Marine Micropaleontology, 7 (1982/83) 517--539 517 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
DISTRIBUTION OF SILICOFLAGELLATES IN PLANKTON AND CORE TOP SAMPLES FROM THE GULF OF CALIFORNIA
DAVID MURRAY and HANS SCHRADER
School of Oceanography, Oregon State University, Corvallis, OR 97331 (U.S.A.)
(Received August 20, 1982, accepted October 22, 1982)
Abstract
Murray, D. and Schrader, H., 1983. Distribution of silicoflagellates in plankton and core top samples from the Gulf of California. Mar. Micropaleontol., 7 : 517--539.
Plankton and surface sediment samples from the Gulf of California were examined to determine the present geographic distribution of silicoflagellate species in this area. Variations in the species composition of the silico- flagellate assemblage were found to be related to water mass distributions. Eight species were identified in these samples. Octactis pulchra is associated with high levels of primary productivity in the surface waters and is found in greatest abundance in the central Gulf of California. Dictyocha messanensis dominates the silicoflagel- late assemblage in stations outside the Gulf of California and increases in relative abundance with decreasing amounts of Octactis pulchra. Dictyocha calida, Dictyocha sp. A, and Dictyocha sp. B are associated with equato- rial waters and have the highest relative abundance near the mouth of the Gulf. Dictyocha epiodon and Di- stephanus speculum are associated with cold California Current Water, and Dictyocha epiodon is present in minor abundance in Gulf samples. Dictyocha sp. 2 has a patchy distribution with low relative abundance.
Introduction
Few studies have attempted to map the present biogeographic distribution of silico- flagellates in the open ocean. Gemeinhardt (1930, 1934) examined silicoflagellate distri- butions in the South Atlantic and noted a correlation to surface water temperature. Poelchau (1976) determined the distribution of silicoflagellate species in North Pacific sur- face sediments and showed a relationship between water mass and species distributions.
Based on the distribution of diatom species in a suite of plankton samples, Round (1967) established three biogeographic zones in the Gulf of California south of 29°N. His results and previous work by Gilbert and Allen (1943) indicate that the phytoplankton assemblage compositions in these zones are related to water mass distributions defined by Roden
and Groves (1959). Two of these water masses originate from the surrounding Pacific Ocean which has associated silicoflagellate species as defined by Poelchau (1976}.
With the information provided by Poel- chau in mind, this study uses surface plankton and sediment samples from the Gulf of Cali- fornia and along the Pacific side of the Baja California Peninsula to determine the geo- graphic distribution of silicoflagellate species and their relationship to water masses. This information will aid in the interpretation of ecologic changes in down-core studies from this region.
Methods and taxonomy
Underway near-surface plankton samples were obtained during the BAM 80 (October 30, 1980--December 2, 1980) cruise (Figs. 1,
Fig. 1. S t a t i o n l o c a t i o n s o f samples used in th i s s t u d y .
Dots r ep resen t p l a n k t o n samples. Stars r ep resen t core locat ions . Sample loca t ions in areas A, B and C are p resen ted in Fig. 2.
2). Areas A, B and C are regions where a detailed sampling survey was made. Non- quantitative samples were collected by pumping seawater through a phytoplankton net with 33 pm mesh size at a rate of 181/min. The pump inlet was located in the ship's hull 3 m below the sea surface. The dura-
tion of the sampling period varied depending upon how quickly the net became clogged with material. Generally, the sampling pe- riod was 20 min for stations outside the Gulf of California and 10--15 min for sta- tions within the Gulf where plankton in the near-surface waters was more abundant. At the end of each sampling period, the mate- rial collected was placed in plastic or glass bottles, and approximately 1 ml of 37% formaldehyde solution was added.
The samples were prepared for light micro- scope analysis using methods similar to those outlined by Fryxell (1975). Instead of using a centrifuge, the samples were left to stand for 24 h before decanting the supernatant. It was observed that very little of the plankton material was lost using this method. Micro- scopic slides were made with 300--400 pl (generally most of the material) from each chemically cleaned sample using Hyrax (R.I. = 1.71) as a mounting medium.
Surface sediment samples in the Gulf of California were obtained from the top 5 cm of 6 Kasten cores (Figs. 1, 2). These samples were prepared for microscopic analysis using the method outlined by Schuette and Schrader (1981) with Hyrax as a mounting medium.
