Plankton of the Red Sea and the Arabian Gulf

969

I'I,ANKTON OF TIIE RED SEA AND THE ARABIAN GULF

Youssef Department

of O c e a n o g r a p h y ,

llalim

Faculty

of Science,

Alexandria

ABSTRACT

T h o u g h p r e s e n t i n g some similarities, the Red Sea and the A r a b i a n Gulf differ much in their c o n f i g u r a t i o n , h y d r o g r a p h y and p l a n k t o n populations. Both the r e c r u i t m e n t of pelagic o r g a n isms into the Red Sea from the Gulf of Aden and their subsequent n o r t h w a r d d i f f u s i o n w i t h i n the Red Sea basin, and the f l u c t u a t i o n s in biomass and primary p r o d u c t i v i t y are g o v e r n e d by the c i r c u l a t i o n pattern, w h i c h is itself d e p e n d e n t on the periodical M o n s o o n w i n d system. The species d i v e r s i t y of Red Sea p l a n k t o n is r e d u c e d relative to the Indian Ocean, but m u c h h i g h e r than that of the A r a b i a n Gulf. An i n d i g e n o u s assemblage of d i n o f l a g e l l a t e s , tintinnids, c o p e p o d s and c h a e t o g n a t h species, however, a p p e a r s to be well adapted to the c o n d i t i o n s of this sea. T h e i r wide d i s t r i b u t i o n s do not d e p e n d on the seasonal inflow. As a rule b o t h the p r i m a r y p r o d u c t i v i t y and the z o o p l a n k t o n b i o m a s s are higher during the NE monsoon. The southern Red Sea is more p r o d u c t i v e than the northern with the two zones being s e p a r a t e d by a l o w - p r o d u c t i v i t y discontinuity zone at about 20" to 25"N, which is the zone of w i n d convergence in summer. More than 95% of the zooplankton biomass OCCURS in the upper 1 0 0 0 m and several endemic species have been reported.

The Arabian Gulf is a vast, relatively shallow lagoon connected t o t h e G u l f o f Oman t h r o u g h the narrow strait of Hormuz. Circulation is anti-clockwise. Along the Iranian coast salinity rises from 36.6~ near the strait to 40.6~ i n t h e NE G u l f a n d e v e n h i g h e r a l o n g t h e A r a b i a n coast. The Indian Ocean bathypelagic species as well as the dlnoflagellate "Schattenarten" are completely missing from the Gulf. The species diversity i s much p o o r e r than that of e i t h e r the Gulf of O m a n or the Red Sea. An indigenous community, however, has d e v e l o p e d w h i c h at first sight appears to be u n i f o r m l y d i s t r i b u t e d especially along the I r a n i a n coast, w h i c h is d o m i n a t e d by copepods and m y o d o c o p i d ostracods. Some h o r i z o n t a l z o n a t l o n is o b s e r v able. Both n u t r i e n t and b i o m a s s c o n c e n t r a t i o n s are higher in m i d - b a s i n than in c o a s t a l waters. The S h a t t - e l - A r a b to Kuwait a r e a where e s t u a r l n e components are not uncommon, and cladocc~-ar~s replace o s t r a c o d s in importance, is m o r e p r o d u c t i v e and more d i v e r s i f i e d than the T r u c i a l coast.

T H E RED SEA A l t h o u g h the Red Sea and the A r a b i a n G u l f are analogous in some ways, they d i f f e r m u c h in their c o n f i g u r a t i o n , h y d r o g r a p h y and p l a n k t o n populations. The Red Sea is in m a n y ways u n i q u e among m a r i n e basins, b e c a u s e of its partial i s o l a t i o n from the Indian Ocean, its arid tropical climate and from the p r e v a i l i n g w i n d system. The shallow sill at B a b - e l - M a n d a h prevents e x c h a n g e b e t w e e n the Red Sea and the d e e p w a t e r of the Gulf of Aden with its b a t h y p e l a g i c plankton. E v a p o r a t i o n is active and there is no river run off, so s a l i n i t y rises r a p i d l y from south to north. From 300m to the bottom, the Red Sea is almost isothermal a r o u n d 2 1 . 7 " C and i s o h a l i n e at 4 0 . 5 ~ . The deep w a t e r s of the Red Sea are far warmer and m o r e saline than in any other m a r i n e basin. A c o m b i n e d o x y g e n m i n i m u m and phosphate m a x i m u m e x t e n d s from 300-600m. N o w h e r e are the p l a n k t o n d i v e r s i t y and biomass fluctuations so c l o s e l y r e l a t e d to s u r f a c e circulation, which is itself g o v e r n e d by the wind system. D u r i n g the SW m o n s o o n in summer, winds blow from the north along the whole Red Sea basin, while in winter, d u r i n g the NE monsoon, winds over the n o r t h e r n and southern Red Sea blow in o p p o s i t e d i r e c t i o n s

970

Halim

Y.

(Fig.l). These winds and the associated surface graphic regions in the Red Sea (Morcos, 1970):

circulation

create

three

I.

North of about 20"N the prevailing wind is from the NNW all the year r o u n d ,

2.

South

3.

