Indian Journal of Marine Sciences Vol. 28, December 1999, pp. 416-423

Analysis of the relationship between phytoplankton biomass and the euphotic layer off Kuwait, Arabian Gulf

D V Subba Rao & Faiza Al-Yamani

Mariculture and Fisheries Department, Kuwait Institute for Scientific Research, P.O.Box 1638, 22017, Salmiya, Kuwait

Received 28 February 1998, revised 16 August 1999

The Kuwait Bay is a shallow tidally well-mixed sub-tropical environment in the Arabian Gulf, and is characteri zed by excessive evaporation, little freshwater input, and several anthropogenic disturbances attendant with oil explorations. From the Gulf we examined 219 Secchi disc readings and profiles of temperature, salinity, chlorophyll a. and nutrients. Features of interest are: a) existence of marked differences in the magnitude of phytoplankton biomass between the nearshore (3.8 -113.4 mg chi a m-2

) and offshore stations (4.5 - 57.9 mg chi a m·2), b) lower algal biomass (18 .5 to 27 .3 mg chi a m-2) at 3 inshore stations located off an industrial belt compared to offshore waters (>42 mg chi a m-2

), c) absence of pronounced seasonal phytoplankton growth and d) small increases in biomass sometime during March - May, August and October - De­cember. An analysis of the relationship between scaled critical depth, integrated chlorophyll a and nutrients support the hy­pothesis that phytoplankton biomass in Kuwait waters, in general , was not restrained by physical environment. The correla­tion coefficient (Spearman Rank) for the sign trend test between scaled critical depth (Z'e<) and integrated chlorophyll a was not significant at 10 stations suggesting dependence of phytoplankton abundance on factor(s ) other than li ght. Only at an offshore station (#18) this correlation was negative and significant. Light profiles yielded 83-275 W m· l near the bottom suggesting availability of sufficient light for algal growth in these well-mixed sub-tropical shallow waters.

Situated in the northwestern Arabian Gulf, the waters of Kuwait Bay are shallow «30m). The overall cir­culation in the Gulf is anti-clockwise that results in a net southerly flow of > 40 cm S-I in the Kuwait Bay l.2. In the Kuwait Bay, tides contribute nearly 90% of the total energy and the waters are usually well-mixed and well-oxygenated. Like the rest of the Gulf, it experiences several environmental perturba­tions such as the spillage of oil, discharge of cargo vessel ballast waters, both concomitant with oil ex­plorations, traditional to this region. Besides, dis­charges from coastal dredging operations, effluents from power and desalination plants, mining, petro­chemical industries, slaughterhouses, dairy plants and sewage treatment plants compound the stress on this unique ecosystem. Surprisingly, long-term plankton studies do not exist for these waters. Only a few studies on the hydrography and phytoplankton of mostly surface waters exise,4. Phytoplankton growth in the tropical-temperate coastal waters usually fol­lows a bimodal distribution with a major peak during spring and a minor one during fa1l 5

. The few phyto­plankton studies off Kuwait are limited to January through March 19793 or between March and May 19784

. Because of these gaps it is hard to discern any

annual progression of phytoplankton growth . Three interrelated concepts i.e. the Critical Depth

Model, Compensation Depth6, and the Critical Mean

Irradiance in the mixed layer? explain the interplay of physical forces on phytoplankton growth in a water column (w), particularly in the temperate waters. Critical depth is defined as the depth at which total photosynthesis for the water column (Pw) is equal to the total respiration (Rw) of primary producers . Com­pensation depth is the depth where photosynthesis equals its respiration. Based on empirical evidence from cold temperate waters, Rile/ suggested that a critical mean light level of about 193.4 W m-~ should be exceeded to promote phytoplankton growth. If the critical depth is greater than the depth of mixing, phytoplankton growth in the column will not be re­strained and results in a net positive (Pw>Rw) pro­duction. Thus the relationship between the mixed layer (Zrru) and the extent of the euphotic layer (as­sumed as represented by depth where the light energy is 1 % of surface energy) is important. The concept of scaled ' critical depth Z'er was developed and was use­ful to model the duration of the annual phytoplankton growth in shallow and deeper watersR