A Leitz Orthoplan microscope (objective: dry 40×/0 .65; ocular: Periplan GW 10 X M) was used to identify and count silicoflagel- lates. Only samples where more than 50 silico- flagellates could be counted were used, and generally 150--200 individuals were counted from each sample. Traverses scanned most of the coverslip area and only individuals where greater than one half of t he skeleton was present were counted.
The data set was subjected to multivariate analysis. A correlation matrix was obtained to show which species commonly occurred together in the samples. Q-mode factor anal- ysis was performed on the data which was transformed to give all species equal weight (Klovan and Imbrie, 1971; Heath and Dy- mond, 1977).
The t axonomy follows Poelchau (1976) for Octactis pulchra Schiller (Plate I, figs. 1--6), Dictyocha calida Poelchau (Plate II,
Fig. 2. Station locations of samples in areas A, B and C in Fig. 1.
figs. 14--15), Dictyocha epiodon Ehrenberg (Plate I, figs. 7--12), Dictyocha messanensis Haeckel (not divided into the two forms distinguished by Poelchau) (Plate I, figs. 13--19), and Distephanus speculum (Ehren- berg) Haeckel (Plate I, fig. 20). Three other species were identified that were not found in North Pacific sediments by Poelchau (1976). Because of the difficulties in delin- eating silicoflagellate species, apparent from the literature, these three species are placed in an open nomenclature. Dictyocha sp. A (Plate II, figs. 1--5) is similar in morphology to Dictyocha perlaevis perlaevis (elongate) identified by Bukry (1980). The length of the major axis is approximately 1.5 times the length of the minor axis. This species char- acteristically lacks supporting spines. Very
short radial spines on the basal ring are a common feature. Dictyocha sp. B (Plate II, figs. 6--10) is similar to Dictyocha perlaevis delicata described by Bukry (1976) although the apical bridge and lateral rods were not al- ways distinctly thinner than the basal ring. The length of the major and minor axes are generally equal and this species has supporting spines which distinguishes it from Dictyocha sp. A. Dictyocha sp. 2 (Plate II, figs. 11--13) is similar in skeletal morphology to Dictyocha calida Poelchau but the minor axis is approx- imately one third of the major axis in Dic- tyocha sp. 2. The radial spine lengths on the basal ring of Dictyocha sp. 2 are generally smaller than those in Dictyocha calida. This species lacks an apical spine.
522
H y d r o g r a p h y
The t h e r m o h a l i n e s t r u c t u r e of the s o u t h e r n G u l f ( s o u t h o f 2 9 ° N ) is s imi la r to t h a t o f the e q u a t o r i a l Pacif ic w i th m o d i f i c a t i o n s d u e to
ex t ens ive e v a p o r a t i o n a t t he su r face a n d by m i x i n g wi th C a l i f o r n i a C u r r e n t Wate r ( R o d e n , 1 9 6 4 ) . Th ree su r face w a t e r t y p e s have b e e n obse rved in the u p p e r 200 m o f t he s o u t h e r n Gul f . Cold C a l i f o r n i a C u r r e n t Wate r w i th l ow
sa l i n i t y (T ~ 22°C , S < 34.60/0o) t u r n s eas t a r o u n d t he t ip of the p e n i n s u l a a n d p e n e t r a t e s i n t o the Gul f . The e x t e n t o f this p e n e t r a t i o n
d e p e n d s u p o n t he the season a n d y e a r of ob- s e r va t i on ( S t e v e n s o n , 1970) . W a r m E a s t e r n T rop i ca l Pacif ic Wate r wi th i n t e r m e d i a t e sa l in i t ies (T > 2 5 ° C , 34 .6 < S ( 34.90/0o ) f lows in f r o m the s o u t h e a s t . W y r t k i ( 1 9 6 7 ) i d e n - t i f ies t he b o u n d a r y o f t r op i ca l su r face wate r s
as the 2 5 ° C i s o t h e r m . D u r i n g t he s u m m e r m o n t h s , its n o r t h e r n p o s i t i o n reaches a b o v e
the s o u t h e r n t ip of Baja C a l i f o r n i a a p p a r a n t l y l i m i t i n g the i n f l u e n c e o f the C a l i f o r n i a Cur- r e n t ( R o b i n s o n , 1 9 7 3 ) . T h e t h i r d t y p e is
w a r m , h igh ly sa l ine G u l f Water , {22 ° ( T 25°C , S > 3 4 . 9 % 0 ) f o r m e d w i t h i n the G u l f
TABLE I
Relative
No.
