International Council for
the Exploration of the Sea
ICES C.M. 1993/L:66
Theme Session 0
DYNAMIC OF TOXIC DINOFLAGELLATES DURING AN UPWElLING EVENT AT THE
NORTHWEST COAST OFF PORTUGAL
A. Jorge da Silva • and M. Teresa Moita
11
* Instituto Hidrografico, Rua das Trinas 49, 1296 LISBOA CODEX, PORTUGAL
# Instituto Nacional de Investiga~ao das Pescas, Av.Brasilia, 1400 LISBOA, PORTUGAL
•
Abstract
The dynamies of Gymnodinium catenatum and Dinophysis acuta was studied during
the development of a weak upwelling event observed in a repeated coverage of a combined
CTD/plankton transect at 41°0S ' N, off the coast of Portugal. Both species were mainly
distributed in the surface wind driven layer within the region of the equatorward coastal jet.
Wind induced mixing, offshore transport and vertical motions were reflected in the
distribution and the number of cells. It was generally observed that wind relaxation was
associated with blooming conditions and shellfish toxicity, while active winds gave rise to
dispersion of cells, reduction of their number and detoxification of bivalves.
INTRODUCTION
•
Since 1985 blooms of Gymnodinium catenatum and of species of the genus Dinophysis,
like D. acuta, D. sacculus and D. acuminata, along the western coast of Iberian Peninsula have
been associated with recurrent toxicity problems involving, respectively, paralytic shellfish
poisoning (PSP) and diarrhetic shellfish poisoning (DSP) (Fraga, Anderson, Bravo, Reguera,
Steidinger and Yentsch, 1988; Reguera, Marino, Campos, Bravo, Fraga and Carbonell, 1993;
Sampayo, 1989; Sampayo, Alvito, Franca and Sousa, 1990). Off Rias Baixas (NW Galicia),
these dinoflagellates belong to phytoplankton assemblages characteristic of late stages of
succession in the coastal upwelling ecosystem (Figueiras and Rios, 1993). Proliferations of
these species in several pulses are also a common feature during summer and autumn in the rias
(Fraga, Reguera and Bravo, 1990) as weIl as in Portuguese coastal lagoons (Sampayo et al.,
1990).
In the Iberian area, significant development of G. catenatum has never been referred to
occur north of cape Finisterre. Until 1990 blooms of this species occurred mainly north of
Lisbon during summer and autumn (Moita, 1993), while in 1992-93 large ceIl numbers have
been observed south of Lisbon even during winter (Sampayo and Vilarinho, personal
communication). In 1985-87 the major outbreaks in the Galician rias took pIace during autumn,
associated with southerly wind events that caused shoreward advection of coastal waters,
eventually penetrating into the rias (Reguera, Campos, Fraga, Marino and Bravo, 1991).
Maximum cell counts were observed in the upper 5 m in the outer parts of the rias. The same
authors mention that, since 1988, weak summer outbreaks of this species have also occurred
in stratified eonditions during moderate upweiling. In such eases surface counts were very low,
the eeil maxima being observed in the thermocline.
Reguera et al. (1993) relate the growth of D. acuta and other species of Dinophysis to
the establishment of a thermocline (10-15 m) under which the maximum concentrations are
found, in the temperature range 15-17.5 °C, at salinities above 35. These authors suggest that
extremely dry, hot summers eombined with moderate upweiling pulses, which allow
thermoclines to persist at suitable depths, may give rise to suitable conditions for unusual
summer blooms of D. acuta in the Galieian rias.
Blooms of flageilates have been associated with different hydrodynamical conditions
within and between water masses. In particular, eoastal currents and costal trapped buoyant
plumes have been referred as playing a major role in the development and advection of the
blooms and the associated toxicity (Franks and Anderson, 1992; Lindahl, 1993). During spring
and summer the Portuguese shelf north of the Nazare eanyon (ca. 39°30'N, Figure 1) tends to
be oceupied by a shallow lens of reduced salinity, roughly limited below by the seasonal
pycnocline. The lens must be the result of the eombined eontributions of water from the NE
Atlantic and the Bay of Biscay, being modified by local river runoff and interaction with Central
Water upweiled near the coast (Jorge da Silva, 1992a). Upwelling events have been observed
to disrupt the strueture of this lens near the coast (Jorge da Silva, 1992a,b) or to push its coastal
limit offshore (Jorge da Silva, 1992b), mainly depending on wind strength and persistence
(Jorge da Silva, 1992e).
