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Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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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 silicoflagellatesin 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, 0377-8398/83/$03.00 © 1983 Elsevier Science Publishers B.V.
Transcript
Page 1: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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,

0377-8398/83/$03.00 © 1983 Elsevier Science Publishers B.V.

Page 2: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

518

PLATE I

l ...................... i 2 ...................... 3 4

.... i

7 6 5 .............

9 10 ............ i l 12

14 15 17 18

50pm , . : |

1--6 Octactis pulchra: 1--4, BAM 80-P 3; 5--6, BAM 80-P 163. 7--12 Dictyocha epiodon: 7, 10--12, BAM 80-P 172; 8--9, BAM 80-G 34 (0--5 cm). 13--19 Dictyocha messanensis: 13, 16, BAM 80-P 179; 14--15,18, BAM 80-P 172; 1 7, BAM 80-P 3; 19, BAM 80-G 34 (0--5 cm). 20 Distephanus speculum: BAM 80-P 3.

Page 3: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

PLATE II

519

| 2 3 4 5

6 7

4

E

10 14

11 12 13 1--5 Dictyocha sp. A: 1--3, 5, BAM 80-P 104; 4, BAM 80-G 34 (0--5 cm). 6--10Dictyocha sp. B: 6, BAM 80-P 104; 7, BAM 80-P 163; 8, 10, BAM 80-F 30 (0--5 cm); 9, BAM 80-P 3.11--13 Dictyocha sp. 2: 11, BAM 80-G 34 (0--5 cm); 12--13, BAM 80-F 30 (0--5 cm). 14--15 Dictyocha calida: BAM 80-P 163.

Page 4: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

520

30 ° N

28 °

26 °

ymas

\

14ow

12 °

/ /

24 ° \ \

I08 °

"~ M a z a f / a n ~ /

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,

Page 5: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

521

¸111040 ' I I I °

1 ' . . ' ~ L e36 " : . • "

59 *36 ~L" • " ' • • 37 "'.. " .L 48 . m . , 7 % :_1

r ~ , , ~ " ' . . * •

t +" 27 ° 40' 30. o40

29- 27.

I

A

112°W I

B

111o20 ' 27 ° N

::. : ~ ~ : y ~ . - - . ...~. B-29

° "+';12a. -]

....)'"

1 26020 '

C

107 ° 20'W

1 ~ i . . : ' : : ' ' . ". .

151. 148-'~.. " " 152. " ~ . -

1,56e 142- ~ " "~.'..

158- I °150 I I

106 ° 20'

23 ° 40'

23°N

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.

Page 6: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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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

2 60.432 30.216 0.000 0.719 8.633 0.000 0.000 0.000 278 18.5 3 31.765 51.176 0.000 4.706 8.824 0.000 3.529 0.000 170 18.0 4 3.947 90.132 0.000 3.947 1.316 0.000 0.658 0.000 152 19.0 5 29.870 68.182 0.000 0.649 0.000 0.000 0.000 1.299 154 20.0 6 20.755 75.472 0.000 0.000 0.000 0.000 3.774 0.000 106 21.0 7 4.898 91.429 0.000 3.265 0.000 0.000 0.408 0.000 245 21.0 8 0.000 96.040 0.000 3.960 0.000 0.000 0.000 0.000 202 21.0 9 2.577 88.144 0.000 9.278 0.000 0.000 0.000 0.000 194 22.0