Between

of

this

latitude,

these

-

zones,

z6

-

winds

an

are

zone

intermediate

of

wind

I

----

I

I

~

I

29

29

develops

during

1

I--

"1

I

-'---

"

~ surface CU~mnt ¢lirectio~ ~ f o c e salinity (ports per tr~us ) Wind directioa Sur face isotherms \~. . -3o -"~ " . - - W ........

28

28

convergence

50 3O

20

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40

60

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Meoo surfoce current direclu~n _.J S u i ' f a . i o l i n i t y l p o r ~ per thOus) i wkid direct ion /

..... #. . . . . . . .

-"

z

j'*"

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~

,

Fig.l.

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Jonuory I

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l

i

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i"

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i/~ ............

i; 'l

I z~"l

-

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--

~:,,, ~__.~, ,...¢~ / v ~ , , , ~~,,. / " <.::;~...

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hydro

in October-April

60

I

27 2.,~7~_

but reverse to SSE

°E 5o

4o I ----- 'I

I

NNW during summer,

distinct

"~%~'"

">....

'

Mean wind and surface current d i r e c t i o n s with surface isotherms and isohaline8 in the Red Sea, G u l f o£ Aden and A r a b i a n Sea irl summer (July) and winter (January). (After Morcos, 1970).

winter

Plankton

of the

Red Sea and the A r a b i a n

Gulf

971

D u r i n g the SW m o n s o o n (May t o O c t o b e r ) s u r f a c e t r a n s p o r t is S S E down the Red Sea basin, but d u r i n g the NE m o n s o o n ( N o v e m b e r to A p r i l ) the c i r c u l a t i o n r e v e r s e s and the mean s u r f a c e current sets north, up the sea. It is d u r i n g the NE m o n s o o n that the c o n v e r g e n c e d e v e l o p s at 20"N, where the n o r t h w a r d c u r r e n t is o p p o s e d by the n o r t h w e s t wind. A d i s c o n t i n u i t y zone in the p l a n k t o n d i s t r i b u t i o n was o b s e r v e d by M a t z e n a u e m (1933) in this region. A few c o n t r i b u t i o n s to the p l a n k t o n of the Redd Sea have a p p e a r e d s i n c e the r e v i e w of Halim (1969). A c o o r d i n a t e d study of the p l a n k t o n and its e n v i r o n m e n t was c o n d u c t e d by M E S E D A I and II (Metalliferous Sediments Atlantis II Deep D e v e l o p m e n t P r o g r a m m e ) (Weikert 1980a,b, 1982). Nasr (1980) r e p o r t e d on a p r e l i m i n a r y study of the S u d a n e s e c o a s t a l plankton. F e d o r i n a and K o r n i l o v a (1970) have added to our k n o w l e d g e on z o o p l a n k t o n , p a r t i c u l a r l y on copepods. The Gulf of A q a b a has been i n v e s t i g a t e d for p r i m a r y p r o d u c t i v i t y ( L e v a n o n - S p a n i e r , Padan and Reiss, 1979), microplankton distribution Kimor and G o l a n d s k y , ]977) and d i n o f l a g e l l a t e s (Natour and Nienhuis, 1980). More i n f o r m a t i o n on the Red Sea p l a n k t o n is now a v a i l a b l e , but the g e n e r a l p a t t e r n as described earlier remains unaltered. P r o d u c t i v i t y i n c r e a s e s from north to south, both in the main basin and in the Gulf of Aqaba. It is a l s o h i g h e r in the r e e f - b o u n d c o a s t a l zone than o f f s h o r e ( L e v a n o n - S p a n i e r et al., 1979). As a rule, the v e r t i c a l p r o f i l e s for c h l o r o p h y l l a ( Y e n t s c h and Wood, 1961; W e i k e r t , 1980a) show a s u r f a c e m a x i m u m in the low s a l i n i t y s u r f a c e layer and a deeper m a x i m u m w h i c h c o i n c i d e s w i t h the upper level of the p y c n o c l i n e (Figs 2 and 3).

"3

• e( -~-\

20--

"-.. -\ "~ 4o

40-\. cl

60--

d".. "\

~

"'-.. "\

.... a

60

80--

iOO O

80

0.2

0.4

I

,oo

0.6

0

J'/

0.2

Chl.orophytt ( m g m -3)

Fig. 2.

/

I

04

I

I 06

Chtorophytt (rnQ m -3)

V e r t i c a l p r o f i l e s at five s t a t i o n s in the Red Sea s a m p l e d in May-Ju, te ( Y e n t s c h and Wood, 1960). The c o n c e n t r a t i o n s u n d e r a s q u a r e ' m e t r e of sea s u r f a c e in the top 100m were: a - 27.2; b - 17.8; c - 17.6; d - 15.8; e - 16.8 m g . m -2.

3e51

4ooi .¢ Z5

20

J

.

/

I

~5

8 3O

'

'

,

=

A

.e

: t

/ zso

Fig.3.

__

x A

'

I

• B

D e p t h s of the c h l o r o p h y l l m a x i m a (shaded zones) in r e l a t i o n to the t e m p e r a t u r e (A) and s a l i n i t y (B) p r o f i l e s , in the v i c i n i t y Of the A t l a n t i s II d e e p in N o v e m b e r 1977 (Weikert, 1980a).