• This model showed the annual phytoplankton growth cycle in the

SUBBA RAO & AL-Y AM ANI: PHYTOPLANKTON EUPHOTIC LAYER 417

tropical estuaries is not limited by light where the Z'cr is larger than the scaled mixed layer depth (Z'rnl)'

This analysis, the first synthesis from this geo­graphical region, is based on profiles of temperature, salinity and chlorophyll a. We test the hypothesis that phytoplankton growth in these well-mixed sub­tropical shallow waters is not restrained by physical environment. Our data, despite shortcomings in the sampling frequency, support the hypothesis that in Kuwait waters phytoplankton growth in general is not limited by interaction between the euphotic layer and mixed layer.

Materials and Methods Model-The scaled critical depth (Z'er) and scaled

mixed layer depth (Z'rnl) model is a modification of the model described by Sverdrup6 and Riley9_ Modifi­cations to this have been discussed earlier by Sinclair et aL 8

_ Critical mean light leveller is expressed as:

I cr = (10 /k Zrru) (l_e-kZrnl)

where

10 = 500 W m-2 total incident radiation k = extinction coefficient (m- I

) deter-mined by 1.441 ZSD (Holmes 10)

Zrru = depth of mixed layer (m) and ZSD =Secchi disc depth (m)

Z'rn! = scaled mixed layer depth is

Zrru + ZSD Ier = critical mean light >39W m-2 re-

quired for phytoplankton growth at Bushehr, northeastern coast of the Gulf of Persia (Hulburt et at. II)

Z'cr = the scaled critical depth: Z'cr = [(10 (l-e-kZ

cr) 1 kIcr)/ZsD (m)] Io and Icr = units of radiation W m-2

When Secchi disc reading is >2 m (l-e- kZ er) ap­proximates unity. For calculating the attenuation co­efficient, 1.44/ZsD (m) is appropriate. In the turbid coastal waters of Galathea Bay 10 and in the northern Arabian Gulf waters at Bushehrll the same factor was used. It is comparable to the 1.5 for Cochin backwa­ters l2 or the 1.48 reported from the estuarine coastal waters off Goa 13 _ Substituting one for (I-e-kZ

er) and 1.441 ZSD for k, Z'cF 17.8 (xIO-3

) 10. The relationship between Z'cr and Z'ml is of interest

while predicting seasonal phytoplankton growth_ When Z'er is > Z'rn! , phytoplankton growth should not be light limited (see Table 1, in Sinclair et at.8

).

St. no

6 7 13 14 16 18 20 23 24 K6 KIO

ISLANDS

F:FAILAKA A:AUHAH K:KUBBAR U : VMM-AL-M>\RADEM

ARABIAN GULF

~oA

-u •. 23 .24

30' 45'

Fig. I-Sampling stations in the Kuwaiti waters

Table I-Sampling stations

Long (E) Lat (N) Depth (m) Sampling (m)

48" 10' 29"20' 22.0 1, 10, 21 48" 10' 29" 10' 22.5 1,10,21.5 48"20' 29"4' 23.0 1,10,22 48" 12' 29"00" 9.0 1,8 48"20' 29" 00" 20.0 1,10,19 48"40' 29"00" 26.5 1, 10, 25 .5 48"30' 28"50" 23.0 1, 10,22 48"30' 28"38" 16.0 1,10,15 48"40' 28"38" 25.0 1,10, 24 47" 58' 29"27" 8.4 I. 7.4 47"50' 29" 25" 11.9 1, 10.9

Temperature and salinity were determined at II stations (Fig. 1) using a Hydrolab Surveyor 3 Water quality logging system. Details of station depths, sample depths, and the total visits to each station are set in Table I. Distinction between inshore (stations 6, 7, 14, 23, K6 and KIO) and the offshore (stations 13, 16, 18, 20 and 24) is in accordance with diction­ary meaning but not oceanographic sense because the Arabian Gulf is a shallow body of water with a mean depth of 35 m. Depth of the water column sampled depended on the amplitude of the tide. A 50 cm di­ameter Secchi disc was lowered and the depth of its

418 INDIAN 1. MAR. SCI., VOL. 28, DECEMBER 1999

disappearance is considered representative of 1 % of surface light or the euphotoic layer. Incident radiation data are from Kuwait weather data 19851198614

.

