Relationship between the phytoplankton distribution and composition and the hydrography in the northwest African
upwelling region near Cabo Corbeiro
DOLORS BLASCO*, MARTA ESTRADA~" and BURTON JONES*
(Received 26 October 1979; in revised form 22 April 1980; accepted 26 April 1980)
Abstract--During the JOINT I experiment, phytoplankton samples were collected along with the hydrographic data from March through May 1974. Analysis of the data by principal component analysis reveals that subtle changes in the hydrographical regime have major effects on the phytoplankton composition. Within the upwelling region the physical processes may play an important role in the variability observed in the phytoplankton distribution.
Evidence suggests that the decrease in diversity observed in the phytoplankton community during the study was induced by the decline of the heterogeneity of the physical environment.
INTRODUCTION
UPWELLING is prominent along the coast of northwest Africa, particularly during winter and spring (ToMCZAK, 1973; FRAGA, 1974; WEICHART, 1974; WOOSTER, BAKUN and MCLAIN, 1976), and in recent years there have been intense studies of the phytoplankton composition of the region (MARGALEF, 1973, 1975, 1978; ESTRADA, 1976, 1978; RICHERT, 1975). The papers have been based on quasi-synoptic surveys over large areas, and they have been useful in describing the phytoplankton in terms of mean distribution and seasonal variation. However, to understand the effect of the physical processes on the phytoplankton, a more detailed knowledge of the space and time variability was necessary.
The JOINT I study of the upwelling system from March to mid-May 1974, over the continental shelf and slope region between Cabo Blanco and Cabo Corbeiro provided a unique data base for such'a study.
The main object of this paper is to describe the variability in the distribution and composition of the phytoplankton during the period and to relate the variability to the hydrographical regime.
MATERIAL AND METHODS
As part of the JOINT I program, 177 hydrographic and 37 productivity stations were occupied from the R.V. Atlantis II during March to May 1974, in the northwest African region, extending from 22 to 21°40'N and from 0 to 100 km offshore. Most stations were along a line at 21°40'N where an array of current meter moorings extended from nearshore
* Bigelow Laboratory for Ocean Sciences, West Boothbay Harbor, Maine 04575, U.S.A. t Instituto Investigaciones Pesqueras, Paseo Nacional s/n Barcelona, Spain, Contribution 79022 Bigelow Laboratory for Ocean Sciences.
799
800 DOLORS BLASCO, MARTA ESTRADA and BURTON JONES
to midslope (Fig. 1). Water samples for phytoplankton analysis were collected at all the productivity stations in 30-1. Niskin bottles from the depths to which 100, 50, 30, 15, 5, and 1~ of the incident light penetrated, and at some of the hydrographic stations of Leg l (40 stations) and Leg 3 (59 stations) with 5-1. Niskin bottles at standard depths down to 200 m. The samples were preserved with Lugol's solution and phytoplankton cells were counted and identified by the Utermohl inverted microscope technique (BLAsCO, 1977). Cells were identified to species when possible but some could be assigned only to a genus or to a group. Temperature, salinity, chlorophyll, ammonia, nitrate, nitrite, reactive phosphorus and dissolved silica were routinely determined at each station and depth (FRIEBERTSHAUSER, CODISPOTI, BISHOP, FRIEDRICH and WESTHAGEN, 1975). The current meter data are from the report by BARTON, PILLSBURY and SMITH (1975). The letters G, O, U, R, and D identify reference locations that received emphasis in the sampling scheme.
Because of the quantity of data collected, principal component analysis was used to synthesize the information and to describe the variability within the hydrographic and phytoplankton data. The technique has been used in similar studies (BLAsCO, 1971 ; WANG and WALSH, 1976; ESTRADA, 1978; ESTRADA and BLASCO, 1979). The approach, the method, and the system of analysis are described in ESTRADA and BLASCO (1979). The phytoplankton data were logarithm transformed [x -~ In (x + 1), x = cells (50 ml)-a]. No transformation was applied to the hydrographic data.