972

Y. Hallm

The d i s t r i b u t i o n of total pigment c o n c e n t r a t i o n in the NW Indian O c e a n and the Red Sea (McGill and Lawson, 1966) d i s p l a y s a s t r i k i n g c o n t r a s t b e t w e e n the two m o n s o o n seasons (Figs 4 and 5)° D u r l n E the N E m o n s o o n the s t a n d i n g Crop is at a m a x i m u m in the Red Sea, with values ranglng f r o m 65 - 91 m g . m -2, but in the m e a n t i m e there is a general decline in the Indian Ocean. In w i n t e r p r o d u c t i o n rises to i~s m a x i m u m in the Gulf of Aqaba a l t h o u g h the main basin remains the m o r e p r o d u c t i v e ( L e v a n o n - S p a n l e r et al, 1979). The situation reverses during the SW monsoon. C h l o r o p h y l l values > 4 0 - 5 0 m E . m - 2 are r e s t r i c t e d to the southern end of the basin w h i l e there is a g e n e r a l rise in chlorophyll c o n c e n t r a t i o n s in the N o r t h - w e s t e r n Indian Ocean. The M a r c h and June m e a s u r e m e n t s ranged from 16.7 - 41.8 m g . m - 2 ( W e l k e r t , 1980a) and in MayJune from 16 27.2 m E . m - 2 (Yentsch and Wood, 1961). The r e s p e c t i v e blooming periods in the Red S e a and the Indian Ocean therefore take place in the opposite seasons. Weikert'a (1980a) conclusion that summer is the productive season in the Red Sea appears to be erroneous.

r5

Fig.4.

30

>30

D i s t r i b u t i o n of total pigment c o n c e n t r a t i o n s (mg.m -2) in the s u r f a c e 200m of the w a t e r column during the southwest m o n s o o n period 1963 (after MeGill and Lawson, 1966).

20

o-~-

Fig.5.

"

/

D i s t r i b u t i o n of total pigment c o n c e n t r a t i o n s (mg.m -2) in the s u r f a c e 200m of the w a t e r column during the northeast monsoor~ period, 196] (after McGill and Lawson, ]966).

Plankton

of the Red Sea and

tile A r a b i a n

Gulf

9[3

Primary p r o d u c t i v i t y m e a s u r e m e n t s are e x t r e m e l y scarce. L e v a n o n - S p a n i e r et al (1979) o b t a i n e d 14C a s s i m i l a t i o n values for the Gulf of Aqaba r a n g i n g from 19.7 - ill mg C . m - 2 d -I with a distinct m a x i m u m in winter. Four p r o d u c t i v i t y stations were worked by Weikert (1980a) in the m a i n basin, r e s u l t i n g in e s t i m a t e s of 115 - I00 mg C.m-~d -I . In the A t l a n t i s II deep region w h i c h lies w i t h i n the d i s c o n t i n u i t y zone, the March productioll was only half that of June, 45 mg C.m-2.d -I , as O p p o s e d to I00 mg C.m-2.d -I . But the conditions in the discontinuity zone are not r e p r e s e n t a t i v e of the rest of tile basin . The upwe] I ing coastal waters of the Indian Ocean y i e l d up to 500 mg C.m-2.d -I . The Hed Sea p r o d u c t i v i t y is c o m p a r a b l e to that of the o l i g o t r o p h i c s u b t r o p i c a l Indian ()teal1, 70-]O0 in~ C.m-2.d -I (Krey, 19"/3). Wet weight biomass m e a s u r e m e n t s of z o o p l a n k t o n cari'ied out by Weikert (1980a) in March showed a significantly larger b i o m a s s in the southern than in the central Red Sea. The vertical biomass p r o f i l e s i n d i c a t e d two maxima, the most important of which o c c u r r e d i n the 0 - 1 0 0 m layer. A m i n o r b i o m a s s peak was found in the core of the oxygen m i n i m u m layer, from where a large p r o p o r t i o n of z o o p l a n k t o n m i g r a t e s up at night. Most of the net z o o p l a n k t o n (about 98%) is d i s t r i b u t e d in the upper lO00m of the Red Sea. A l t h o u g h the Red S e a is poorer in species than the Indian Ocean, its p l a n k t o n i c p o p u l a t i o n is still very diverse, and m u c h more so than that of the A r a b i a n Gulf. There are, however, p r o n o u n c e d n o r t h - s o u t h g e o g r a p h i c a l , as well as seasonal v a r i a t i o n s in diversity. Two types of a s s e m b l a g e s can be r e c o g n i s e d . A c o n s i d e r a b l e p r o p o r t i o n of the d i n o f l a g e l l a t e , tintinnid, copepod, e u p h a u s i i d , c h a e t o g n a t h and a p p e n d i c u l a r i a n p o p u l a t i o n s afford no e v i d e n c e of the s o u t h e r n inflow i n f l u e n c i n g their seasonal or g e o g r a p h i c a l distributions, but beside these well a d a p t e d y e a r - r o u n d e d i n d i g e n o u s species, there are w i n t e r - r e c r u i t m e n t species. The w i n t e r - r e c r u i t m e n t s p e c i e s enter the Red Sea via the inflow from the Gulf of Aden which o c c u r s during the NE m o n s o o n and which plays an important role not only in the r e c r u i t m e n t of surface pelagic o r g a n i s m s from the Gulf of A d e n but also in their n o r t h w a r d d i f f u s i o n through the Red Sea basin. A l t h o u g h some species appear only t e m p o r a r i l y and are r e s t r i c t e d to the v i c i n i t y of B a b - e l - M a n d a b strait, a p r o n o u n c e d g r a d i e n t is e s t a b l i s h e d in w i n t e r from south to north. The s o u t h e r n Red Sea b e c o m e s c o n s i d e r a b l y r i c h e r in species than the n o r t h e r n w i t h the two basins being s e p a r a t e d by the c o n v e r g e n c e zone which is poorer in species than both. Both the Gulfs of Suez and A q a b a are less d i v e r s e than the m a i n Red Sea, but the diversity of their p l a n k t o n communities is also s i g n i f i c a n t l y higher in winter. This pattern is evident from the d i s t r i b u t i o n s of several taxa of phyto- and z o o p l a n k t o n of which the c o p e p o d s and the d i n o f l a g e l l a t e s are the best known (Halim, 1969). About 92% of all Red Sea c o p e p o d s p e c i e s were r e c o r d e d in w i n t e r and only 62% in s u m m e r - a u t u m n (Tables 1 and 2). TABLE