10 0.000 98.333 0.000 1.667 0.000 0.000 0.000 0.000 60 23.0 11 0.000 93.427 0.000 6.573 0.000 0.000 0.000 0.000 213 23.5 12 0.952 92.857 0.000 6.190 0.000 0.000 0.000 0.000 210 24.0 13 0.000 94.915 0.000 5.085 0.000 0.000 0.000 0.000 118 25.0 21 26.368 50.249 2.488 0.000 0.000 17.413 3.483 0.000 201 27.5 23 43.575 39.106 10.615 0.000 0.000 3.911 2.793 0.000 179 27.0 24 84.791 12.928 0.000 0.000 0.000 0.380 1.901 0.000 263 26.3 25 61.111 35.185 0.000 0.000 0.000 3.704 0.000 0.000 54 26.0 27 83.966 15.612 0.000 0.000 0.000 0.000 0.422 0.000 237 24.6 28 75.684 16.717 0.912 0.000 0.000 2.432 1.824 2.432 329 26.5 29 94.884 5.116 0.000 0.000 0.000 0.000 0.000 0.000 215 24.7 30 87.591 11.436 0.000 0.000 0.000 0.000 0.973 0.000 411 25.0 31 89.951 9.804 0.245 0.000 0.000 0.000 0.000 0.000 408 25.1 35 78.908 20.844 0.000 0.000 0.000 0.000 0.248 0.000 403 24.6 36 81.378 17.602 0.255 0.000 0.000 0.255 0.510 0.000 392 24.0 37 84.277 14.465 0.000 0.000 0.000 0.000 1.253 0.000 159 25.0 38 71.498 28.019 0.000 0.000 0.000 0.000 0.483 0.000 414 25.0 39 76.190 23.308 0.000 0.000 0.000 0.251 0.251 0.000 399 25.5 40 78.358 21.393 0.000 0.000 0.000 0.000 0.249 0.000 402 24.6 47 80.645 18.362 0.000 0.000 0.000 0.000 0.993 0.000 403 27.0 48 80.798 18.204 0.000 0.000 0.000 0.249 0.748 0.000 401 25.7 49 79.208 20.297 0.000 0.000 0.000 0.000 0.495 0.000 404 25.5 50 84.615 15.385 0.000 0.000 0.000 0.000 0.000 0.000 364 25.0 51 79.750 20.000 0.250 0.000 0.000 0.000 0.000 0.000 400 25.0 52 79.250 20.750 0.000 0.000 0.000 0.000 0.000 0.000 400 26.0 53 76.980 21.782 0.000 0.000 0.000 0.495 0.743 0.000 405 26.7 54 77.510 20.080 0.000 0.000 0.000 1.205 1.205 0.000 249 26.0 55 72.727 23.445 0.000 0.000 0.000 0.478 3.349 0.000 209 25.0

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T A B L E I ( con t . )

NO. O. PUL D. MES D. C A L D. EPI DIST. D. SA D. SB D. S2 T O T TEMP

56 7 6 . 5 0 0 23 .500 0 .000 0 .000 0 .000 0 .000 0 .000 0 .000 200 24.5 57 7 5 . 8 3 6 24 .164 0 .000 0 .000 0 .000 0 .000 0 .000 0 .000 269 24.5 58 73 .684 26 .316 0 .000 0 .000 0 .000 0 .000 0 .000 0 .000 95 24.0 59 75 .401 18 .717 0 .000 0 .000 0 .000 1 .604 4 .278 0 .000 187 24.0 60 80 .315 18 .110 0 .000 0 .000 0 .000 0 .787 0 .787 0 .000 127 26.0 61 8 2 . 4 5 6 17 .105 0 .000 0 .000 0 .000 0 .000 0 .439 0 .000 223 26.5 62 7 6 . 6 2 3 22 .078 0 .433 0 .000 0 .000 0 .000 0 .866 0 .000 231 26.0 63 89 .241 9 .494 0 .000 0 .000 0 .000 1 .266 0 .000 0 .000 158 26.0 64 8 8 . 6 2 3 10 .180 0 .000 0 .000 0 .000 0 .000 1 .198 0 .000 167 26 .0 65 82 .075 15 .566 0 .000 0 .000 0 .000 0 .943 1 .415 0 .000 212 26.0 66 7 6 . 3 2 9 21 .739 0 .000 0 .000 0 .000 0 .000 1 .932 0 .000 207 25.5 67 78 .788 19 .192 0 .000 0 .000 0 .000 0 .000 2 .020 0 .000 198 26.0 68 8 1 . 3 8 3 15 .426 0 .000 0 .000 0 .000 0 .000 3 .191 0 .000 188 26.2 70 8 2 . 9 4 6 13 .953 0 .000 0 .000 0 .000 0 .000 3 .101 0 .000 129 26.0 71 7 5 . 8 7 9 18 .090 0 .000 0 .000 0 .000 0 .000 6 .030 0 .000 199 26.5 72 8 2 . 2 2 2 15 .556 1 .111 0 .000 0 .000 0 .000 1 .111 0 .000 90 26.5 73 55 .556 4 1 . 5 2 0 0 .000 0 .000 0 .000 0 .585 2 .339 0 .000 171 25.9 74 76 .617 20 .896 0 .000 0 .000 0 .000 0 .000 2 .488 0 .000 201 25.7 75 7 7 . 4 3 6 22 .051 0 .000 0 .000 0 .000 0 .000 0 .513 0 .000 195 26.0 76 27 .451 4 1 . 1 7 6 2 .941 0 .000 0 .000 11 .765 16 .667 0 .000 102 27.5 78 18 .841 6 0 . 8 7 0 5 .797 0 .000 0 .000 4 .348 10 .145 0 .000 69 27.5 79 28 .767 54 .795 5 .479 0 .000 0 .000 4 .110 6 .489 0 .000 73 27.5 80 13 .158 3 0 . 2 6 3 5 .263 0 .000 0 .000 34 .211 17 .105 0 .000 76 28.0 81 7 .882 30 .542 1 .478 0 .000 0 . 0 0 0 37 .438 22 .660 0 .000 203 28.0 85 7 .792 33 .766 1 .299 0 .000 0 .000 4 8 . 0 5 2 9 .091 0 .000 77 28.0 89 5 .021 39 .331 1 .255 0 .000 0 .000 5 0 . 2 0 9 4 .184 0 .000 239 28.0 92 4 .938 3 6 . 4 2 0 0 .617 0 .000 0 .000 5 2 . 4 6 9 5 .556 0 .000 162 28.5 96 37 .079 51 .685 1 .124 0 .000 0 .000 8 .989 1 .124 0 .000 89 28.0