The principal component scores were calculated using the formula"
In IX( i ) - X(i)]F(i,j)
S(j) = s(i)
i=l E(j)
where
S(j) = score for principal component j X(i) = In IX(i)+ 1] for species i )((i) = mean of In IX(i)+ 1] for species i s(i) = standard deviation of In Ix(i)+ 1] for species i
F(i,j) = factor loading to species i on principal component j E(j) = eigenvalue for principal component j
as described by COOLEY and LOHNES (1971). In addition to the total scores, partial scores were calculated independently for the
species having positive or negative loadings. Given the method of calculating the scores, negative scores of the positive group or positive scores of the negative group indicate average abundances of the species are lower than their means. Use of partial instead of total scores enables one to distinguish the contribution of high abundance of the species with positive or negative loadings from that of low abundance of the species with loadings of opposite sign.
HYDROGRAPHY
As our main objective is to relate to phytoplankton distribution to the hydrography, only certain aspects will be emphasized. Detailed discussions of the current regime and of the variability of the hydrographical and chemical variables can be found in MITrELSTAEDT,
Phytoplankton distribution and composition 801
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Fig. 1. Station locations for the R.V. A t l a n t i s II Cruise 82 (JOINT-I) March to May 1974, and the Oregon State University current meter array locations.
802 DOLORS BLASCO, MARTA ESTRADA and BURTON JONES
Table 1. Correlation coefficients of the hydrographic variables with the first four principal components of the hydrographic analysis
PC I PC II PC III PC IV Total variance explained 62.1~ 18.8~ 10~ 5.3~o
PILLSBURY a n d SMITH (1975); BARTON, HUYER a n d SMITH (1977); a n d CODISPOTI and FRIEDRICH (1978).
We have used principal component analysis to define the major trends of variability and to facilitate the comparison with the phytoplankton data. In the analysis the data from 102 hydrographic stations have been considered. The variables and the results are summarized in Table 1. The first principal component (PC I) is related to the upwelling process. This can be deduced from the fact that most of the variation in the component is accounted for by temperature on the positive side and by the major nutrients, NOB, PO,~, and SiO 4 on the negative side, and from the similarity that exists between distribution of PC I and sigma-t (density) at the mid-shelf station U (Fig. 2). The opposite behaviour of nutrients and
Fig. 2. Vertical distribution of sigma-t and first principal component of the hydrography during the entire cruise period at location U.
13
Phytoplankton distribution and composition 803
salinity in this component suggests that PC I is also related to the presence of the lower salinity, more nutrient-rich South Atlantic Central Water (SACW) in the area. The relationship between PC I and the richness of the upwelling source water can be more clearly seen in Figs 3 and 4. We conclude that the PC I reflects not only physical upwelling but also the intensity of the enrichment caused by the upwelling. Thus, we will refer to PC I as the upwelling enrichment component. The high negative values of PC I observed at depth (Fig. 2), which do not follow the density distribution, indicate the stronger influence of SACW in the upwelling source during the early part of the study, as has been reported by CODISPOTI and FRIEDRICH (1978). In Fig. 2 it can also be observed that three periods of enhanced upwelling ('events') and four relaxations occurred during the study.
The second principal component (PC II) shows positive correlations with NH4 and NO2, temperature, salinity, and silicic acid and a negative correlation with NO3. High
STATION 5 57 5 8 59 6 0 611
SIGMA t / 24 MARCH- LEG, 150
~ - ~26 ,8o I I • i f . I | I l I I 1 2 0 0
9O 80 70 60 ,5O 4O 50 20 I0
~5 " 57 5 8 59 60 61 , ,0-4. .4--_ -if'o" o
bJ
/ - I . " / ~ PRINCIPAL COMPONENT I / / / PRINCl AL COMPONENT 3: I-- • / / . -20 ( HYDROGRAPHY ) 150 I~ / / / 24MAROH-LEGI C~
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- - ~ J ~ I0" i - • • " 5 0
• • 20
#i • I I _ I I I I I 1 200 90 8 0 7 0 60 50 40 ;50 20 I0
Km OFFSHORE
Fig. 3. Cross-shelf distribution of sigma-t, first principal component, and nitrate (Ixg-at. l - x ) at the 21°40'N line during a relaxation of the upweUing intensity during Leg 1 of the cruise.