•
In this paper the eross.-shelf and vertical distributions of D._ acuta and G. catenatum
during their nearly simultaneous blooming in the northwest coast of Portugal is discussed in
relation to the development of an upwelling event and the associated dynamics, as weil as with
the presence of near-surface stratification conditions.
THE DATA
From 19 to 27 August 1987 a eombined hydrographie/plankton section was repeated
daily over the western Portuguese shelf and slope roughly off the city of Oporto (Figure 1).
CTD data, using a NBIS Mk IIIB instrument, were eoileeted along the section from on board
RlV Almeida Carvalho. When the third repetition had just begun the CTD hit the bottom and
the temperature sensor was damaged. The observations were, therefore, concluded using
Nansen bottles equipped with reversing thermometers.
A GeneralOceanics 101512 rosette sampIer (or Nansen bottles from the third repetition
on) was used to coilect sampies for nutrients and phytoplankton studies at every other CTD
station over the shelf and every third station over the slope, starting at the eostalmost one.
Sampling levels were 5, 10, 20, 30, 40, 50 and 75 dbar (or near bottom in shallow areas).
During the whole study, eoastal winds were measured at an Aanderaa automatie weather
station installed near Ferrel, at an altitude of 17 m in a weil exposed area with no distinet
topographie features, some 200 km to the south of the seetion (Figure 1).
2
•
.
Phytoplankton sampIes were preserved with hexamethylenetetramine buffered formalin
to a final concentration of 2 % (Throndsen, 1978). SubsampIes of 100 ml were allowed "to setUe
for 3 days. Cells were identified and count~ by the Utermohl technique (HasIe, 1978). In this
paper the authors' concern will essentially be related to two "dinoflagellate species, namely
Dinophysis aellta and Gynmodinillm eatelwtum.
CTD data were processed as described in Jorge da Silva, Marreiros and Almeida (1992).
The wind time series, collected at 30-min intervals, was filtered to produce hourly values, and
subsequently low-pass filtered (fe = 0.028 cph) with a third order Bwtenvonlz filter timt
preserves 95% of the signals with 50-hour periods and 5% of signals with 20-hour periods.
Prior to low-pass filtering, the eastward and northward wind stress components (TE' T N) were
ca1culated tising the quadratic drag law
(T E' TN)
•
= p. Co IVI (ll , v)
where v is the wind vector, (ll, v) are, respectively, the eastward and northward wind
components, p. is the air density (1.22 kg m-3) and CD is the drag coefficient (0.0012).
RESULTS
The summer upwelling season of 1987 was rather weak, the main favorable wind events
having occurred In mid May to mirl iüne (VItorino, 1989) and iritermittency being the main
characteristie of the remaining months (Jorge da Silva, 1992c). Somehow reflecting this
situation, at the beginning of the observations, on 19 August, blooms of large autotrophie
dinoflagellates, in particular Cerati1ll1l!llSliS (1.3 x 1<f cells I-I), dominated a characteristic late
summer I early autumn plankton commtinity. At the same time, G. eatenat1ll1l and D. aClita
reached, respectively, 36xH)3 cells }"I and 69xlOl cells 1\
•
The wind was \veak to moderate from an upwelling favorable direction during the
investigations (Figure 2). The near surface response of the ocean, clearly revealed by the
offshore displacement of the isotherms over the shelf (Figure 3), occurred within 1 day of the
onset of a significant forcing. The effect of the weak wind pulse of 17-19 August could still
be noticed as inshore temperature values Iower than 15°C. Iri the water column the inducect
vertical motion near the coast was quite obvious as an uplifting of isohalines and isopycnals,
with surface outcropping at the innennost 20 km (Figure 6).
Dinophysis aellta was basically distributed over the inner and mid shelf, the Iargest niean
values being observed inshore duririg periods of wind relaxation (Figure 4), with water column
maxima being found in association with the largest inshore vertical density gradients (Figtire 7).
Gymnodinillm catenatum was distributed over the whole shelf. Mean values tended to reflect
the cross-shelf water transport associated with wind pulses both as an offshore displacement of
the maximum and as a reduction of the average number of cells (Figure 5).