Water samples were collected using a Niskin sam­pler from surface (S), mid (M) and one meter above the bottom (B) and were processed for nutrients and chlorophyll a. Nitrate-N, phosphate-P, and total sili­cate were determined following Strickland & Par­sons l5 . Chlorophyll a determinations were based on the fluorometric method l5 on duplicate samples. Phaeopigments were excluded. Chlorophyll a was integrated under m'2 area and expressed as mg m'2. Because of the pronounced tidal amplitude an accu­racy within 0.5 m is expected for sample depths and therefore nutrient values and phytoplankton biomass values are rounded off to one decimal point.

Results Secchi disc depths (SD) ranged between 1 and 16.5

m (Table 2) of which 88%were > 2 m, 11 % were> 1 and < 1.5 m and 0.8% with 0.5 m. The mean Secchi disc readings (Table 2) for nearshore stations (sts. 6, 7, 14, 23, K6 and KIO) ranged between 1.5 and 6 compared to 1.5 and 10.5 m for the offshore (sts. 13, 16, 18, 20 and 24). The higher turbidity in this shal­low inshore environment may be due to resuspension of silt resulting from the dumping of dredged spoil, and by the tides and tidal currents. Exception to this is st. 14; although an inshore station there were occa­sions when the whole water column was transparent.

In this sub-tropical coastal environment vertical gradients in the temperature and salinity profiles were absent suggesting that the water column was well­mixed. The mixed layer (Zml) ranged between 7.4 and 25.5 m (Table 2). For inshore stations the scaled mixed layer (Z'rn!) extended between 0.9 (st. 14) and 23 .8 (st. KIO) and in the offshore between 1.38 (st. 20) and 16.67 (st. 24).

The incident solar radiation (10) at noon ranged between 617 W m'2 (December) and 1060 W m-2 dur­ing July (Rasasa & Aburshaid I4

). Using Z'cr=17.8 (xlO'3) 10 (see model) the calculated Z'er was mini­mum (11.9) during December and steadily increased till July to attain a maximum (18.9). A gradual de­crease followed till December.

Phytoplankton biomass variations were high . In discrete samples it ranged from 0.0 I to 12.8 Ilg chI a rl. Integrated chlorophyll values ranged be­tween 3.8 mg chI a m-2 at station 14 on 21 May 1985 and 113.4 mg chI a m'2 at station 10 on 12 July 1987

Table 2-Data on mixed layer depth and Secchi disc depth off Kuwait

St. no Mixed layer (m)?(!!:!:hi~~s!:_ ~(!p!~ (fllL_ .. _ Min Max Median

6 21.0 1.0 4.5 1.5 7 21.5 1.5 9.0 5.0 13 22.0 3.0 14.5 7.3 14 8.0 2.0 10.5 4.5 16 19.0 2.0 14.0 7.5 18 25.5 2.0 16.5 10.5 20 21.0 3.0 15.0 7.0 23 15.0 4.0 14.5 6.0 24 24.0 1.5 16.0 7.0 K6 7.4 0.5 4.5 2.0 KIO 10.9 0.5 4.5 1.5