804 DOLORS BLASCO, MARTA ESTRADA and BURTON JONES
STAr ION 154 t55 156 157 158 159 160
: / : i 14-_'____:---, o
~ / / ~ / I / IO MAY-LEG 3 50
.I / 1 ~ I . If I I I I :)00 90 8 0 " 7 0 60 50 4 0 30 20 I0
Fig. 4. Cross-shelf distribution of sigma-t, first principal component, and nitrate (~tg-at. l- 1 ) at the 21°41'N line during a relaxation of the upwelling intensity during Leg 3 of the cruise.
positive values of the component are observed (Fig. 5) throughout the water column at the inner parts of the shelf and subsurface levels offshore. Negative values are associated with the upwelled water and appear at the surface at the shelf break stations. This distribution, along with the fact that NH4 and NO2 have the highest correlations with this component, suggests that PC II reflects variability arising from biological activity.
The same assumption seems valid for principal component III. Only NO2 and NH4 show significant correlations with this component. The association of NH 4 and NO2 in relation with PC II and their opposite correlation with PC III indicates that at least two biological processes contribute to the distribution of the two nutrients.
In the fourth principal component (PC IV) an increase in the values of the conservative properties is accompanied by an increase in nutrients (Table 1 ). The association of salinity with the nutrients seems to indicate that PC IV reflects the presence of a different water mass.
Phytoplankton distribution and composition 805
Fig. 5.
STATION 38 39 40 41 42 43
: . ,
~ ~ / PRINCI/ABLM~#MCpHoNENT Tr :
, , ,T:: 90 80
68
70 60 50 40 250 20 10
67 66 65 62 63 64
0
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, ,</ 90 80 70 60 50
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f . . . . . . . . . . i I I ;,:, 40 30 20 l0
Km OFFSHORE
2OO
0
50
I(:X:)
150
2OO
Distribution of the second principal component of the hydrographic data in repeated transects at the 21°40'N line.
Comparison of the vertical distribution of PC IV (Fig. 6) with the salinity distribution (Fig. 7) shows that positive scores are associated with water of salinity above 36.3% o, while negative scores are associated with lower-salinity water. However, the associations hold only at the surface. It is clear in the figures that the low salinity water observed at depths which correspond to the presence of the SACW has no negative scores in this component.
To obtain a more general picture of the temporal and spatial variability of the upweUing system during the entire JOINT I study we have projected the scores for surface samples from all the stations on the plane defined by the first and fourth principal components, upwelling enrichment and the high salinity water mass (Fig. 8). The distribution of the stations on the two axes shows significant differences between each leg of the cruise, especially between the first and the third legs. Legs 1 and 3 differ in the extent of strong
STATION 58 59 40 41 42 l
806 DOLORS BLASCO, MARTA ESTRADA and BURTON JONES
n~
W
W
Fig. 6.
~50 K) tSO b(2 40 ~ EU It,)
Km OFFSHORE
Distribution of the fourth principal component of the hydrographic data in three sections at the 21°40'N line.
upwelling. During Leg 1 upwelling enrichment occurred at all the positions as indicated by the stations having low positive or negative values for PC I. However, during Leg 3 upwelling enrichment was strong only at the nearshore positions R and D, except for two stations at position U. The scores of G and O for PC 1 were also consistently higher than in Leg 1. Another difference between Legs 1 and 3 is that during Leg 3 all the stations at a given geographic position are clustered within a relatively small region on the PC I and PC IV planes. The Leg 1 stations are more widely dispersed on the plane and only the stations at position U are closely clustered. We conclude that a change took place in the hydrography during the study, from a variable period with strong upwelling events and relaxations, to a more stable period when upwelling was more persistent but enrichment was less intense and restricted to the shelf region. CODISPOT~ and FRIEDERICH (1978) explain this apparent paradox as resulting from a decrease in the proportion of SACW in the upwelling source waters.
Fig. 7. Distribution of salinity in three sections at the 21°40'N line.