During the relaxation of the'short wind event that peaked on 18 August the maximum
number of cells of D. aeuta occurred inshore in the upper 10 or 20 dbar" (Figure 7), usually in
"water with salinity above 35.4 and density anomaly greater than 26.0 kg m-3 (Figure 6).
3
G. catenatum showed a similar behaviour but significant cell numbers were also found at mid
shelf, extending vertically down to the basis of the seasonal pycnocline.
On 22 August the surface water was less saline and less dense, particularly at the
innermost 10 km (Figure 6). Maximum cell numbers of G. catenatum on that day extended
over deeper levels and greater distance from the coast than 2 days before, significant values still
being found dose to the bottom of the pycnocline (Figure 7). On the other hand, maxima of
D. acuta were absent from the near surface, in an apparent indication that the lighter inshore
water contained no cells of this species although it was rich in G. catenatum (Figure 7).
The northerly wind event that peaked on 22 August (Figure 2) gave rise to coastal
upwelling and surface mixing. On 24 August the density anomaly in the surface layer was
greater than 26.0 kg m- 3 all over the mid and inner shelf and the salinity was above 35.6
everywhere but at the surface in the innermost station (Figure 6). Cell numbers of D. acuta
had the same order of magnitude from the near surface to 20 dbar at the innermost 10-15 km
(Figure 7). Below that level the number of cells was drastically reduced, in contrast with the
situation of 22 August.
On 23 August (not shown) cell numbers of G. catenatum were maximum in a band
extending from 10 dbar at 10 km to 30 dbar at 30 km, roughly following the 26.8 kg m-3
isopycnal. On 24 August, however, the values were lower, evidencing greater vertical
homogeneity with the exception of the innermost station where no cells were found below
10 dbar (Figure 7).
Wind relaxation occurred again on 25 August (Figure 2) and the average cell number
of D. acuta was seen to increase near the coast (Figure 4). This contrasts with the distribution
of G. catenatum which showed a minimum dose to the coast (Figure 5). On 26 August,
D. acuta was basically confmed to the upper 20 dbar, with maximum values at 5 dbar in the
innermost 10 km. Instead, the largest cell counts of G. catenatum were obtained in the upper
10 dbar at 20 km from the coast (Figure 7).
On 27 August the number of cells of both species decreased again (Figures 4 and 5), in an
apparent association with the wind peak the day before (Figure 2).
DISCUSSION
Based on direct current measurements conducted at three mooring positions covering the
whoie shelf along the CTD/plankton section, Jorge da Silva (1992c) described the shelf
circulation associated with the upwelling process in the area. Matching his interpretations of
the current system on 19 August, just after the short northerly wind peak (Figure 2), with the
present vertical distributions (Figures 6 and 7) on 20 August, it is possible to arrive at the
following:
- The wind driven surface (Ekman) layer was roughly limited below by
the isopycnal of 26.8 kg m-3 ;
- The offshore extension of the equatorward costal jet was likely to
coincide approximately with the 35.7 isohaline;
4
•
- The poleward undercurrent had its core cit 50-60 km from the coast,
along and below the coastal limit of the offshore salinity maXimum;
- The onshore subsurface motion was most importaTIt beneath nie offshore
salinity maxiinum;
- Near bottom onshore motion was likely to bring deeper water to
pressurc levels above 100 dbar at mid shelf;
,
' - Both dinoflagellate species werc niainly distributed in the wind driven
layer within the coastee'll equatorward jet;
- A secondary maximum of G. catenatum was preserit near the bottom
within the coastal salinity maximum, where the motion tended to be generally
poleward and onshore.
As a cönsequence of the wind event that began on 22 August, the coastal equatorward
jet intensified and underwent offshore displacement, with an inshore counter-current being
noticeable on 25-26 August (Jorge da Silva, 1992c). The wind driven surface layer was,
howev~r, restricted to thc upper 30 m or less, at least over the inner and mid shelf (Figure 6).
The ,water column reniained two-layered after the upwelling event; which Jorge da Silva (1992c)
. atirlbuted to the relatively mild wind conditions.
Positive cell counts were essentially found in the wind driven surface layer, and reflected
thc vertical oscillations of Hs base. This was particularly evident on 26 August in the
distributions of both dinoflagellate species (Figures'6 and 7). It is interesting to compare the
distributions of cells on that day with those on 20 August, also obtairied after wind relaxation
.(Figure 2) and when surface salinities were similar elose to the coast. It is tempting to interpret
the differences in cell distributions as due to either totally different stratification conditions,
resulting from the different strength of the upwelling events, or different longshore dynamics
dose to the coast: equatorward jet on 20 August, and poleward counter-current on 26 August
(Jorge da Silva, 1992c).