Table 3-Ranges of Z' cp Z'IIII and integrated phytoplankton biomass

St.no. Z'cr Z'ml Chi ({

6 11.90, 18.90 4.70,21.00 15.X ' 83.5 7 12.50 - 18.90 2.50 - 15.00 17.02,40.49 13 11.90 - 18.90 1.59 - 9.20 10.0 - 57.9 14 11.90 - 18.90 0.90 - 4.5 H2 - I X.45 16 5.00 - 18.90 1.43 - 10.00 7.61 - 26.96 18 12.50 - 18.90 1.61 - 6.63 17.6H ,43.65 20 12.50 - 18.90 1.38 ' 11.00 11 .27 - 42.26 23 12.50 - 18.90 1.10 - 4.57 7.38 - 27.26 24 12.50 - 18.90 1.56, 16.67 12.45 - 42.35 K6 11.90 - 18.90 1.4 - 16.8 4.51 - 30.56

KIO 11.90, 18.90 1.70 - 23.H 12.H2 - 113.40

(Table 3). In general, there were marked differences in the magnitude of algal biomass between the near­shore (3.8 - 113.4 mg chi a m- l

) and offshore stations (4.5 - 57.9 mg chi a m'\ Stations 13, 18 and 24 al­though located in the offshore had attained chloro­phyll a values> 42 mg chI a m-2

, much higher than the maximum (28 mg chi a m-2) at the near shore sta­tions 14 and 23.

Nutrients in this shallow well-mixed water column were never exhausted (Table 4). Maximum levels (f.1 mol) at all stations were 1.6 for P04, 46.2 for Si02

and 4.8 for N03 (Table 4). Their median ranged from 0.01 to 0.3 (P04); 0.3 to 14.4 (SiOz) and 0. 1 to 0 .2 (N03). Although there were occasions when nitrate and phosphate levels at all stations attained zero , and the minimum silicate level was 0.1 f.1 mol, it should be remembered that water column was never stripped of these nutrients on any day.

Discussion That out of 219 observations. on 208 occasions the

Z'er off Kuwait was > 5 and larger than Z 'ml shows

(

SUBBA RAO & AL-Y AMANI: PHYTOPLANKTON EUPHOTIC LAYER 419

20 Stn. K6 120

80

N 10

40

Z'crll mJAChl. Stn.14

80 ';'8

N 10 eo 8 ..

::a 40

u ,

• l I I II • • I I I I I III I I I I I 6 I I

20 . Icz'cr .. Z'mJ A Chi. I Sto.18 120

80 ~ .;.

N 10 8 .. ::a u

40

o o

I I ! ! .. I I Date

Fig. 2-Seasonal variations in the scaled ertieal depth (Z 'er 0 ), scaled mixed ·Iayer (Z'm~ ) an integrated chlorophyll (/ (Ill)! Ill ·: .A. ) at sts. K6, 14 and 18.

420 INDIAN J. MAR. SCI., VOL. 28, DECEMBER 1999

light cannot be limiting phytoplankton growth. This is valid for most stations (Fig. 2) . Unpublished data collected with a Sea bird electronic profiler 25 yielded about 325-575 W m·2 at I m comparable to that at Bushehr ll

. The critical mean light> 39 W m'2 required for phytoplankton growth in the waters off Bushehr, II is within the range of I % light measured by the Sea bird electronic profiler. In these shallow waters light in general should not be a limiting factor for algal growth, consistent with observations in the

Table 4-Nutrient levels (~ mol) at sampling stations

SI. NOrN SiOz-Si P04-P

no Max Median Max Median Max Median

6 4.8 0.2 34.4 4.6 0.7 0.2 7 1.7 0.1 46.2 4.4 0.5 0.01 13 3.5 0.2 25.5 5.6 0.4 0.1 14 2.2 0.1 27.5 5.6 0.3 0.1 16 1.9 0.1 34.2 0.4 0.5 0.01 18 3.1 0.2 13. 1 4.4 0.7 0.1 20 1.0 0.1 28.9 0.4 1.2 0.01 23 1.0 0.1 3.7 0.3 0.4 0.01 24 2.2 0.1 15.8 3.9 0.5 0.1 K6 3.7 0.2 41.9 4.5 1.3 0.2

KIO 2.7 0.2 37.4 4.7 1.6 0.3

N.D - not detennined.; minimum level of nitrate and phosphate was zero at all stations. Minimum silicate levels ranged between 0.1 to 0.5

northern Arabian Gulf at Bushehr ll .