101 5 .224 27 .612 0 .000 0 .000 0 .000 67 .164 0 .000 0 .000 134 28.0 102 10 .309 24 .742 0 .000 0 .000 0 .000 63 .981 1 .031 0 .000 97 28.0 103 12 .800 7 5 . 2 0 0 0 .800 0 .000 0 .000 6 .400 4 .800 0 .000 125 27.0 104 1 .015 4 1 . 6 2 4 0 .508 0 .000 0 .000 4 0 . 6 0 9 16 .244 0 .000 197 27.0 112 54 .237 31 .638 0 .000 0 .000 0 .000 0 .000 14 .124 0 .000 177 - - 113 4 1 . 4 2 0 37 .278 1 .775 0 .000 0 .000 4 .734 14 .793 0 .000 169 - - 114 72 .436 23 .718 0 .000 0 .000 0 .000 0 .641 3 .205 0 .000 156 - - 120 79 .052 15 .711 0 .000 0 .000 0 .000 0 .000 5 .237 0 .000 401 - - 121 66 .667 16 .790 0 .000 0 .000 0 .000 0 .000 16 .543 0 .000 405 25.0 122 8 2 . 8 4 3 11 .765 0 .980 0 .000 0 .000 0 .490 3 .922 0 .000 204 24.8 123 78 .475 19 .283 0 .000 0 .000 0 .000 1 .345 0 .897 0 .000 223 24.5 124 83 .203 15 .234 0 .000 0 .000 0 .000 1 .172 0 .391 0 .000 256 - - 125 76 .974 19 .737 0 .658 0 .000 0 .000 0 .658 1 .974 0 .000 152 - - 126 76 .648 15 .385 0 .275 0 .000 0 .000 0 .000 7 .692 0 .000 364 - - 127 80 .198 15 .347 0 .990 0 .000 0 .000 0 . 0 0 0 3 .465 0 .000 202 - - 128 87 .121 12 .879 0 .000 0 .000 0 .000 0 .000 0 .000 0 .000 132 25.0 130 5 4 . 0 0 0 3 0 . 0 0 0 0 .000 0 .000 0 .000 0 .000 16 .000 0 .000 50 24.5 134 4 7 . 2 3 6 27 .638 14 .070 0 .000 0 .000 4 .523 6 .533 0 .000 199 - - 136 23 .111 31 .556 21 .333 1 .333 0 .000 8 .000 14 .667 0 .000 225 - - 137 14 .729 34 .109 1 3 . 1 7 8 0 .000 0 .000 12 .403 25 .581 0 .000 129 - - 138 24 .725 29 .670 6 .044 0 .549 0 .000 31 .319 7 .692 0 .000 182 - - 139 7 .097 11 .613 1 .290 0 .000 0 .000 7 7 . 4 1 9 2 .581 0 .000 155 - - 140 36 .014 1 0 . 8 3 9 1 .049 0 .000 0 .000 5 1 . 3 9 9 0 .699 0 .000 286 - - 141 48 .675 13 .012 1 .205 0 .000 0 .000 34 .217 2 .892 0 .000 415 27.0 142 4 7 . 1 7 0 11 .321 0 .000 0 .000 0 .000 3 7 . 7 3 6 3 .774 0 .000 53 26.5 143 6 8 . 2 5 0 10 .250 1 .000 0 .000 0 .000 17 .750 2 .750 0 .000 400 26.5 144 38 .462 12 .903 0 .744 0 .000 0 .000 4 3 . 4 2 4 4 .467 0 .000 403 26.3