PHYTOPLANKTON
Biomass
The mean cross-shelf distribution in the upper 10 m of total cells, chlorophyll a, and the major taxonomic groups for the entire period is shown in Fig. 9. The mean temperature section is given in the same figure to illustrate the background conditions during this period. Maximum cell concentrations occurred between mid-shelf and the shelf break. Diatoms show the same pattern; their maximum contribution to the total phytoplankton, 85% of the total, is at the shelf-break. Farther offshore and inshore, their contribution is 29 and 49%, respectively.
Mean chlorophyll distribution in the upper 10 m shows a similar pattern, but with less dramatic changes. At the inshore stations (D) the mean chlorophyll per cell was 72.2 pg. The mean chlorophyll per cell for the other stations was 18.2 pg, and the minimum, 12.9 pg(cel l ) - ' occurred at the shelf-break (O). The rather high value observed at the
808 DOLORS BLASCO, MARTA ESTRADA and BURTON JONES
22"00 '
21" 30'
21°00 ' N
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-I 0 I PRINCIPAL COMPONENT "rV"
-2 +1 O I
- I 0 I 2
PRINCIPAL COMPONENT T~7
0
-I
Fig. 8. Relative positions of the surface samples of all the hydrographic stations taken along the 21°40'N line during the three legs of the cruise on the plane defined by the first and fourth principal components of the hydrographic data. The letters indicate the location of the stations on the shelf as
it is presented on the above left corner of the figure.
Table 2. Mean phytoplankton cell concentrations at three locations over the shelf integrated vertically. The intervals of integration are defined by the mean depth of the change of cross-shelf flow from onshore to offshore
F i g . 9 . Mean temperature distribution for the entire period of the cruise at the 21°40'N line (above). Average number for the upper 10m of total cells, diatoms, dinoflagellates, coccolithophorids, cells of other groups, chlorophyll a concentration, and chlorophyll a per million
cells at the 21°40'N cross-shelf line (below).
inshore station could indicate the presence of shade-adapted cells or of chlorophyll in the resuspended particulate matter that was observed during this cruise (MILLIMAN, 1977). Although the average biomass concentration for the whole period shows a definite trend across the shelf, the individual values at any given position demonstrated a large degree of variability with time (Fig. 10). In general, the highest cell concentrations seem to be associated with the periods of relaxation of the upwelling or an increase in the stratification of the water column (Fig. 2).
The lowest cell concentrations were observed at the surface during March and the highest from mid-April to the beginning of May. At depth the distribution was reversed. Cell concentrations were higher at the beginning than at the end of the period (Table 2).
8 l 0 DOLORS BLASCO, MARTA ESTRADA and BURTON JONES
STA 0 I0
"~ zO r r
~ 4c
N m
8c
STA 0
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,6 33 4, 55"00 • 6z" 70"
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y :o I 14 18 22 I .5 17 21 2.5 29 I 5 9
16 55 41 55~60 62 ~ 78 ~' I01104 125 132 139 158
• U Tota l d i a t o m s 10 3 l i ter ' l
I 0 14 18 22 2 30 5 I 21 25 9 5 9
MARCH APRIL MAY
89 I01104 125 132 139 158
~oo : : : : i qoo
I
I U, Total ce, lls, lO l i te r - , , ] i 13 ' 13
Fig. 10. Vertical distribution of total cells and total diatoms during the entire cruise (March through May) at location U.
Table 3. List of the most abundant and most frequent phytoplankton species during the JOINT-1 cruise, March to May 1974, in the northwest African upwelling region near Cabo Corbeiro
• / • ' ° ° q l " : # . i) o • ~ . .i. u I / / " i ~ \ / / / ~ - - ~ , o ~ \ "~2 "/" q\\ " / / • "~-"-~,"~,-2"~'-~ \ • £ . '~. . • \ U Rhtmsolen/o dehco/ulo I 03 cel ls l i ter -= ° "
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I 0 14 18 2 2 2 6 3 0 I 5 9 1:5 17 21 25 2 9 I 5 9
MARCH APRI L MAY
13
13
F i g . 11 . V e r t i c a l d i s t r i b u t i o n o f Thalassiosira rotula, Ceratium furca, a n d Rhizosolenia delicatula d u r i n g t h e e n t i r e c r u i s e p e r i o d ( M a r c h t h r o u g h M a y ) a t l o c a t i o n U .