We justify using a critical mean light of >39 W m'2 for phytoplankton growth in the sub-tropical Gulf waters and distinguish it from the 193.4 W m'] used in temperate estuaries7

. In the high latitudes phyto­plankton is regulated by low temperature during the early spring, although the nutrient levels are high. In these waters a higher insolation (> 193.4 W m'2) would be required to warm the surface mixed layer during early spring thaw and to initiate active phyto­plankton growth as in Tokyo Ba/6

, Fram Strait l 7 and in the Rhine estuar/ 8

, Off Kuwait where the waters are usually warm i.e. > 15°C and < 32°C, further in­solation to elevate the temperature to sustain high phytoplankton growth is unnecessary .

Nutrients could also limit phytoplankton growth. On 28 occasions although Z'er > Z'mJ, algal biomass level was low i.e. < 10 mg chI a m'2 on II occasions each at sts. 14 and K6, 4 times at st. 23 and twice at st. 16 (Fig. 2). The choice of < lOa mg chi m 'l as an indicator of low algal biomass is justified based on the literature values (Table 5) . If light was not limit­ing, in' this tidally well-mixed bay there was depletion of silicate on 2 occasions or nitrate on 4 occasions (Table 6). On these six occasions the prevailing phy­toplankton biomass levels were between 5.4 and 9.2 mg chi a m-2 comparable to the 3.8 and 9.9 mg chi a m' 2 observed during the other 22 events when either

Table 5-Comparison of integrated phytoplankton biomass and primary production in selected waters

Region Biomass Production Reference (chi a mg m'2) (g Cm,2 day' I )

Kuwait 3.8 - 113.4 Present study

Eastern tropical Pacific 8.8 - 60

Gulf of California 26.0-118 0.7 - 4.6 Gaxiola-Castro et a/. 27

Gulf of California 23 .7 - 62.8 0.5 - 1.9 ?X Valdez- Holguin et a/.-

Gulf of Tehuantepec, Mexico 4.5 - 135 .5 0.7-1.4 Robles-Jarero & Lara,Lara2'}

Gulf of Mexico 3.3 - 22.7 0.02 - 4 .2 EI-Sayed & Turner 30

Gulf of Thailand 15.0 - 24 0.4 - 0.8 Subba Rao 3J

San Antonio Bay, Texas 0.1 - 2.5 Macintyre & Cullen .12

Gulf of Thailand 0.4 - 1.5 Steemann Nielsen & Jensen .1.1

Southwest coast of India 7.6 - 30.4 0.4 - 1.1 Radhakrishna 34

£'

SUBBA RAO & AL-Y AM ANI: PHYTOPLANKTON EUPHOTIC LAYER 421

Table 6-- Occasions when Z'eT> Z' mi ., and integrated chlorophyll a were < 10 mg m·2. (Nutrient levels in the column are in Ilmol)

St. no Date Z'ml Z 'eT

K6 02-13-85 4.2 15.3 05-20-85 4.2 18.5 06-24-85 8.4 18.9 01-19-87 8.4 12.5 03-19-87 8.4 16.5 05-18-87 5.6 18.5 12-06-87 8.4 11.9 01-20-88 8.4 12.5 05-23-88 1.4 18.5 10-17-88 3.4 15.5 04-01-89 5.6 17.8

14 02-11-85 1.8 15.3 04-14-85 1.6 17.8 05-21-85 1.5 18.5 06-25-85 0.9 18.9 08-04-85 1.6 18.4 11-04-85 2.3 13.6 02-05-86 2.7 15.3 04-07-86 1.2 17.8 04-14-87 2.1 17.8 05-18-87 1.5 18.5 07-12-87 1.5 18.1