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TABLE I (cont . )

NO. O. PUL D. MES D. CAL D. EPI DIST. D. SA D. SB D. $2 TOT TEMP

145 26.549 22.124 2.655 0.000 0.000 39.381 9.292 0.000 226 -- 147 4.869 8.614 0.375 0.000 0.000 82.772 3.371 0.000 267 -- 148 3.774 6.918 0.629 0.000 0.000 86.164 2.516 0.000 159 27.0 149 3.268 1.307 0.000 0.000 0.000 95.098 0.327 0.000 306 26.0 151 65.920 22.388 1.741 0.000 0.000 6.716 3.234 0.000 402 26.0 152 30.513 14.872 0.769 0.000 0.000 44.615 9.231 0.000 390 25.0 153 27.273 25.799 4.177 0.246 0.000 39.066 3.440 0.000 407 - - 154 50.000 20.625 5.625 0.000 0.000 20.625 3.125 0.000 160 25.5 155 19.318 31.818 18.182 1.136 0.000 20.455 9.091 0.000 88 26.5 156 40.750 40.000 1.000 0.000 0.000 12.000 6.250 0.000 400 36.0 157 5.085 42.373 11.864 0.000 0.000 32.203 8.475 0.000 59 26.5 158 32.203 11.138 2.663 0.000 0.000 47.458 6.538 0.000 413 24.5 159 75.676 16.216 2.162 0.000 0.000 3.243 2.703 0.000 185 26.0 160 29.825 45.614 15.789 1.754 0.000 5.263 0.000 1.754 57 26.5 161 41.667 50.000 6.061 0.000 0.000 0.758 0.758 0.758 132 26.0 162 60.377 20.755 15.094 0.000 0.000 3.774 0.000 0.000 53 26.0 163 51.542 33.480 11.454 0.000 0.000 0.881 2.643 0.000 227 25.5 164 64.324 27.027 5.405 0.541 0.000 0.000 2.703 0.000 185 25.0 165 43.373 45.783 9.036 0.000 0.000 0.602 1.205 0.000 166 25.5 166 26.415 59.434 10.377 0.943 0.000 0.943 1.887 0.000 106 27.0 167 28.333 48.333 20.833 1.667 0.000 0.833 0.000 0.000 120 26.5 168 3.271 85.514 2.804 8.411 0.000 0.000 0.000 0.000 214 23.9 170 8.088 75.000 2.941 6.618 0.735 6.618 0.000 0.000 136 23.5 171 1.562 89.062 0.521 7.812 0.000 1.042 0.000 0.000 192 24.5 172 0.000 87.793 0.000 11.268 0.000 0.939 0.000 0.000 213 24.2 173 2.703 90.270 0.000 6.486 0.000 0.541 0.000 0.000 185 24.5 174 10.606 85.354 0.000 3.030 0.000 1.010 0.000 0.000 198 24.5 176 1.389 92.593 0.000 6.019 0.000 0.000 0.000 0.000 216 23.5 177 1.810 86.425 0.000 11.765 0.000 0.000 0.000 0.000 221 23.5 179 10.549 85.232 0.000 3.376 0.000 0.000 0.000 0.844 237 21.5 180 9.319 82.796 0.000 7.885 0.000 0.000 0.000 0.000 279 21.0 182 1.630 84.239 0.000 13.043 0.000 1.087 0.000 0.000 184 20.0 184 23.841 62.252 0.000 3.974 0.000 0.000 9.934 0.000 151 19.3 186 22.430 69.626 0.000 0.000 0.935 0.000 6.542 0.467 214 18.5 187 44.177 40.161 0.402 1.606 2.008 0.000 11.647 0.000 249 18.3 190 55.140 39.720 0.000 0.467 0.467 0.000 4.206 0.000 214 18.0 192 6.478 91.093 0.000 0.810 0.000 0.00O 1.619 0.000 247 18.7 193 10.152 86.294 0.000 1.523 0.000 0.000 2.030 0.000 197 18.0 194 38.811 57.343 0.000 1.399 0.350 0.000 2.098 0.000 286 17.5 198 15.859 81.933 0.000 0.441 0.441 0.441 0.881 0.000 227 18.8 199 33.333 62.911 0.000 3.756 0.000 0.000 0.000 0.000 213 17.5 200 27.830 69.811 0.000 1.887 0.472 0.000 0.000 0.000 212 17.5