However, if the whole water column is considered, no significant changes occurred during the entire period of the study.
Species composition and distribution
The 200 species observed during the study have been reported before as components of the phytoplankton of this region (MARGALEF, 1973, 1975; RICHERT, 1975; ESTRADA, 1978). Table 3 presents the list of the most abundant and the most frequent species. Most of the species have been reported in other upwelling regions with the exception of the three Coccolithophorides: Coccolithus pelagicus, Coccolithus huxleyi, and Calyptosphaera spp., which were also listed as common species for this region by MARGALEF (1973).
Figure 11 shows the time series distribution, at the mid-shelf station U, of the most abundant species, Rhizosolenia delicatula and the two most frequent species, Thalassiosira rotula and Ceratium furca. Rh. delicatula and Th. rotula were more abundant during the
812 DOLORS BLASCO, MARTA ESTRADA and BURTON JONES
second part of the study, while the dinoflagellate C. furca was more abundant during the first period. It is also interesting to notice that C. furca was generally restricted to the upper 30 to 40 m, with its maximum abundance between 0 and 10 m, but the two diatoms had a deeper distribution, with maxima often at 20 m or even deeper. While R. delicatula was frequently absent from the samples below 50 m, Th. rotula was always present at all depths, although in low numbers. Other species not among the most abundant or most frequent species, but which have been described as characteristic of this region are: Brachydinium capitatum, Paralia sulcata, and Thalassiosira partheneia. During our study, B. capitatum was always observed in the stations 50 to 80 km offshore, although in low numbers (20 cells 1- ~). P. sulcata was almost always present in the inshore stations D and R, but its distribution never extended past the mid-shelf. Based on the observed distribution Th. partheneia was considered to be a mid-shelf to shelf-break species.
Principal component analysis of the species and relationship with the hydrography
During the study 740 samples were analysed and more than 200 species were identified. To compress the information of the original lists we have used two different approaches. One, the more classical, has been presented in the above section, and the second is the application of principal component analysis.
Of the 740 samples, 627 were collected at hydrographic stations and the rest at productivity stations. Since the sampling depths and the time scales were different at each set of stations only the hydrographic set has been used for the principal component analysis.
STATION 57 58 59 60 61
90 80 70 60 ,50 40 50 20 t0
155 156 157 158 159 160
90 83 70 60 50 40 50 20 I0 Km OFFSHORE
50
0 3
150 t~ -o -d
K)OZr o
ro -q 0 m
5 0
100
150
200 0
Fig . 12. Cross-shelf distributions of the first principal component of the phytoplankton data during relaxations of the upwelling intensity. Leg 1 above, Leg 3 below.
P h y t o p l a n k t o n dis t r ibut ion and compos i t ion 813
Table 4. List of species included in principal component analysis. Species ordained according to their correlation coefficient with P C II
Because a large number of zero-abundance values in the data may distort the analysis only 104 species have been considered. The selection includes all the species present in more than 50 samples. The fraction of the total variance accounted for by the first three principal components was 14.9, 7.5, and 5.1%. The low variance explained by the first three components can be expected given the high number of variables (104) and samples (627) included in the analysis, and the complicated interrelationships among densities of phytoplankton species (MARGALEF and GONZALEZ BERNALDEZ, 1969). However, even if the
ST~ 0
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16 3:3 41 5560 I01104 125 132 139 158 • : " " " "
\ " o i -"'-- o ~
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I I z I 1 1 I I I I I I I I I I I I I I I I I I I I I 1 I I 1 I J I
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0 PRINCIPAL COMPONENT 11" (PHYTOPLANKTON)
POSITIVE SPECIES - - NEGATIVE SPECIES
102 126129133 140151 157
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-0. ~ I0 v • •
I I l I l I I I l I I I I l I I I I [ l l I I I I I I I I I I l I I
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I i I0 14 18 22 26 :30 I
MARCH
65 7073 85 929697
I I i 1 I 1 i
102
I i I I I I I
5 9 13 17 21 25
APRI L
126 129133 140151 157
~ I I 1 I i 2 9 I 5 9 13
MAY
Fig. 13. Distribution of the second principal component of the phytoplankton analysis at locations U and O and distribution of the fourth principal component of the hydrographic analysis
at location O during the entire cruise.