percent of silicoflagellate species in plankton and core top samples
O.PUL D.MES D.CAL D.EPI DIST. D.SA D.SB D.S2 TOT TEMP
No. = s ta t ion num ber : All p l ank ton samples have pref ix BAM80 P-, A-5 and B-29 have prefix BAV79 and E-17, F-30, G-34, and 1-42 have the pref ix BAM80. O. PUL = Octac t i spu l chra ; D. MES = D i c t y o c h a mes- sanensis; D. CAL = D. calida; D. EPI = D. ep iodon; D. SA = D. sp. A; D. SB = D. sp. B; D. $2 = D. sp. 2; DIST = Dis tephanus specu lum; TOT = tota l silicoflagellates counted per sample; TEMP = measured tem- perature (° C) 3 m below sea-surface at t ime of sampling.
525
by evaporation of Equatorial Water (Roden and Groves, 1959).
Sea surface temperatures were obtained at most stations (Table I) and show an equa- torward shift during the s tudy period. This temperature distribution closely follows the mean sea surface temperature pattern estab- lished by Robinson (1973) for the same time period (November).
Plankton blooms have been observed in many locations in the Gulf of California throughout the year, mainly near steep slopes and in the lee of capes and islands (Byrne, 1957i Van Andel, 1964). These blooms are caused by the upwelling of nutri- ent-rich waters in association with the season- al wind patterns. Revelle (1950) indicates that upwelling is less intense on the west side of the Gulf than on the east side where it is at a maximum in the winter and early spring. Zeitzschel (1969) summarized primary pro- ductivity data from six cruises to the Gulf of California and reported an average inte- grated rate of 0.382 gC m -2 d -1 (range: 0.002--0.952) for the whole Gulf. Both Zeitzschel's data and work by Round (1967) indicate that the central Gulf had the highest phytoplankton levels.
Results and discUssion
Phy toplankton
Silicoflagellates generally make up less than 2% of the siliceous phytoplankton as- semblage, which is dominated by diatoms. In a few samples where "b looms" of Dictyocha messanensis or Octactispulchra were observed, silicoflagellates comprised up to 5% of the sil- iceous phytoplankton assemblage. The sam- ples are considered to be time equivalent with respect to the general areal distribution of the species.
Octactis pulchra occurs in the eastern tropical Pacific in areas associated with high primary productivi ty (Poelchau, 1976). This species is most abundant in the central Gulf north of 26°N (Figs. 3a, 4a). It is also pres- ent in high abundance at the mouth of the
Gulf and be tween 29°N and 30°N in the Pacific coastal stations. Highest abundance of Octactis pulchra occurs in surface waters of the Guaymas and Carmen basins, which coincides with the area showing highest levels of primary productivity in earlier studies (Round, 1968; Zeitzschel, 1969). Samples in these regions are also indicative of high levels of primary production based on the diatoms present (abundant Chaetoceros spp. vegetative cells, Thalassionema spp., Thalas- siothrix spp., a.o.). Previous studies have documented that upwelling of nutrient-rich waters occurs on the east side of the Gulf of California in association with strong north- west winds. The wide distribution of Octactis pulchra across the Gulf implies that there were high nutrient levels on both sides of the central Gulf. It is possible that mixing, more than upwelling, is responsible for bringing nutrients to the surface to support high levels of primary product ion (Donegan and Schra- der, 1982).
Dictyocha messanensis is a cosmopolitan species in North Pacific surface sediments (Poelchau, 1976) and dominates the silico- flagellate assemblage in the Pacific stations west of the Baja California Peninsula (Figs. 3b, 4b). Some "b looms" of this species were observed at stations along the northern part of the Baja California Peninsula. This species also dominates in samples where silicoflagel- lates are rare, such as stations at the mouth of the Gulf. It was generally observed that a smaller form of Dictyocha messanensis (Plate I, figs. 13--18) dominated in samples from the Pacific Ocean, whereas a larger form of Dic- tyocha messanensis (Plate I, fig. 19) was more prevalent in Gulf stations. Within the Gulf this species increases in relative abundance when Octactis pulchra decreases in abun- dance. This observation is supported by a correlation of --0.729 between Octactis pulchra and Dictyocha messanensis.