A-5 32.035 38.961 12.121 3.896 0.000 9.091 2.165 1.732 231 B-29 43.043 47.826 2.174 0.435 0.000 3.043 2.609 0.870 230 E-17 49.561 35.526 8.333 0.439 0.000 2.632 2.632 0.877 228 F-30 30.769 34.615 12.821 0.641 0.000 7.692 5.769 7.692 156 G-34 0.429 42.489 2.146 11.159 0.000 25.751 9.442 9.013 233 1-42 28.743 45.509 11.377 1.769 0.000 4.192 7.186 1.198 167

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.

Page 9: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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

Page 10: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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

Page 11: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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-

Page 12: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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-

Page 13: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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

Page 14: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

5 3 0

Octactis t 1 1 o 4 © ' I I I o

:: i :/: ! i:/: ::::::::::::::::::::::::::::::::::::::::::::: .!:: !ili!i;i~iiiiiii~iiiiiiiii~ii'~iiiiiii!ilili~ili~i~i~i !::!i:: ' : . " :i:i:i*/:i:i:i:i:iiii:':'ii:::iiiii!!!~:i:i:!:!:::... "' .

o i:i i i i i i i ! ;~:i:i:i:i:~i:: :i::: :!::: :!:!:!:!:!:~¢~:. : . 2 8 N ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: • : -

Iilil ~i ::::::::::::::::::::::::::::::::::::::::::: i~::ii~i::i:: ::i i:::: ::::ii!~ !::!::i::i liiiii iiiiiiiiiiiiii

pulchra 112°W

iiiiii iiiiii iiiiii ii iiii ii iiiii ii iiii ii iiiii iiiiii::! ii::! iii iiiiiii{i i{i !i iiiiii ii i !{!!i iii!ii{iii{i i i 1 1 o 2 0 '

[ 7 ° N

! 6 ° 2 0 '

(a)

1 0 7 ° 2 0 ' W 1 0 6 ° 2 0 '

\ :i:i:i:!:!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' • " .

iii!. ::!:i:i:i~i:i:i:i:!:i:i:i:=:i:iii:~::. ." . r- ii:i:i:iiiiiiiii!::::.... "::::iii!ii!!iiiii!ii!iiiiiiili!~iiiiiii:: ".. :i:i:i::i:i:!:!:i:i:ii:i:!:!:i~ i:i:i:i:i:i:i:!!ii::::!:i:i:!i:i. , ". '

2 3 ° 4 0 '

2 3 ° N

[ ' - ] 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

Page 15: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

531

Dictyocho 111o40 '

28 ° N -" .~!..

27 ° 40'

Ill o

o o

1

messonensis 112°W

I

:." :../..

• •C: .e

II 1°20 ' 27 ° N

26 ° 20'

(b)

107 ° 20' W

:.:.!.:.:+ :,:.:,:,:,:,:,:,:,:,:,:,:,:,: ::.:.:. •

iiiiiiiiiiiiii!i!i!i;iiiiiiiiiii!i;:;~iil

106 ° 20' I

"~"' - ~ i "" I 25 ° 40'

.-%. • ' - .

• • " .