1

Monthly variations Sea (Halim, 1969).

in the n u m b e r

of c o p e p o d

species

in the Red

I Month

January February-March April May-June August October

G u l f of Suez

39 33 18 19 8 24

I I

Red Sea North

of 18"N 65 52 33 23 28 33

South of 18"N 66 62 26 39 37

Seasonal and/or diurnal vertical m o v e m e n t m a y a c c o u n t for part of this s u m m e r p a u c i t y in species. Some s u r f a c e s p e c i e s common in winter, such as E u c h a e t a m a r i n a b e c o m e r e s t r i c t e d to the m e s o p e l a g l c z o n e in summer. S i m u l t a n e o u s echo s o u n d e r records and m i d - w a t e r trawls s h o w that the d i u r n a l v e r t i c a l m i g r a t i o n of the z o o p l a n k t o n is almost i d e n t i c a l to the vertical m o v e m e n t s of the d e e p s c a t t e r i n g l a y e r (Weikert, 1980a). Observations r e s t r i c t e d to s u r f a c e samples, therefore, m i g h t be m i s l e a d i n g . However, the f l u c t u a t i o n s in d i v e r s i t y a r e m a i n l y g o v e r n e d by the c i r c u l a t i o n p a t t e r n and the e x c h a n g e w i t h the Gulf of Aden, as s h o w n by the v e r t i c a l d i s t r i b u t i o n of E u e a l a n u s spp down to 450m in the south and central b a s i n (Welkert, 1980a). The species w e r e a b u n d a n t in M a r c h both in the e p i p e l a g i c and the O x y g e n m i n i m u m zones, but d i s a p p e a r e d c o m p l e t e l y in N o v e m b e r (Table 3).

Y.

974

TABLE

2

Monthly species

v a r l a t i o n s in the number of d i a t o m in the Red Sea (Balim, 1969).

Month

Diatom

January February March April May August October November

*the genus

TABLE

3

Halim

spp.

and d l n o f l a g e l l a t e

Dinoflagellate

32 31 4 22 1 6 45

Ceratium

Abundance different

is not

spp

43 18 22 26 39 62" 40 51

included.

of E u c a l a n u s spp (Numbers seasons. (After Weikert,

S o u t h e r n Basin Djebel Tair March

Central

in the Red Sea

Basin A t l a n t i s June

(m)

o -

50

2800

1259

10

0

50 - I00

I000

150

0

0

November

- 150

i00

o

o

0

150 - 200

lOG

10

0

0

50

lo

0

o

150

50

IO

o

200 - 250 250

- 300

300 - 550

40o

50

lo

o

350 - 40o

4oo

lO

o

0

400 - 45o

450

lo

IO

o

ARABIAN

in

II Deep

Depth

100

March

10Om -3) 1980a)

GULF

The A r a b i a n Gulf, on the other side of the A r a b i a n Peninsula, is a m u c h smaller and s h a l l o w e r sea. It is m o r e llke a vast l a g o o n c o n n e c t e d to the Gulf of Oman t h r o u g h the n a r r o w strait of Hormuz. Its m e a n depth is 35m and the m a x i m u m depth in the central basin does not reach 200m. Like the Bed Sea, it is l o c a t e d in a w a g m and arid zone where e v a p o r a t i o n exceeds p r e c i p i t a t i o n and runoff. U n l i k e the Red Sea, however, the G u l f r e c e i v e s some f r e s h - w a t e r input via the Shatt el A r a b estuaPy. A l t h o u g h this input has a p e r c e p t i b l e effect o n the b i o l o g i c a l p r o d u c t i o n in the n o r t h - w e s t e r n Gulf, it does not c o m p e n s a t e for the e v a p o r a t i v e loss. As a r e s u l t of the d e n s i t y gradient, surface water from the G u l f of Oman p e n e t r a t e s along the I r a n i a n coast. Its s a l i n i t y g r a d u a l l y i n c r e a s e s w i t h d i s t a n c e from the inlets from about 3 6 . 6 ~ n e a r the S t r a i t s to about 4 0 . 6 ~ in the NE G u l f (Seibold, 1973) and further to 42.2 - 4 2 . 5 ~ off Q a t a r and 4 4 . 3 ~ in the stagnant w a t e r s off the Trucial coast (Yamazi, 1974) . S u m m e r air t e m p e r a t u r e s in the central Gulf rise to 36"C and fall to ]9"C in winter. Evaporation and w i n t e r c o o l i n g r e s u l t s in the f o r m a t i o n of dense water in the inner G u l f which sinks to the bottom. T h i s h i g h e r s a l i n i t y and cool b o t t o m w a t e r flows along the deeps near the A r a b i a n coast and finally forms the d e e p o u t f l o w through the Gulf of Oman. The surface layers tend to be m i x e d by the p r e d o m i n a n t NW winds (tile "Schemal"). As a result of t11ese circulations the Gulf w a t e r s are well o x y g e n a t e d down to the b o t t o m (Seibold, 1973) N:~ oxygen m i n i m u m layer exists in this shallow Gulf (Figs 6-1]).