16 03-18-85 2.5 16.5 04-10-90 2.5 17.8

23 06-29-88 2.1 18.9 03-21-89 1.9 16.5 05-22-89 l.l 18.5 06-19-89 2.3 18.9

Ud=Undetectable; L=Sample lost

nutrient was not depleted. This suggests these nutri­ents do not in general seem to restrain algal abun­dance. Some factor not determined by us must have limited phytoplankton biomass. It is of interest to note that off Mandovi and Zuari Rivers, off Goa, on the west coast of India, on several occasions the euphotic zone extended right to the bottom13

• It is of interest to note that growth of phytoplankton cultures from the turbid northern waters of the Arabian Gulf off Busher could be stimulated to an optimal level over a tem­perature range of 12-34°C even in media with low nitrate and high phosphate 11. According to these authors II, these perpetually bright sunlight turbid waters receive on the average 194 W m-2 similar to that needed for spring phytoplankton bloom initiation in nutrient rich temperate bays I9.20. Observations in the coastal tropical waters off Cochin l2

.21 showed

availability of sufficient light for sustaining phyto­plankton growth. Based on their data12 for Cochin backwaters we calculated compensation light (i.e. the

ChI a NO)- N Si02 - Si P04-P

9.3 L L L 8.2 <0.1 4-8 0.2 - 0.3 9.2 <0.2 3 0.2 - 0.4 5.6 0.2 - 0.4 < I 0.3 - 0.4 4.5 1.0 - 1.5 L 0.3 - 0.4 9.9 0.2 - 0.6 8.9 - 9.3 0.4 - 0.5 9.6 0.1 13.7 - 13.9 0.1 - 0.4 7.7 < 0.1 - 0.3 <10 0.1 - 0.2 4.8 <0.10 <5 0.1 - 0.3 8.9 0.6 - 0.8 Uct 0.3 - 0.6 6.9 Ud <I 0.1 - 0.2 6.3 L L L 9.8 <0.2 <7 0.1 3.8 0.1 - 0.3 <2 <0.1 9.4 <0.1 I - 3 0.1 7.7 0.1 - 0.3 2-5 0.2 7.4 <0.1 <7 0.1 7.0 0.2 - 0.3 0.5 0.1 - 0.2 5.6 <0.1 <7 0.1 - 0.3 5.4 <0.1 Ud <0.1 6.5 <0.2 4-5 0.1 - 0.2 8.5 Trace 3-4 Trace 8.2 L L L 7.6 L L 0.1 - 0.2 8.6 Ud 3-4 <0.1 9.2 Ud <I <0.1 7.4 Uct <I <0.1 9.9 <0.1 <I <0.1

lower limit of light which is usually I % of the surface radiation where photosynthesis equals respiration) and the optimum light for maximum photosynthesis. The former ranged from 1.41 W m-2and 3.53 W m-2

while the latter between 70.7 and 106.1 W m-2. In Kuwait waters measurements with a Seabird elec­tronics profiler 25 yielded light 83-275 W m-2 near the bottom (AI-Yamani unpublished) sufficient for algal growth. It is possible that the optimum light for maximum photosynthesis was> 275 W m-2 compara­ble to the range 139.6 and 418.8 W m-2 determined for 10 species of tropical phytoplankton cultures21

•

It is of interest that phytoplankton biomass levels > 10 and up to 79.65 mg chi a m-2 were observed once each at sts. K6, K 10, 24 and 9 events at st. 6 when Z'er < Z'm1. Bulk of the algal biomass was from samples taken from mid- and near bottom which suggests ac­cumulation of phytoplankton below the euphotic layer. Reasons for such an accumulation remain un­known.