•
,
Toxicity by both DSP and PSP was observed duririg the cruise in the musseI Mytillis
.edulis (intertidal) and the elam Spisula solida (caught at the bottom inshore of the 35 m
isobath)~ The sampIes were coIlectCd on 24 August in the area off Oporto; and a bari was
imposed on shellfish harvest. It seems reasonable to link the shellfish toxification with the high
numbers of both D. aClita and G. catenatum observed irishore at the beginning of the cruise.
Conversely, the 100ver abundance ofboth dinoflagellates after 23 August, partiCularly in inshore
near-bottom waters (Figurc 7), is likely to be the inain reason of the confirmed absence of
shellfish toxicity after 31 August.
The lower cell counts obtained after 23 August, and the subsequent absence of shellfish
toxicity, may be due to replacement of the inshore bottom water, in a direct consequence of the
upwelling event. Alternatively, one may think that the offshore displacemerit of the
equatorward jet, as weIl as of the upwellirig source region (Jorge da Silva; 1992c), may have
replaced the water in the coastalmost 10 km, when the inshore counter-current was established
in the last days of the cruise.
'. Whenever the observations happeried to have exteride<i to the bottom at inid shelf they
tended to reveal a weak maximum in cell mimber of (at least) G. catenatunz (20, 22 August,
Figure 7). Thc developinerit of upwellirig, giving rise to a shoreward compensation current at
depth, may have carried this maximum inshore (26 August). Actually, thc intensification of thc
equatorward jet may have originated, through bohom friction; onshore motion in the near
5
bottom layer (Jorge da Silva, 1992c). It may, thus, be hypothesized that cyst re-suspension and
germination is likely to underlie the presence of this maximum, although the possibility of
offshore sinking in a vertical circulation cell should not be excluded as a possible cause of its
presence.
CONCLUSIONS
At the mid-term of a weak summer upwelling season off Oporto (Portugal), dinoflagellates dominated a plankton community characteristic of early autumn in which Gymnodinium
catenatum and Dinophysis acuta represented, at most, 5% ofthe most abundant species. These
two species were distributed over the shelf, D. acuta being absent from its outer part. They
were mainly present in the surface layer within the region of the equatorward jet characteristic
of coastal upwelling. During a short lasting wind induced upwelling event of moderate
intensity, the water column remained two-Iayered and the distribution of those dinoflagellates
reflected the vertical displacement of the pycnocline.
Upwelling and the associated wind mixing apparently caused offshore transport,
dispersion and reduction in the number of cells, which was particularly evident with G.
catenatum due to its wider distribution area. It is suggested that stronger stratification
conditions favor the development of both dinoflagellate species, but it is not clear whether the
nearshore dynamics plays also a relevant role.
Shellfish toxicity was positively linked to blooming of both dinoflagellates in inshore
waters. Subsequent detoxification was associated to the decline of the blooms which appeared
to be caused by development of coastal upwelling.
Acknowledgmellts - The authors are indebted to Augusto Carreira for bis help on the identifieation and eounting
of phytoplankton sampies. Thanks are also due to the captain and crew of RN Almeida Carvalho for their
eollaboration during the field phase of the studies. Tbis work was partly funded by Junta Naeional de Investiga<;äo
Cientifiea e Tecnol6giea through Project STRD/A/MAR/224/92.
REFERENCES
Figueiras, F.G. and A.F. Rios (1993) Phytoplankton sueeession, Red Tides and the hydrographie regime in the
Rias Bajas of Galieia. In: T. Smayda and Y. Shimizu (Eds.) Toxie Phytoplankton Blooms in the Sea,
Elsevier, Amsterdam, 239-244.
Fraga, S., B. Reguera and I. Bravo (1990) Gymnodinium catenatum bloom formation in the Spanish Rias.
In: E. Graneli, B. Sundstmrn, L. Edler and D.M. Anderson (Eds.) Toxie Marine Phytoplankton,
Elsevier, New York, 149-154.
