera observed in the samples were Ceratium, Protoperidinium and
Polykrikos. Other algae were Dictyocha fibula, filaments of cyanobacteria and
small flagellates and coccoid forms.
944
Hydrography and pbytoplankton of Bnuil-Malvinfls Confluence
Six of the stations showed phytoplankton bloom conditions, with total cell concentrations ranging between 1.4 and 6.2 x 10511 (Table I). The bloom stations were
characterized by a well-defined assemblage of species dominated by the diatom
Thalassiosira delicatula Ostenfeld emend. Hasle (Hasle, 1980). This species
accounted for 61-89% of the total number of cells in the bloom stations (Table I).
The delicate valvar structure, the arrangement of strutted processes and the position of the labiate process in these cells coincided with T. delicatula cited by Rivera
(1981, Figures 68-69, 75-77). This author considered Thalassiosira chilensis
Kraaske as a synonym of T.delicatula. The T.delicatula cells were 11-25 n-m in
diameter, with a pervalvar axis of 12-22 \i.m, showing radial areolae arrays, one
ring of occluded processes and 2-3 rings of strutted processes (4-5 in 10 u.m), and
one labiate process situated between the valvar surface and the mantle. Thalassiosira simonsenii was also identified in the bloom samples, but was not included in
the total cell counts because it looked like T.delicatula through the inverted microscope. Its occurrence was detected using acid-cleaned samples and phase-contrast
microscopy, and represented <0.2 % of the total number of cells registered in each
sample.
Other diatoms, such as Lauderia annulata and Chaetoceros sp. 1, and dinoflagellates such as Gymnodinium sp., Gyrodinium sp. and Polykrikos sp., were also part
of the species assemblage associated with bloom stations. A similar group of species was observed at station 12, but with lower cell densities (11 x 103 cells I1)Phytoplankton abundance at the non-bloom stations fluctuated between 0.9 and
60 x 103 cells I"1, and they generally showed a mixture of subantarctic and warmwater taxa.
Principal component analysis
The first four principal components (PCI, PC2, PC3 and PC4) respectively
explained 24, 19, 14 and 13% of the total variance, or a cumulative 69%. The
remaining principal components explained an increasingly smaller fraction of the
variance. Discussion of the PC A results centers mostly on PCI and PC2 (43% of
the total variance).
The PCA results corroborated the pattern of species distribution and abundance described above. The loadings for each taxon for the first (PCI) and second
(PC2) principal components are shown in Figure 2. PCI showed high negative
loadings for a group of species associated with the bloom stations: T.delicatula,
L.annulata, Chaetoceros sp. 1, Polykrikos sp. and Gymnodinium sp. In contrast,
Corethron criophilum, Thalassiosira sp. and Nitzschia sp. showed positive loadings
on PCI. PC2 had positive loadings for Rhizosolenia setigera, Nitzschia closterium
and Ceratium fusus. Dichtyocha fibula and Ceratium lineatum showed slightly
lower loadings on this component.
The distribution of stations in the plane defined by thefirsttwo principal components is shown in Figure 3. The figure also shows four major clusters of stations
identified using the scores of thefirstfour principal components as input to a hierarchical cluster analysis using an average linkage technique (Gould et al., 1986).
Figure 3 shows the clear separation of bloom stations 2, 3, 23 and 24, with large
945
A.M.Gayoso and G.P.Podesti
•
Rs
Cf •
nc
0.4
Df
1
Hs
0.2
Cl
La
A
Td
CM
o
Q_
0.0 A
Qs
Cd
Pk
Cc
Ca
T
Qr
-0.4
-0.3
-0.2
-0.1
0.0
0.1
*#.
Ns
0.2
PC1 Loadings
Fig. 2. Loadings of the major taxa or algal groups in the Confluence (acronyms listed in Table II) on PCI
and PC2. Triangles represent bloom taxa; circles indicate non-bloom taxa.
negative scores on PCI. The stations listed are dominated by the warm-water species T.delicatula and the bloom-related group of species discussed above. Station 1
is another bloom station, but shows a relatively low score along PCI and is identified as a distinct entity by the clustering procedure. Despite being dominated by
T.delicatula, station 1 appears to have low numbers of the bloom-related dinoflagellate species. Stations 5 and 6, with intermediate total cell counts (23-60 x 101
cells H), are clustered together. These stations are characterized by high diversity,
with a mixture of warm-water species, cosmopolitan and subantarctic elements.