~ ~ . • : i -

I I

[ ] present- 25%

25 ° N

0>25-75%

TABLE II

Varimax factor score matrix output from Q-mode factor analysis performed on the phytoplankton data

V a r i a b l e 1 2 3 4 5 6

O. pulchra 0.965 0.009 --0.058 0.025 --0.056 --0.012 D. messanensis 0.243 0.059 0.475 0.058 0.058 0.168 D. calida ---0.046 0.029 --0.013 0.987 ---0.060 0.029 D. epiodon --0.067 --0.015 0.873 --0.004 --0.023 --0.064 Dist. speculum --0.036 --0.026 0.032 --0.101 0.097 0.771 D. sp. A -0 .028 0.988 --0.010 -0 .044 --0.133 0.013 D. sp. B 0.035 0.135 -0 .026 0.071 0.974 -0 .012 D. sp. 2 --0.005 -0 .004 ---0.081 0.068 --0.118 0.609

Variance 37.476 15.035 15.430 10.819 9.828 5.433 Cure. var. 37.476 52.511 67.941 78.760 88.589 94.022

Page 16: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

532

I l l O q 0 '

28 ° N

27 ° 40'

Dictyocha I I I °

° • I " . • , ~

:

AI,

I

sp. A ]I2°W 1t1°20 '

:.. J ] 27 °N

I • 9 . ~ --

1 " " :L:X 26 ° 20'

(c)

107 ° 20 'W

.... " i ii,, • . '!:i~:i:. ': .

106 ° 20'

2:5 ° 40 '

23°N

• absent [ ] >25-75%

[ ]present-25% [ ] >75% Fig. 4 (continued)

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

Page 17: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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

Page 18: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

5 3 4

111o40 '

2 8 ° N

2 7 ° 4 0 '

Dictyocha I I I °

i:i:i:i:::i:i:i:i:i?i:!:i:!:!:!~!?!:i?i:i::f: ".. " ii!i!!ii!iiiiii!!ii!i!ili!ii!i!!!ii!i!i!ililililiiiiiiii:'.'. • " :i:i:)!!ii:i:i:!:i:i:i~:i:!:i:i:i:i:i:iii:ii$! . •

iiiili~iiiii?:::iiiiiiiii~i!ili!iiiiiiii!iiii~iiiiiiiii~ii~:i... . . . . ~ : : . : : : ~ : . . --.=====================================. , . . .

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

~]i]~]i]i]~]~i~i~]]]~i~]~]~i~]~!i]i!]]!]~]i!iil]~ii]i]]]iii]]i!]]]i~i]i[i:.': - ~i~]~]~i~!~!~i~i~!~!~i~[~i~i~i~]~i~i~i~i~i~i~i~i~]~i~!~.~ ' . . " " !i~iii~ii~iii!~!iiiii!i!i!~iiii!ii~i!iii!i!iiiiiiiiii~iii~i!iii!i!i~i!i~i!i!i!i!~iii~ ". • : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : • ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: • . i~!~!~]~i~iii~i~i!ii~ii~iiii]i~iii~iii!i!i~i~ii~!i~!~i~iiiiiii~i!!~i!i~i!ii~!i!i~i~i~i~!!!~i~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

Page 19: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

5 3 5

2 8 ° N

2 7 ° 4 0 '

o 4 0 '

• • I e

• e •

Dictyocha I I I °

o •

I

epiodon 112°W

I

• ';.'• ea~O

111020 ' 2 7 ° N

2 6 ° 20 '

( f)

107 ° 2 0 ' W

• O ' % . • : : : : : : : : : ::::::::::::::::::::::::::::::. • " ..

:i:?:i:i:i:i:!:i:i:!:i~!iiii i i i i iiiii i!i:: : : • " iliiiii!iiiiiiiilili!i:,:.:.:.:.:.:ii:,:.:.:" '" • : . . ::::::::::::::::::::::::::::::::::: • • • ": . . - ~ i : : : : : : : : : : : : : : : : : : : : : : : : : . ~ ~ • ,.

I • I I

106 ° 2 0 '

2 5 ° 4 0 '

I 125 ° N

[ ] absent [ ] present

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),

Page 20: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

5 3 6

s o ~ BAV79 % ~ B - 2 9

0 ~ : : : 2 1 . ; , ~ . : . : . : . : . : . z . : . : . : . : . : . ~

5 O 1 ~ BAM80

:H - . ~ r~ ~> m DO

CI ~ --.

: ~ --" 0 e~

0

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

Page 21: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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

Page 22: Distribution of silicoflagellates in plankton and core top samples from the Gulf of California

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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