Phytoplankton distribution and composition 815
explained fraction of the variance is low, the projection of the variables within the reduced space determined by the corresponding components may be perfectly valid (LEGENDRE and LEGENDRE, 1979).
The first principal component was strongly related to the total cell abundance and most of the species had a positive correlation with it. The correlation coefficient between this component and the logarithm of the total cell counts was 0.84 for the first leg and 0.88 for the third leg of the cruise. The species that had the highest positive correlation with this component were mostly dinoflagellates and flagellates, together with some species of diatoms. The species with low or negative correlation were mostly diatoms and coccolithophorids. The microzooplankton organisms included in the analysis (Tiarina fusus, ciliate oligotriches, and copepodites) also had high positive correlations.
We have said before that total cell concentration seems to be negatively correlated with upwelling strength, but the comparison of the cross-shelf distribution of PC I, cell abundance component (Fig. 12), with the distribution of the upwelling enrichment component (PC I hydrography, Figs 3 and 4) shows that the conjecture is not completely valid. Up to 1 an increase in the scores of the upwelling enrichment component corresponds also to an increase in the cell abundance component, however, when the enrichment upwelling component has scores above 1, the biomass component decrease again.
STATION 58 59 40 41 42 45 ^
80 70 60 50 40 :30 20 I0
)
155
t I J
I 7o
156 157 158 ~59 16o • ,~- , , • 0
•
•
• I 0 0
I I 150 60 50 40 30 20 IO
Km OFFSHORE
Fig. 14. Cross-shelf distributions of the second principal component of the phytoplankton data at three sections of the 21°40'N line.
816 DOLORS BLASCO, MARTA ESTRADA and BVRTON JONES
Table 5. List of species included in principal component analysis. Species ordained according to their correlation coefficient with P C I I I
Neither the ordination of the species with respect to the second principal component, nor the ecological characteristics of the species gives insight into the significance of the component (Table 4), but the time distribution of the component at the mid-shelf, position U, and at the shelf break, position O (Fig. 13), shows a distinct trend. Species with negative loadings were always present at the mid-shelf station U but not at the shelf-break station O. On the other hand, the positive species were only present in the area during Leg 1 and during a short period of Leg 3. The distribution indicates that the total number of species in the region was greater during the first leg and suggests that PC II separates organisms that persisted for different periods in the system.
The existing parallelism between the distribution of this component and the PC IV of the hydrography (Fig. 12, and Figs 14 and 6) suggests that the species with positive loadings are associated with the presence of the high-salinity water mass over the shelf, while the negative species seem to dominate when the surface water has lower salinity.
The third principal component (Table 5) separates benthic and large centric diatoms from more pelagic diatom species such as those belonging to the genera Chaetoceros, Rhizosolenia, and Nitzchia and some others with flotation devices, e.g. Planktoniella sol, Dactyliosolen mediterranea. This ordination of the diatom species suggests that the third principal component expresses the influence of the coastal and of the bottom environments.
The cross-shelf distributions of the third principal component of the phytoplankton (Figs 15 and 16) support the suggestion that this component separates neritic and benthic from pelagic species. Species with the positive loadings on this component are always present inshore with their maxima usually at depth. On the contrary, the species with the negative loadings are found usually offshore at the surface and are absent from the inner part of the shelf.
Another observation is that the distribution of the neritic benthic and pelagic groups are strongly related to the cross-shelf flow. This point has been approached in more detail in BLASCO, ESTRADA and JONES (submitted). If there is near-surface offshore flow and deep onshore flow (Fig. 15), the neritic-benthic species extend further offshore and the pelagic group is restricted to the most offshore stations. If the surface flow is mainly onshore (Fig. 16), then the pelagic species appear closer to the coast. The result is that at the shelf break and mid-shelf stations (O and U) the predominant group will switch between the neritic-benthic group or the pelagic group depending mostly on the current regime. It is important that the pelagic species are associated with a stratified water column, and the neritic ones predominate where strong mixing occurs.