Fraga, S., D.M. Anderson, I. Bravo, B. Reguera, K. Steidinger and C. M. Yentseh (1988) Influence of upwelling
relaxation on dinoflagellates and shellfish toxicity in Ria de Vigo, Spain. Estuarine, Coastal and Shelf
Sciellce, 27, 349-361.
Franks, P.J.S. and D.M. Anderson (1992) Toxie phytoplankton blooms in the southwestern Gulf of Maine: testing
hypothesis of physical control using historieal data. Marine Biology, 112, 165-174.
6
•
•
Hasle, G.R. (1978) Using the inverted microseope. In: A. Sournia (Ed.), Phytoplankton Manual, Monographs Oll
Oceallic Methodology, 6, UNESCO, Paris, 191-196.
Jorge da Silva, A. (1992a) Contribui9äo do Instituto Hidrognifico para 0 Projecto JNICT 87344 - Resultados do
cruzeiro CECIR XII, Maio 1987. REL TF-OF-7/92; Instituto Hidrogr:ifieo, Lisbon, 29 pp. (unpublished
manuscript) (in Portuguese).
Jorge da Silva, A. (l992b) Contribui~ao do Instituto Hidrogr:ifico para 0 Projecto JNICT 87344 - Resultados do
Cruzeiro CECIR XIII, Agosto 1987. REL TF-OF-8/92, Instituto Hidrognifico, Lisbon, 43 pp.
(unpublished manuscript) (in Portuguese).
Jorge da Silva, A. (1992c) Dependance ofupwelling related circulation on wind forcing and stratification over the
Portuguese northem shelf. International Council for the Exploration of the Sea, Council Meeting 1992,
Paper C:17, 12 pp.
Jorge da Silva, A., M. Marreiros and S.L. Almeida (1992) Processamento de dados de CTD. Allais do Imtituto
Hidrograjico, 11, 7-18 (in Portuguese).
Lindahl, O. (1993) Hydrodynamical processes: a trigger and source for tlagelIate blooms along the Skagerrak
eoasts. IIl: T. Smayda and Y. Shimizu, (Eds.) Toxie Phytoplankton Blooms in the Sea, Elsevier, 775-781.
Moita, M.T. (1993) Development of toxie dinotlagellates in relation to upwelling patterns off Portugal. In: T.
Smayda and Y. Shimizu (Eds.) Toxie Phytoplankton Blooms in the Sea, Elsevier, Amsterdam, 299-304.
Reguera, B., J. Marino, M.J. Campos, I. Bravo, S. Fraga and A. Carbonell (1993) Trends in the oeeurrenee of
Dinophysis spp. in Galician Waters. IIl: T.Smayda and Y.Shimizu (Eds.) Toxie Phytoplankton B100ms
in the Sea, Elsevier, Amstenlam, 559-564.
Reguera, B., M.J. Campos, S. Fraga, J. Marino and I. Bravo (1991) The Spanish monitoring of harmful algal
blooms in Galieia. Proeeedings of the CNEVA International Symposium on Marine Biotoxins, Paris,
16 pp.
Sampayo, M.A. (1989) Red tides offthe portuguese eoast. IIl: T. Okaichi, D.M. Anderson and T. Nemoto, (Eds.)
Red Tides: Biology, Environmental Scienee, and Toxicology, EIsevier, New York, 89-92.
Sampayo. M.A.M., P. Alvito, S. Franca and I. Sousa (1990) Dillophysis spp. toxicity and relation to
aceompanying species. 111: E. Graneli, B. Sundstmm, L. Edler and D. Anderson (Eds.) Toxie Marine
. .•..•.
Phytoplankton, Elsevier, New York, 215-220.
Throndsen, J. (1978)Preservation and storage. 111: A.Sournia (Ed.), Phytoplankton Manual, MOllographs 011
Oceallic Methodology, 6, UNESCO, Paris, 69-74.
Vitorino, J.P.N. (1989) Circula~ao residual ao largo da costa NW de Portugal durante a esta<$äo de atloramento
de 1987. Allais do Imtituto Hidrogr!ljico, 10, 25-37 (in Portuguese) .
•
7
13
12
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42 r-----,,"'--,---.-_____.-rr-n-r---.-----, 42
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Longitude W
8
7
Figure 1. Bathymetrie ehart of the eoast of Portugal (isobaths in metres), position of the repeated section and approximate
loeation of the eoastal automatie weather station of Ferrel.