All remaining stations are clustered together. Interestingly, this large cluster
includes station 16 which, despite having a high abundance of T.delicatula (81% of
the total number of cells correspond to this species), seems to share the characteristics of the non-bloom stations, because it lacks many of the species in the bloom
assemblage.
Surface hydrographic features associated with phytoplankton distribution
The satellite-derived SST data (Figure 1) provided a useful oceanographic context
to interpret some of the phytoplankton biomass variability among stations. To
facilitate the discussion in this section, a schematic description of the main hydrographic features in Figure 1 is presented in Figure 4.
The Confluence area showed a fairly complicated SST pattern. The Brazil Current separated from the shelf break at ~35.5°S, where it encountered a narrow
filament of Malvinas water flowing along the shelf break. The SST pattern suggested the presence of a cyclonic eddy just off the shelf break, between the latitude
of the Brazil Current separation and ~38°S. This feature was evidenced by the
southward return flow of the Malvinas filament, together with a filament of subtropical water being wrapped northwards along the inshore edge of the eddy. The
three stations with the highest phytoplankton abundances were associated with
the eddy: station 1 (3.6 x 103 cells I"1) was located in the inshore filament of subtropical water, station 2, showing the maximum abundance (6.2 x 105 cells I 1 ) . w a s
946
Hydrography and pbytoplankton of Brazil-Malvinas Confluence
{' 6
res
6
v
4
8
CO
3
\
\\
\'
«%
{ A
2
•"3
.'A
2 4
'
0 %
\
;
23
/
13
i
N
\
14
4
-
2
17 ""
,•21
|«12
16
--. A---- '"''
-
i
0
25V •
2
PC1 Scores
Fig. 3. Distribution of the stations in PCI versus PC2 space. Triangles represent bloom stations (total
cell concentrations >1 4 x 105 cells I'); circles indicate non-bloom stations.
in the boundary between the Malvinas water filament and the edge of the Brazil
Current; station 3 (3.4 x 10s cells 1"') was located slightly inside the core of the
Brazil Current.
The contact between the Malvinas and Brazil currents continued south of the
cyclonic eddy. Another filament of Malvinas waters turning offshore was found at
~39.5°S. Increased phytoplankton densities (1.4-1.7 x 105 cells 1"') were observed
at stations 23 and 24, located along the northern boundary of this flow. Very strong
temperature gradients were detected near these stations: SST changed from ~6.5
to 14.5°C in 12-17 km. In contrast, station 25, located in Malvinas Current waters
and farther from strong thermal changes, showed cell concentrations two orders of
magnitude lower.
A high phytoplankton concentration was also registered at station 16 (2.6 x 105
cells I"1), farther offshore, away from the main interface of Brazil and Malvinas
waters. It occurred near a thermal discontinuity dividing a branch of the Brazil
Current warmest waters and slightly cooler waters of unclear origin, probably
associated with an intrusion of cooler waters from the Confluence. Such intrusions,
flowing along the edge of the Brazil Current for a long distance, have been
reported by Gordon (1989), and they are clearly detected in satellite imagery of
ocean color as long, skinny filaments of high phytoplankton concentrations along
the returnflowsof the Malvinas and Brazil currents.
Table I shows surface temperature and salinity values for the stations studied.
Two end member types of water could be distinguished: the core of the Brazil
Current (SSTs >18°C and salinities >36 p.p.t.) is represented by stations 13-14,
and the Malvinas waters (SSTs ~7°C and salinities ~34 p.p.t.) are represented by
station 25. Temperatures and salinities at most other stations (especially those with
high cell concentrations) showed intermediate values, demonstrating the mixing
of both water types.
947
A.M.Gayoso and G.P.Podesti
36° S
40° S
W
Fig. 4. Schematic description of hydrographic features in Figure 1 (the area east of 47°W is excluded).
Dotted lines indicate boundaries of the Brazil (BC) and Malvinas (MC) Currents. The dashed line
indicates boundary of cold water intrusion east of the Brazil Current core. Large white circles, small
white circles and black circles indicate, respectively, bloom stations (i) bloom conditions (total cell
concentrations >1.4 x 10* I ' ) , (ii) intermediate cell concentrations (0.2-0.6 x 10* \~') and (iii) low cell
concentrations (<0.2 x 10* I'). Arrows indicate flow direction as presumed from SST patterns and
historical data.