Dictyocha calida, associated with the water of the Equatorial Countercurrent (Poelchau, 1976), occurs in relatively high abundance at the mouth of the Gulf and is present in low numbers in waters extending up to 26°N
526
Octactis
: ..
2 8 °
2 6 °
2 4 °
2 2 ° N
(a)
Fig. 3a--h. Relative
pulchra /
/ I i 4 ° W
I12 °
I I 0 °
I 0 8 °
3 0 ° N
2 8 °
2 6 °
2 4 "
Dictyocha messanensis /
22 °
106°W
(b)
percent distribution of each silicoflagellate species. Data given in Table I.
I 1 4 ° W
112 °
I I 0 °
108 °
I06°W
within the Gulf (Figs. 3d, 4d). It is also found in a few isolated areas further to the north. This species is present on the Pacific side of Baja as far north as 24°N, corresponding to the 24°C isotherm (Table I).
In North Pacific surface sediments, Dic-
tyocha epiodon mainly occurs in the Alaskan Gyre and in the western North Pacific. It is found along the west coast of North America in association with the California Current (Poelchau, 1976). The distribution of Dic- tyocha epiodon (Figs. 3f, 4f) moves further
Dictyocha \ / '~A~' l l ,
28 °
26 °
24 '
22 ° I~
(c)
sp. A /
l l14°W
12 °
I0 o
lOB °
106°W
30 ° N
28 °
26 °
24"
22 ° N
(d)
Dictyocha calida
30 ° N L
527
114°W
112 °
II0 o
IOB °
106°W
to the south along the Baja Peninsula during the sampling period in association with the 25--26°C isotherms (Table I, and Murray, 1982). Its distribution extends across the mouth of the Gulf by the end of November. Wyrtki (1967) uses the 25°C isotherm to
mark the northern boundary of tropical sur- face waters. Thus, variations in the southward extent of Dictyocha epiodon in the surface waters of the eastern Pacific could be an indi- cation of the movement of the front between the California Current and Subtropical Sur-
5 2 8
Dictyocha sp. B
I /
5 0 ° N
~ o
2 6 °
2 4 °
114°W
112 °
110 °
108 °
Dictyocha epiodon
"t /
5 0 ° N "- f
< ..
2 8 °
2 6 °
2 4 °
1 t 4 ° w
t i l 2
I I0 °
1 0 8 °
2 2 ° N
(e) F i g . 3 c o n t i n u e d )
I 0 6 ° W
22_ ° N
(f)
I 0 6 ° W
face Water. The presence of both Dictyocha epiodon and Dictyocha calida near the mouth of the Gulf indicates mixing of Equatorial waters and California Current Water in this region.
Distephanus speculum, a cold water spe-
cies, is found in minor abundance at the Pa- cific stations along the north coast of Baja California north of 28 .5°N and also near the coast between 23.5 ° and 24.5°N (Fig. 3g). This southern region had measured temper- atures between 25 ° and 26°C which is gener-
3 0 ° N
Distephanus speculum
28 °
26 °
24 ~
/
2 2 ° ~ x
absent * > present ,~
(g)
14°W
\
/
\
/ 12 °
\
/
x ,
/ I10 °
%
/
\
108 ° /
II06°W
50 ° N
28 °
26 °
24 °
22 ° N
(h)
Dictyocha
529
/ • . \
q
/
\
[] absent
/ [ ] present "X
sp. 2 /
• •~
• o • : .
• • , ' .
• i
/
114°W
12"
\
/
N
/ I10 ° \
/
\
108"
" ~ / 1 1 0 6 ° w
ally considered as an upper growth limit for Distephanus speculum (Poelchau, 1974).