Plankton

48"

49 °

of

50 °

the

Red

5~~

Sea

and

52"

the

Arabian

55 °

Gulf

54"

975

55 °

56 °

20m ---- 4 0 m 60m

K U W A T- ~

/

~,o-fl--,~l'.'

\\L

A "

'\

km

80 m - -- l O O m 120m

0

50

I00

150

I

I

I

I

\~\\::.,. I

\. ~-.~,,%",x : ",'"." ~\~z~._X'. ., ",. ', ~ ~,. -.-W'-.

27"

57"

__.j--

',.h~

_,

~o e-~,?. ,',k

26"

"/ , il,,~/.

~~>/'°

Omon

25"

•

.::...%

i)j

24"

Truciat i Coast

Fig.6.

The

Arabian

Hartmann,

Gulf

showing

the

main

Lange,

Seibold

and

Walger,

Shatt-eL-Arab

Fig.7.

topographic

features

Central, basin

Hormuz s t r a i t

Longitudinal profile of temperature (~C) d i s t r i b u t i o n Arabian Gulf. The hatched area shows the occurrence depleted

water

enriched

with

(after

1971).

phosphate

(after

Hartmann

in the of oxygenet

ai.1971)

976

Y.

Shott-et-Arob \

~

k.~

Centrat

Halim

basin

Hormuz

1 V ' G 9 -5 " - ' - ~ 9 o ~ \

",.~zs~

strait

~36.5

5O

Satin

C3

i

~

-

-

~

~

I00

o

Fig.8.

5o

,oo

2oo

3oo

"

'

")

/~-----~

L o n g i t u d i n a l p r o f i l e of s a l i n i t y ( ~ ) d i s t r i b u t i o n in the A r a b i a n Gulf. The h a t c h e d a r e a s h o w s the o c c u r r e n c e of o x y g e n - d e p l e t e d w a t e r e n r i c h e d w i t h p h o s p h a t e ( a f t e r B r e t t s c h n e i d e r e t a l, ]970)

Shatt-et-Arab 28.4 Centratbasin ., m~.2\~:~ .~ ~ / ~ / ~ 4 f 2,

% ~ '

Hormuzstrait I

I

II

''

'

\

5 E3

tOO

Fig.9.

5 o n g i t u d i n a l p r o f i l e of d e n s i t y ( G S e a ( a f t e r H a r t m a n n et al, 1971}.

t) d i s t r i b u t i o n

in the Ar'abian

i

Plankton

of the

I

~:..I

24

52 °

the A r a b i a n

I

Gulf

°

53 °

°

o

o

I

~ ~_o 51 °

977

I .

54"

55 °

D i s t r i b u t i o n of s u r f a c e t e m p e r a t u r e of Q a t a r a n d and a l s o off K u w a i t (in i n s e r t ) , d u r i n g D e c e m b e r

~o

Fig.ll.

and

_

51°

Fig.10.

Red S e a

52"

~°~ O o

Yo/ 55"

54 °

o

the T r u c i a l c o a s t , (after Yamazi, 1974).

II 1 426

~ -I~. 55"

D i s t r i b u t i o n of s u r f a c e s a l i n i t y ( ~ ) o f f Q a t a r a n d the T r u c i a l c o a s t , and a l s o o f f K u w a i t ( i n s e r t ) , d u r i n g D e c e m b e r ( a f t e r Y a m a z i ,

1974)

Y.

978

Halim

The o f f s h o r e w a t e r s are h i s h e r in n u t r i e n t c o n t e n t and in p h y t o p l a n k t o n and z o o p l a n k t o n s t a n d ing crop and d i v e r s i t y than those inshore. O f f K u w a i t and in the v i c i n i t y of the Shatt el Arab, the w a t e r s are c o n s i d e r a b l y r i c h e r in s i l i c a t e than off Q a t a r and t h e T r u c i a l coast (Table 4). The NE Gulf a p p e a r s to be p o o r e r in b o t h s i l i c a t e and p h o s p h a t e (Pig.12 and 113).