Discussion
The confluence of the Brazil and Malvinas currents is part of the subtropical front
in the South Atlantic Ocean, and constitutes an important biogeographic boundary between organisms of subtropical and subantarctic origin. Near the Confluence, there is a mixture of organisms associated with both sides of the boundary
(Boltovskoy, 1981,1986; Deacon, 1982; Olson, 1986). The strong influence of the
Brazil Current was revealed in this study by the dominance of warm-water species
such as the diatom T.delicatula and subtropical species such as L.annulata. The
mixing of waters was demonstrated, on the other hand, by the presence of subantarctic components like the dinoflagellate Ceratium Hneatum and the diatom
Corethron criophilum.
PCA results showed that the largest proportion of variability among stations
was dominated by the cell counts of the bloom-related species, which included
T.delicatula, L.annulata, Chaetoceros sp. 1, Polykrikos sp. and Gymnodinium sp.
Most of the remaining variability among stations appears to be related to the presence of cosmopolitan and subtropical species.
The main finding of this study was an extended bloom dominated by diatoms.
The bloom was clearly associated with the mesoscale circulation in the Confluence.
Five of the six largest cell concentrations found occurred near strong thermal
fronts separating Malvinas and Brazil waters. Many authors have discussed the
association between high phytoplankton biomass and vicinity to ocean fronts (Le
Fevre, 1986; Olson et al., 1994). The enhanced phytoplankton biomass near the
thermal fronts can be explained by both the accumulation of plankton as a result of
convergent flow and by higher production supported by nutrient-rich waters
advected by the Malvinas Current. The supply of nutrients to the fast-growing
948
Hydrography and phytoplankton of BrazU-Mahinas Confluence
diatoms associated with the normally oligotrophic Brazil Current waters could
have induced the bloom.
Early studies reported poor productivity and phytoplankton biomass in the
Confluence. Hentschel (1932), based on the study of bottle samples, found poor
phytoplankton densities in the region. More recently, Brosin and Nehring (1967)
reported high chlorophyll concentrations (80-120 ^.g I"1) at the boundary of the
Malvinas and Brazil currents. Balech (1971a, 1976) found rich phytoplankton samples in some localities near the west side of the Confluence, between 38 and 40°S.
Hubold (1980a) observed high chlorophyll values and zooplankton densities at the
northern end of the Malvinas Current, close to Brazil Current waters.
The complex mesoscale circulation of the Confluence may have played a role in
the formation of the diatom bloom by enhancing the retention of cells in a geographic area. Maximum cell concentrations (stations 1-3) were associated with a
cyclonic eddy located just off the shelf break and centered at ~37°S. Theflowsof
the Brazil and Malvinas currents turn offshore after their convergence (Olson et
al., 1988). Therefore, despite their rapid growth, diatom cells occurring along the
Confluence would normally be advected offshore by the strong return flows of
both currents. However, the cyclonic eddy could provide an adequate retention
mechanism, delaying the advection of phytoplankton cells and allowing them to
develop a large bloom. In a similar situation, Olson (1986) described a recirculation cell associated with the Brazil Current which significantly enhanced the residence time of drifting buoys. The pseudo-Lagrangian drifters provided a crude
measure of the drift patterns expected in the plankton community.
This study is thefirstrecord of a diatom bloom in the confluence of the Brazil and
Malvinas currents described within the oceanographic context provided by satellite-based sensors. The heterogeneity in the spatial phytoplankton distribution
described in this study would be hard to explain based only on data obtained from
traditional sampling platforms. The strong contrast in the surface temperatures of
the Brazil and Malvinas currents allowed the identification of the main circulation
features using satellite-derived data. In turn, patterns of phytoplankton distribution and abundance could be interpreted in relation to the mesoscale circulation
in the area. Unfortunately, at present there are no satellite-borne ocean color sensors that would allow frequent monitoring of phytoplankton biomass in the Confluence region. However, two ocean color instruments will be launched by the
USA and Japan in the near future. Additional field studies, together with data
from the new satellite sensors, will enhance our understanding of the patterns of
phytoplankton variability in the complex and interesting Confluence region.
Acknowledgements
We thank J.C.Elgue (Uruguay), who collected and kindly offered the samples for
phytoplankton analysis. Ms K.Kilpatrick provided useful editorial comments. The
AVHRR data used in this study were recorded by the Servicio Meteorol6gico
Nacional of Argentina as part of a cooperative agreement with the University of
Miami (USA). Satellite data collection and analysis were funded by the US
National Science Foundation. G.P. was supported by the US National Oceanic and
Atmospheric Administration during the preparation of this manuscript.
949
A.M.Gayoso and G.P.Podesta
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Received on October 19, 1995; accepted on January 19, 1996
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