Dictyocha sp. A dominates the silicoflagel- late assemblage near the mouth of the Gulf on the mainland side and is present in only minor amounts in other areas (Figs. 3c, 4c). Dic-
tyocha sp. B also has its greatest abundance near the mouth of the Gulf on the mainland side and decreases in abundance to the north (Figs. 3e, 4e}. Neither of these species were reported in North Pacific surface sediments examined by Poelchau (1976} nor were they
[ ' - ] p r e s e n t - 2 5 % D>25-75% [ ] > 7 5 %
F i g . 4 a - - f . Relative percent d i s t r i b u t i o n o f silicoflagellate species i n p l a n k t o n s a m p l e s f r o m a r e a s A , B a n d C. Dictyocha sp . 2 a n d Distephanus speculum w e r e a b s e n t i n s a m p l e s f r o m t h e s e a r e a s . D a t a g i v e n i n T a b l e I .
found in plankton samples from Monterey Bay by O'Kane (1970). They were found in surface sediment samples from the Panama Basin and in the upper-slope mud lens off Peru {Murray, 1982). Based on these limited data in the eastern Pacific, these species seem to occur in tropical to subtropical regions along the continental margin. Their presence in the Gulf of California may be indicative of warm Equatorial waters moving up the coast of Central America in association with south- easterly winds or E1 Nifio type events.
Dictyocha sp. 2 shows no apparent pre-
ferred distribution (Fig. 3h). This species never reaches abundances greater than 2%.
Maps of the relative percentages of species in each sample indicate that each species has a distinct geographic distribution. This obser- vation was supported by the results of Q- mode factor analysis of the data. Six factors account for 94% of the variance in the data (Table II), and all but two of the factors are monospecific. Dictyocha messanensis and Dictyocha epiodon have a correlation of 0.757 and have high positive loadings in fac- tor 3. Both species are found in greatest
abundance outside the Gulf of California. The last factor combines Dictyocha sp. 2 and Distephanus speculum which are found in minor abundance at less than ten stations apiece. By mapping the factor loadings of each of the six factors, the same distribu- tion patterns seen in Figs. 3 and 4 emerge.
As was noted earlier, Round (1967) de- fined three water masses south of 29°N within the Gulf of California based on phyto- plankton distributions. A southern water mass south of 25°N has an assemblage strongly influence by Eastern Tropical Pacific
Water. Roden and Groves (1959) and Steven- son (1970) indicate the presence of California Current Water and Gulf Water also in this re- gion. Based on silicoflagellate data, all three water masses are present at the mouth of the Gulf during the sampling period. Roden (1958) noted that old Pacific Water, which is depleted in nutrients, accumulates in this southern region and thus gives rise to low plankton production. This may be the reason that relatively small amounts of plankton material was observed in BAM 80 samples from many of the stations in the southern
533
111040 '
28 ° N •
27 ° 40 '
o."
Dictyocha
iii °
o o
I
calida
l l2°W I
.:'.....
• . ' ' : .
111o20 ' ~_7 ° N
~6 ° 20'
(d)
107 ° 20'W 106 ° 20 '
" " o I 25 40
25 ° N
[ ] absent [ ] p r e s e n t - 1 0 %
[ ] > I0%
region. Round (1967) divides the central region,
between 25 ° and 29°N, into two zones or water masses. The southern zone has a plank- ton assemblage transitional between southern region and the north central region. A transi- tion in this region is also seen in the silico- flagellate assemblage in our study where it changes from species characteristic of Pacific water masses in the southern section to an assemblage dominated by Octactis pulchra in the productive region to the north. The north central region of Round (1967) is char-
acteristic of large phytoplankton blooms. This is noted as a region where strong upwelling of nutrient-rich waters occurs and Octactis pulchra dominates the silicoflagellate assem- blage in the BAM 80 samples.
Sediments
Sediments exhibiting millimeter to sub- millimeter thick alternating dark and light laminations have been obtained from the slopes of the basins in the Gulf of California (Van Andel, 1964; Calvert, 1966; Schrader
i ! i i i!iiiiii iii!iiiiiii!!!!i!i!ii iiiii!;i ! i iii i i!i!iiiiiiii i i !iiiiiiiiiiiii i i i iii!i 1 i:!:i:!:!:i:!?!:•:i:i:i:i:!:•:•:•:[:i:i:•i•:i:i:i:i:i:!:!:!!:!:•:!:!:i:•:•:•:•?•:i:•:•:i:!:ii:••
:i?i:i:i:i:i:i:i:i:i:i:i:i:i:!:!:!:i:i'Q!:i:!:i:i:i:i:i i:i i:i:i:i:i:i:!:i~i:! !:i:!: i i i i i i i i i i i i i i ! ! ! i l i i i i i i i i i i i i i i i i i i i ! ! i !