T A B L E 4:

Range of p h o s p h a t e A r a b i a n Gulf.

and s i l i c a t e

concentrations

(ug.l -l)

in the

A. D e c e m b e r 1968 (after Y a m a z i , 1974) B. April 1965 (after B r e t t s c h n e i d e r , 1970)

Locality

Silicate

Phosphate

Kuwait

0.33

- 0.56

Qatar

0.13

- 0.55

1.0 -

3.0

0.14

- 1.32

0.0 -

1.2

Trucial

Coast

Locality

10.5

Phosphate

- ]8.4

Silicate

Off Iran: N o r t h of 27"N

0.07

- 0.54

0.00 - 0.08

S o u t h of 27"N

0.29 - 0.42

0.00 - 0.07

I

I

~'~:_,~'-_ ...

I

I

o

2~

24 °

51"

Fig. 12.

52 °

53"

54"

55"

D i s t r i b u t i o n o f s u r f a c e s i l i c a t e ( U g - a t . S i 0 ~ - S i . l -I) o f f Q a t a r and the T r u c i a l coast, and a l s o off K u w a i t ~insert) d u r i n g D e c e m b e r (after Yamazi, 1 9 7 4 )

Plankton of the Red Sea and the A r a b i a n Gulf

979

04 f--.O5

0

i:~ :. . . ~

o

o

o

O

O

,

o

i 5fo

Fig.13.

~ •

:

52 °

o

J2

0.8

o

26 °

04 o

-. 53°

~

54 °

55 °

24

D i s t r i b u t i o n of surface phosphate (ug-at. PO4-P.I -I) off Qatar and the Trucial coast, and also off Kuwait (insert) during (after Yamazi, 1974).

The Gulf p l a n k t o n is less well known than that of the Red Sea. Only a few surveys have been c a r r i e d out (Yamazi, 1974; AI-Kaisi, 1976; Gibson, G r i c e and Graham, 1980) and are often e i t h e r l o c a l i s e d or r e s t r i c t e d to a p a r t i c u l a r taxonomic g r o u p (Bohm, 1931; Weigmann, 1970; Fenaux, 1973; F u r n e s t i n and C o d a c c i o n i , 1968; A I - H a n d h a l , personal communication; Halim, u n p u b l i s h e d data). Our k n o w l e d g e of the structure and d i v e r s i t y of the Gulf population r e m a i n s a f u n c t i o n of the number and t h o r o u g h n e s s of surveys. W o o d (1963) in his check list of the Indian O c e a n p h y t o p l a n k t o n s p e c i e s reports only two d i n o f l a g e l l a t e s from the Gulf, a l t h o u g h B o h m (1931) had r e c o r d e d 34. Later, Halim (unpublished data) a d d e d 57 and A i - K a i s i (1976) a f u r t h e r II, r a i s i n g the total to 102. There are good g r o u n d s to believe, however, that the w a t e r s of the A r a b i a n Gulf are poorer than those of the Gulf of O m a n both in biomass and in d i v e r s i t y , and also poorer than the Red Sea. W e i g m a n n (1970), r e c o r d e d 24 Euphausiacae from the A r a b i a n Sea, of w h i c h 13 are known to occur in the Red Sea (Halim, 1969) and only one, P s e u d e u p h a u s i a latifrons in the A r a b i a n Gulf, but in e x c e p t i o n a l l y large n u m b e r s (i000 s p e c i m e n per l O O m 2, Weigmann, 1970). S e v e n a p p e n d i c u l a r i a n species are known from the Gulf ( O i k o p l e u r a l o n g l c a u d a t O. r u f e s c e n s , O. f u s i f o r m l s S t e g o s o m a ma~num~ F r l t i l l a r l a formica~ M e g a l o c e r c u s huxleyi) (Fenaux, 1973; Yamazi, 1974) c o m p a r e d w i t h II in the Red Sea and 19 in both the G u l f of Aden and the O m a n Sea.

The C h a e t o g n a t h a a r e r e p r e s e n t e d by a t l e a s t six species, (Sagitta pulchra, S. n e g l e c t s , S. b e d o t i , S. s p p ) ( F u r n e s t i n and Codaccioni, p a r e d w i t h 13 i n t h e Red S e a . The known c o p e p o d p o p u l a t i o n , however, still u n r e p o r t e d f r o m the Red Sea (Table 5).

inflata, S. r o b u s t a , S_:. 1 9 6 8 ; Y a m a z i , 1974) comincludes several species

The Gulf p o p u l a t i o n is t y p i c a l l y i n t e r t r o p i c a l , e p i p l a n k t o n i c and neritic w i t h but few characteristic I n d o - P a c i f i c species. No d i a t o m o r d i n o f l a g e l l a t e " s h a d e " s p e c i e s a nd no m e s o p l a n k tonic zooplankton forms have been recorded. The s t r u c t u r e a nd p a t t e r n of distribution as shown by H a l i m ( u n p u b l i s h e d ) for dinoflagellates (A1-Kaisi (1976), for phytoplankton, ffanazi, 1974) and f o r z o o p l a n k t o n (Gibson e t a l . , 1980), present characteristic features. Unlike t h e Red S e a , t h e i n s h o r e w a ~ c r s a r e d i s t i n c t l y poorer in species a nd i n b i o m a s s t h a n t h e offshore waters, and t h e p o p u l a t i o n s are relatively d e n s e r and more d i v e r s i f i e d in t h e i n n e r Gulf than in the vicinity of the Strait. There is a small assemblage of dinoflagellate and z o o p l a n k t o n s p e c i e s w h i c h e x t e n d s t h r o u g h o u t t h e G u l f , h o w e v e r i t s c o m p o s i t i o n a nd d i s t r i b u t i o n

980

Y. Hal im

TABLE

5:

C o p e p o d species r e c o r d e d from the Gulf but not (compiled from Yamazi, 1974, and G i b s o n et el.