3
3
-;;~
N
I
-'"
0
.....
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-1
l-
-3
15
20
25
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-3
August 1987
_
2
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0
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August 1987
Figure 2. Coastal wind stress at Ferrel. August 1987.
8
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•
t
(Oe) -
5
dbar
9,._5_ _---=-
18 1,-°.,--
.-9
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--=.8.5 18
""
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26
27
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L-
----'
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Longitude W
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----'
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Figure 3. Space-time representalion of temperature at 5 dbar. 'The verticalline indicates the position of lhe 200 m isobath.
Dinophysis acuta
19
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Figure 4. Space-time rep' esentation of the mean cell count of D. acuta in the water column (log no. cells 1'1).
Gyrnnodiniurn catenatu1n
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Figure 5. Space-time representation of the mean cell count of G. catellatum in the water column (log no. cells t ).
9
Slalions
Slations
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100
Distance offshore (km)
"'1~
. : .".~
U. .
~
0..
100
1-v.:~::~
Ql
IIJ
I-t
I-.
20
Stations
393
Ul
Ul
0..
Distance offshore (km)
397
60
-268
Ul
Ul
Q)
S
22 Aug
100
I-t
;::l
0
100
Distance offshore (km)
...
,,;
'J
425
417
.
Stations
412
425
421
412
417
"...,
20
35.8j
I-t
20
III
.0
'0
'1J
.......
IIJ
Q)
I-t
;::l
I-t
60
rn
S
100
80
60
40
Distance offshore (km)
20
Ul
.,
0..
0..
(/)
I-.
0
::l
IIJ
Ul
26 Aug
III
.0
.......
60
L:'
I-.
100
100
80
60
40
Distance offshore (km)
20
0
Figure 6. Vertical distributions of salinity and density anomaly at the repeated seetion (Lat 41 °OS'N), August 1987.
10
100
_
..---------
--- ---
--
Slations
341
Slations
332
337
327
341
337
332
327
,....,
,....,
I-.
20 III
I-.
20 III
'0
.....,
'0
.....,
.c
.c
l1J
l1J
60 ;:l
60 ;:l
Q)
l1J
I-.
I-.
f/l
f/l
D.acuta
20 Aug.
80
100
60
40
I-.
I-.
e..
0
20
f/l
f/l
0..
100
100
80
361
365
eh
356
369
~1
,
20
J'<?
8
365
f/l
f/l
r;-.
60
40
0
20
356
20
100
100
80
60
40
l1J
I-.
f/l
f/l
l1J
20
I-.
e..
0
100
Dislance offshore (km)
Slations
Slations
389
393
,
0
'
393
397
384
c(l :
2·
20
J'
384
389
~C~~:
'i:'
III
.c
'0
.....,
20
'0
CF,:
l1J
I-.
l1J
I-.
60 ;:l
f/l
f/l
-
80
20
40
60
f/l
f/l
l1J
D.acuta
24 Aug.
100
100
80
'
6
Slations
20
40
0
100
Slations
"
,
425
412
417
421
60
I-.
e..
Dislance offshore (km)
Dislance offshore (km)
425
l1J
C. catenatum
24 Aug.
I-.
e..
0
'i:'
III
.n
'0
.....,
2
60 ;:l
100
'i:'
III
.c
'0
.....,
60 ;:l
'G. catenatum
22 Aug.
I-.
0..
Dislance offshore (km)
397
100
" /.2-
Q)
D.acuta
22 Aug.
361
~~
l1J
I-.
80
0
(,(9'
'i:'
III
.c
'0
.....,
60 ;:l
100
20
Slations
Slations
-
40
Dislance offshore (km)
Dislance offshore (km)
369
60
412
417
421
/~3
~2'
20 'i:'
III
~;"
.\/,
.n
"
~
Q)
l1J
I-.
I-.
60 ;:l
60 ;:l
(
100
80
60
.
40
f/l
f/l
Q)
f/l
f/l
1-
llJ
D.acuta
26 Aug.
20
I-.
I-.
e..
0
0..
100
100
80
60
40
20
o
100
Distance offshore (km)
Dislance offshore (km)
Figure 7. Vertical distributions of D.acuta and G. catenatum (Log no. cells '-1) at the repeated section (Lat 41 °OS'N), August
1987 (station levels with positive cell counts are represented by dots and those with zero cell counts by crosses).
11