sp. B 112°W i11°20 '
27 ° N
2 6 " 20 '
(e)
F i g . 4 ( c o n t i n u e d )
107 ° 2 0 ' W 106 ° 20 '
iii]iiiii!i!iii]i~i]ii]]iiii!i!]!i!iii!iii!]!iii!iii!ii~i~!ii}i:i.:..., i
~ii!iiiiiiiiiiiiiili!i!iiiiiii!iii!iiiii!iiiiiiii!iliiii~ • .- iiii!i!iii!iiiii!i!iiiiii ! i •iii•i•i•iii!i•iiiii•ii)iiiii••;i•ii!;•iii!i•i!•ii!ii•i•ii•••ii;iii•ii•iiiiiii;iiiiiii•i•;•i•iiii!i••iii;•ii;•iiiii•;i;•iii!i•iii•iiiiiii
25 ° 4 0 '
2 5 ° N
[ ] absent [] present -10% [ ] > 10%
et al., 1980). These sediments were deposited within the region where the intense oxygen minimum zone intersects the sediment-- water interface. Because of the low oxygen values, an infauna is not present (Calvert, 1966) and the sediments accumulate rela- tively undisturbed. The sediments are gen- erally diatom-rich with the dark laminae con- taining more clay material than the light laminae (Calvert, 1966; Donegan and Schra- der, 1982). Sediment texture estimated in material obtained from the central Gulf (Donegan and Schrader, 1981) indicates
diatoms generally comprise 60--90% of the sediment and terrigenous material gener- ally comprises 5--25%. Results from Murray (1982) show that terrigenous clay content in- creases down the Gulf on the mainland side. This is consistent with increasing rainfall towards the mouth of the Gulf (Roden, 1958). Diatoms in sediments from the central Gulf are relatively well preserved (Calvert, 1966; Round, 1967; 1968; Matherne, 1982) with core top abundances on the order of 166--108 diatoms per gram of dry sediment. Similar abundances have been found beneath
the upwelling areas of the west coast of South America and South Africa (Schuette and Schrader, 1979; 1981}.
Variation in the composi t ion of the silico- flagellate assemblage in the 0--5 cm compos- ite samples from the six Kasten cores (Table I) is documented in Fig. 5. Each of these sam- ples represents an average of approximately 15 years of deposition. The relative percent- age of Octactis pulchra decreases down the Gulf from greater than 40% to less than 5%. All other forms increase in relative abundance from the Gulf with greatest abundances of
Dictyocha sp. A, Dictyocha sp. B, Dictyocha sp. 2, and Dictyocha epiodon in BAM 80 G-34.
Absolute abundances of silicoflagellates per gram of dry sediment were determined in the surface sediments from the six Kasten cores (Table III). Abundances range from 2.2-106 silicoflageUates per gram of dry sediment in the surface of BAM 80 E-17 to 0.4.106 silicoflagellates per gram of d ry sediment in BAV 79 A-5. Abundances in BAM 80 E-17 are higher than those reported from the North Pacific by Poelchau (1976),
Fig. 5. The relative percent of silicoflagellate species Table I.
BAM80
8Av79
l BAM80
BAM80
0 ~ ~
o
- - ' o
m - .
o
in the 0--5 cm sample from six Kasten cores. Data given in
except for the area beneath the Western Convergence off Japan. These values give accumulation rates of approximately 3--16-104 silicoflagellates cm -2 year -1 (Table III). The decrease in absolute abundance of diatoms and silicoflageUates and an increase in clay Content towards the mouth of the Gulf is associated with poor preservation. This may explain why Octactis pulchra and Dictyocha calida, which are more delicate forms, are in such low abundance in BAM 80 G-34.