Acartia

dense

Labidocera L.

Fontella

detruncata

Lucicutia

karachiensis

F. s e c u r i f e r

krq~eri

Oithona

from the Red Sea. 1980).

P0ntellopsis acute

herdmanni

P. r e g a l i s

fallax

Sapphirina Tortanus

angusta

fo~cipatus

s u g g e s t s no r e c r u i t m e n t occurs from the Gulf of Omen in e i t h e r w i n t e r or spring. Data for the summer are lacking. Such f e a t u r e s c o n t r i b u t e to d i f f e r e n t i a t e the Arabian Gulf ~rom the Red Sea. The z o o p l a n k t o n biomass, in terms of d i s p l a c e m e n t v o l u m e of net catches, is lower in the Gulf than in the Gulf of Omen. But u n l i k e the Red Sea, the g r a d i e n t reverses tn the m l d - b a s l n and the inner Gulf. The m i d - b a s i n is h i g h e r in b i o m a s s than the southern basin in the v i c i n i t y of the Strait, and the w a t e r s off Kuwait are h i g h e r than off Qatar (Table

6).

TABLE 6

Z o o p l a n k t o n b i o m a s s (cc.m -3) i n t e g r a t e d in the A r a b i a n G u l f and Gulf of Omen A. B.

Plankton Plankton

column

net 7 0 c m in d i a m e t e r w i t h 2 4 3 ~ m m e s h (Gibson et al. 1980j net ll3cm in d i a m e t e r w i t h 330 ~ m m e s h (Yamazi, 1974).

Range

Location

Arabian

o v e r the w h o l e water

Gulf:

South

Basin

O.II 0.31

- 2.00

N o r t h basin

0.22

- I.Ii

0.52

- 2.27

Gulf of O m a n

Off Q a t a r Off Kuwait

- 0.95

Mid-basin

and the T r u c i a l

coast

0.29

- 1.59

(mean 0.37)

0.67

- 1.81

(mean

1.00)

e t el. 1980, r e p o r t e d e x t e n s i v e shoals of j u v e n i l e fish, jellyfish, salps and d o l i o l i d s occurrlng in March, t o g e t h e r w i t h a v a r i e t y of i n v e r t e b r a t e larvae. The higher o f f s h o r e Gibson

b i o m a s s r e s u l t s from the a b u n d a n c e of salps and d o l i o l i d s (Yamazi, 1974; G i b s o n et al 1980). C o p e p o d s a/'e n u m e r i c a l l y dominant, c o n t r i b u t i n g up to 50% of the p o p u l a t i o n with o s t r a c o d s ( C o n c h o e c i a sp. and C ~ p r i d l n a n0ctiluca) ranked s e c o n d in abundance. The o s t r a c o d s are scarce in the G u l f of Omen, and their a b u n d a n c e a p p e a r s to be a characteristic feature of the Arabian Gulf. Off Kuwait, they are r e p l a c e d by cladocerans, P e n i l i a a v i r o s t r i s and Evadne tergestin ~. (Figs. 14 and 15).

P l a n k t o n o f t h e Red Sea and t h e A r a b i a n G u l f

~

T

r

I00

1

~

l

I00

50

•

52 °

53 °

"'

2 0 0 50

54 °

26 °

55"

Abundance o f o s t r a c o d s i n t h e s u r f a c e G u l f ( a f t e r Yamaz±, 1 9 7 4 ) .

J

]

I00 I00

51 °

Fig.14.

l

981

plankton of the Arabian

I

B

o6

oo

~ oo

ill i.

'. ::.~

.

~?

48"

°0

o 4

I~

o

o0

o

I 49"

o

g o 4

o

o

3~ o

o o

~.

So

o

~

~';:

I

D:~"."

I

o ' ~ ~ 51 °

Fig.15.

52"

53"

54"

Abundance o f c l a d o c e r a n s i n t h e s u r f a c e G u l f ( a f t e r Yamazi, 1 9 7 4 ) .

55"