Principal component III of the phytoplankton, sigma-t, and onshore-offshore current velocities (cm s-1) sections occupied 15 March 1974.
DISCUSSION
The principal component analysis of temperature, salinity and nutrients data has shown, as suggested by CODISPOTI and FRIEDRICH (1978), that the character of the upwelling changed during the time of the study. From Leg 1 to Leg 3 the following differences occurred:
(1) The proportion of SACW in the upwelling source water decreased. (2) The intrusion of a high-salinity water mass over the shelf also decreased or
disappeared.
Phytoplankton distribution and composit ion 819
STA 38 39 4 0 41 42 43 _
80 70 60 50 4 0 30 20 I0
STA 38 59 40 41 42
80 70 60 50 40 30 20 I0
L 0 UW R I I 1 I
_vO
43
5 0 ~ rr" Od
IOO ~- lad
150 :x I-- rl
2oo c~
Fig. 16.
__O 80 70 60 50 40 30 20 I0
Km OFFSHORE
Principal component III of the phytoplankton, sigma-t, and onshore-offshore current velocities (cm s-a) sections occupied 18 March 1974.
(3) The physical variability diminished, the upwelling events and relaxations were less intense.
We have shown that, in general, relationship exists between the hydrography and the phytoplankton distribution. To see if the general trend observed in the hydrography results also in a change of the phytoplankton, we have projected the scores of the surface samples from all the stations on the plane of the second and third phytoplankton principal components, influence of high salinity water mass and the proximity of the coast, respectively (Fig. 17). From the distribution of the stations in this figure, the following conclusions can be drawn:
(1) The shift in dominant species observed in the depth-time plots of PC II at the positions U and G (Fig. 13) occurred over the entire area.
820 DOLORS BLASCO, MARTA ESTRADA and BURTON JONES
-3 -2 -I 0 I 2
PRINCIPAL COMPONENT TIT
Fig. 17. Relative position of the surface samples of stations taken along the 21°40'N line during the first and third leg of the cruise, on the plane defined by the second and third principal components of
the phytoplankton data. The letters indicate the location of the stations on the shelf.
(2) The variability across the shelf was less pronounced during Leg 3. (3) The coastal effect during Leg 1 was important only at the inshore stations D and R,
while during Leg 3 it expanded and reached as far as the shelf-break stations O and G. As could be expected the differences can be related to the changes observed in the
hydrography, however, there was another interesting difference in the phytoplankton composition between the two legs that is not so easily explained. As was shown by the time distribution of the phytoplankton principal component II, the number of species decreased from Leg 1 to Leg 3. The hydrographic data (Fig. 8) suggest that during Leg 1 the structure of the upwelling system was more heterogeneous and this could result in the existence in the area of more ecological environments or niches. Consequently, one could expect more different phytoplankton species to be in the region during this period.
A second mechanism that would explain the differences between the two legs is that because the physical environment appeared to be more constant during Leg 3, a selective process between the original pool of species may have occurred, with the negative species on PC II, which persist during the entire period, being the winners of the selection. The latter group is mostly composed of diatoms that have often been reported as typical of upwelling regions and they were always present at the U location, which showed less hydrographic variability even during Leg 1 (Fig. 8).
It is important to recognize that the two mechanisms, heterogeneity of the environment and selective pressure, proposed to explain the observed differences are not necessarily
Phytoplankton distribution and composition 821
exclusive but rather complementary. Consequently, one can assume that the phytoplankton composition during the study partially was affected by both processes.
Acknowledgements--We thank N. BREITNER, J. GARSIDE, J. BOTTA and J. ROLLINS for their technical assistance, and L. A. COt)ISPOTI and R. L. SMITH for permitting us to use their data. We would like to express our appreciation to R. C. DUGDALE, J. J. MACISAAC, L. A. CODISPOTI, and T. T. PACKARD for their encouragement through this work. Financial support was provided by the Office of International Decade of Ocean Exploration under National Science Foundation Grant No. 75-23718 AOI (CUEA-12) and National Science Foundation Grant No. BED 76-00861.
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