The relative distribution of Octactispulchra, Dictyocha messanensis, Dictyocha sp. A, and
Dictyocha sp. B in the surface sediment sam- ples is generally consistent with the assem- blage obtained from phytoplankton samples. Variations noted in the presence of Dictyocha epiodon and Dictyocha calida are probably a reflection of the extent to which their asso- ciated water masses influence the Gulf of Cali- fornia over a sampled time interval. Chelton (1981) indicates that recent variation in the strength of f low of the California Current is correlated to E1 Nifio events. This f low pat- tern has an influence on the extent to which Equatorial waters and California Current Water enters into the Gulf and analysis of the
537
T A B L E III
Silicoflagellate accumulation rates from core top samples in the six Kasten cores
Kasten core Sedimentation Dry bu lk Silicoflagel- A c c u m u l a t i o n ra te 0--5 cm ra te ( cm yr -1 ) ,1 density lates (X l 0 s / g silicoflagellates samples ( g c m -2 ) d ry (x 10 4 cm -2 yr -1 ) ,3
sediment) .2
B A V 7 9 A-5 0.27 0 .27 *4 0 .43 3.1 B A V 7 9 B-29 0 .36 0.21 *4 1.5 11.3 BAM80 E-17 0 .39 0 .19 2.2 16.3 BAM80 F-30 0 .33 0 .26 1.0 8.6 B A M 8 0 G-34 0 .24 0.27 0 .55 3.6 B A M 8 0 1-42 0.31 0.21 2.0 13.0
,1 D e t e r m i n e d using l aminae c o u n t s in t he 0--5 cm interval (2 l aminae = 1 year) . ,2 Af t e r Sch rade r and G e r s o n d e (1978) . ,3 Af t e r DeVries a n d Sch rade r (1981) . ,4 Data no t avai lable. Dry bu lk dens i ty f r o m cores w i th s imilar l i thologies were used
( B A M 8 0 G-34 and B A M 8 0 1-42 for B A V 7 9 A-5 and B A V 7 9 B-29, respectively).
fine scale variations in the silicoflagellate as- semblage might reflect these events.
The relative abundance of Dictyocha mes- sanensis increases toward the mouth of the Gulf. This abundance is consistent with the phytoplankton results, although there is a noted enrichment of Dictyocha messanensis in the sediments. This enrichment may be a function of the time of year that sampling took place and indicate that Dictyocha messanensis is more common in surface waters at other times of the year. Poelchau (1974) noted greater abundance of Dictyocha mes- sanensis in the sediments compared to the surface waters in the region off the California coast. He discusses reasons for this enrich- ment and noted preferential fecal pellet transfer or seasonal blooms of Dictyocha messanensis as the possible mechanisms for the difference. Preferential dissolution of more delicate species such as Octactispulchra may also be a factor in creating an enrich- ment of Dictyocha messanensis in the sed- iments.
Downcore analysis in the six Kasten cores indicated that major variations in the relative composi t ion of the assemblage did not occur over the last 1,000 years. Small scale changes have been noted when the cores were examined in detail (Murray, 1982).
Conclusions
Areal distributions of silicoflagellate species in plankton and surface sediment samples from the Gulf of California are indicative of variations in water mass characteristics. Oc- tactis pulchra is associated with areas of high surface primary productivity. Dictyocha mes- sanensis increases in abundance with decreas- ing numbers of Octactis pulchra. Dictyocha calida, Dictyocha sp. A and Dictyocha sp. B, which are associated with tropical waters, are in highest relative abundances near the mouth of the Gulf and decrease to the north. The distributions of Distephanus speculum and Dictyocha epiodon correspond to that of the California Current and decrease in abundance to the south along the Baja Peninsula. Dic- tyocha epiodon is present in minor abundance in Gulf of California surface sediments. The ecologic distributions of silicofiagellate spe- cies within the Gulf are consistent with zones established by Round (1967) for other plank- ton components .
Acknowledgements
This work was supported by the Climate Dynamics Division of the National Science Foundat ion through grants ATM 79 19458 02
538
a n d A T M 81 2 1 7 7 5 . Dr . G u s t a v o C a l d e r o n a n d t h e c r e w o f t h e M e x i c a n R e s e a r c h vesse l H-1 M a r i a n o M a t a m o r o s w i t h i t s c a p t a i n P o m p e y o L e o n H e r r e r a have m a d e t h i s s t u d y p o s s i b l e t h r o u g h o u t s t a n d i n g c o o p - e r a t i o n a n d h e l p d u r i n g t w o f i e ld seasons . G r a n t M i t m a n h e l p e d w i t h s o m e o f t h e s a m p l e c o u n t s . W i l l i a m G i l b e r t a n d A n n e M a t h e r n e a s s i s t ed w i t h t h e d r a f t i n g . We t h a n k Dr . N i c k Pis ias a n d A n n e M a t h e r n e fo r t h e i r h e l p f u l c o m m e n t s o n th i s p a p e r , a n d Dr . D a v i d B u k r y f o r his c o m m e n t s o n
t a x o n o m y .
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