plankton

of the Arabian

982

Y, Halim

REFERENCES A1-Kaisi, K.A. (1976). On t h e p h y t o p l a n k t o n of the Arabian Gulf. Abstracts of papers presented at the Joint Oceanographic Assembly. Edinburgh, UK, 1 3 - 2 4 S e p t e m b e r 1976. ;hO, R o m e , 1976. B o h m , A. ( 1 9 3 1 ) . Peridineen a u s dem P e r s i s c h e n G o l f u n d dem G o l f Yon Oman. Archly fur )~ro testenkunde 74, 188-197. Brettschneider, G., K. G r a s s h o f f , P . H . K o s k e a n d L. v o n T r e p k a , (1970). Physikaliscbe und chemiche Daten nach Beobachtungen des Forschungsschiffes 'METEOR' im P e r s i s c h e n Golf 1965. 'METEOR' Forschun~ser~ebnisse Reihe A~ 8 , 43-90. Fedorina, A.I. and G.N. Kornilova (1970). The zooplankton of the Red Sea. Trud~ hzovskoChernomorsko~o Vauchno-Issled0vatel'sko~o Instituta Rybno~o Khozyaistva~ Okeano~r~fii~ 30. Special issue. Fenaux, R. (1973). Appendicularia from the Indian Ocean, the Red Sea and the Persian Gulf. In: The B i o l o ~ of the Indian Ocean. B. Zeitzschel, editor, Springer-Verlag, 409-414. Furnestin, M.L., and Codacclonl, J.C. (1968). Chaetognathes du nord-ouest de l'oeean Indien (Golfs d'Aden-Me~ d'Arable-Golfe d'Oman-Golfe Persique). Cahiers de l'Orstom ~ Oceans5 graphie, 6, 143-171. Gibson, V.R., G.D. Griee and S.J. Graham (1980). Zooplankton investigations In Gulf wa~ers north and south of the straits of Hormuz. In: Proceedings of a s~mposium on coastal and marine environments of the Red Sea~ Gulf of Aden and tropical western Indian Ocean__ Vol.2. Khartoum, 501-517. Halim, Y. (1969). Plankton of the Red Sea. In: Oceanography and Marine Biology, Annual Review t Vol.7. H. Barnes, editor, George Allen & Unwln, 231-275. Hartmann, M., H. tangs, E. Seibold and E. Walter (1971). Surface sediments in the Persian Gulf and in the Gulf of Oman. I. Geologlc-hyd1"ologic setting and first sedimentological results. 'METEOR' Fopschungsergebnisse I Eelhe C~ 4, 1-76. Kimor, B. and B. Golandgky (1977). Microplankton of the Gulf of Elat: Aspects of Seasonal and Bathymetrlc Distribution. Marine Biology, 42. 55-67. Krey, J. (1973). Primary production in the Indian Ocean. In: The Biology of the Indian Ocean. B. Zeitzschel, editor, Sprlnger-Verlag, 115-126. Levanon-Spanier, I., E. Padan, and Z. Relss, (1979). Primary production in a desert-enclosed sea - the Gulf of Elat (Aqaba), Red Sea. Deep-Sea Research F 26, 673-685. Matzenauer, L. (1933). Die Dinoflagellaten des Indischen Ozeans. Botanisches Archlv~ ~5, 437-509. McGi11, D.A. and T.J. Lawson (1966). The distribution of chlorophyll in the western Indian Ocean during the Northeast monsoon period. Woods Hole Oceanographic Institution, Ref. 66-12, 69 pp. (unpublished manuscript). Morcos, S.A. (1970). Physical and chemical oceanography of the Red Sea. In: Oceanography and Marine Biology Annual Revlew t VoI.8, H. Barnes, editor, George Allen & Unwin. ?3202. Natour, R.M. and H. Nienhuis, (1980). Some phytoplanktonic studies in Aqaba Gulf off Jordan. Nova Hedwigla, 33, 433-443. Nasr, D.H. (1980). Coastal plankton fauna of the Sudanese Red Sea. In: Proceedings ol a s~mposium on the coastal and m~hrine environments of the Red Sea~ Gulf of Aden and tropical western Indian Ocean. V o l ~ 2 , K h a r t o u m , 561-581. Selbold, E. (1973). Biogenic sedimentation of the Persian Gulf. In: The Biology of the Indian Ocean, B. Zeitzschel, editor, Springer-Verlag, I03-126. WeIEmann, R. (1970). Zur Okologle und Ernahrungsblologie der Euphauslaceen ICrustacea) in Arablschen Mee~. 'METEOR' For~chu~g~ergebnlsee~ Relhe D. 5, 11-52. Welkert, H. (1980a). On the plankton of the central Red Sea. A first synopsis of results obtained from the cruises Meseda I and Meseda II. In: Proceedlngs of a symposlum on the coastal and m~rlne @nvi~onments of the Red Sea, Gulf of Aden and troplcal western Indian Ocean~ vol.~, Khartoum, 13~-167. Welker~, H. (1980b). The oxygen minimum layer in the Red Sea: Ecological implications of the zooplankton occurrence in the area of the Atlantis II Deep. Meereeforschung, 28, (i), 1-9. Welkert, H. (1982). The vertical distribution of phytoplankton in i~latlon to habitat zones in the area of the Atlantis II Deep, central Red Sea. Marine Ecology Progress Seriest 8 (2), 129-IS3. Wood, E.I.F. (1963). Cheekllst of the dinoflaEellates recorded from the Indian Ocean. ~eport of the Division of Fisheries and Oceanography, CSIRO, Australia~ 28, 3-63. Yamazi, I. (197~). Analyses of the data on temperature, salinity and chemical propert~e~ of the surface water, and the zooplankton communities ~n the Arabian Gulf in Decemb(~r 1968. Transactions of the Tokyo University of Fisheries, i. 25-51. Yentseh, C.S. and L. Wood (1961). Measurements of primary productivity in the Red Sea. ~ . ~ of Aden and Indian Ocean. Woods Hole Oceanographic Institution. Ref. 61-6. Appendi~ ~ ~r,c,