422 INDIAN J. MAR. SCI., VOL. 28, DECEMBER 1999

Anthropogenic actIvItIes appear to affect phyto­plankton biomass in the nearshore waters. At sts. 14, 16 and 23 maximum chlorophyll values corresponded to 18.45, 27.0 and 27.26 mg chi a m·2

, considerably lower than at other stations. In fact higher biomass levels were observed even in offshore waters. These nearshore stations are located off an industrial belt. Perhaps effluents from several oil refineries, oil loading terminals (Mina Abdullah, Mina AI-Ahmadi and Mina Az-Zor) and power station are dampening the growth of phytoplankton. It is reasonable to hy­pothesize that continuous environmental perturba­tions resulted in sustenance of algal biomass at a much lower level in this nearshore zone, than in the offshore. This needs be substantiated by comparative data based on precise bioassays and ecophysiological studies of algae isolated from these two areas.

The correlation coefficient based on the Spearman Rank for the sign trend test22 between the scaled criti­cal depth and integrated chlorophyll a was insignifi­cant at all stations except at st. 18. Only at this off­shore st. 18 it was significant (-0.85, P < 0.001) and negative. This suggests that at 10 stations phyto­plankton abundance depended on factor(s) other than light; only in the offshore waters with an increase in the Z'cr algal standing crop decreased.

Our data suggest absence of pronounced seasonal evolution of phytoplankton growth. Slight increase in biomass is indicated sometime during March-May, and August and October-December (Fig. 2 sts. K6,14, and 18). However, there were no spectacular and sharp increase in algal biomass, characteristic of wa­ters from other regions in the ecological equatorial zone23

. Unique to this distinct arid zone biotope are the absence of seasonal upwelling, seasonal (April -May) run off from major rivers and lack of monsoon . h l" • bl 14 2S H Impact t at act as lorcmg varIa es - , , owever, data from sts, K6 and K10 which are more abundant than the rest suggest subtle interannual variations in algal biomass (Fig. 2- st. K6). At st. K6, algal bio­mass maxima (mg chi a m·2

) for the different years ranged between 21.3 (1988) and 30.6 (1989). The time of their attainment varied i ,e. December (1985), April (1986, 1987), March (1988, 1989) and August (1989). Phytoplankton biomass levels were usually higher at st. K 10 than at st. K6 as evident from their maxima for the different years. The mg chI a m·

2

ranged from 27.2 (1985) to 113.4 (1987), The timing of these annual high values differed similar to K6: for example during December (1985), May and August

(1986), August and November (1988), March and May (1989) and April (1990), However, at certain localities in this homogeneous, shallow, eutrophic well-lit waters off Kuwait, a potential for a high level of sustained primary production exists.

Integrated chlorophyll a levels in Kuwait waters (3.8 to 113.4 mg chi a m·2

) are of the same order of magnitude summarized for other waters (Table 5 ref.26

-34

). Integrated primary production rates for wa­ters off Kuwait do not exist. However, estimates of daily primary production based on several empirical relationships between total chlorophyll and day length for these waters ranged between O. I and 0.4 g C m·3 day"' during January - March4 and 0.9 g C m·3 day"' for March-Ma/. Note the availability of a high level of algal biomass, photosynthetically active radiation and nutrients in these coastal waters. Tbese being the essential ingredients of primary production, it is reasonable to sustain that a high magnitude of phytoplankton biomass and primary production com­parable to the values for several coastal waters (Table 5, ref. 26.34) can be sustained off Kuwait. More recent data3S yielded some of the highest chi a and produc­tion values (55.4 - 500.7 f-lg chi a r l

, 6.09 - 6.85 g C m·3 day"1 ).

A word of caution about present data is in order. Because our sampling frequency was low, particU­larly during certain seasons, a possibility exists for misrepresentation of the actual seasonal cycle of phytoplankton. The lack of any bloom suggested by low chlorophyll a at sts. K14, KI6 and K23 serves as an example. It is also probable that grazing by zoo­plankton including the microzooplankton or by the bottom fauna or loss due to lateral advection account for the low biomass levels. Similar to the observa­tions of Storm & Storm36 from the eutrophic coastal waters of northern Gulf of Mexico, in the waters off Kuwait also microzooplankton community grazing could be a significant source of phytoplankton mor­tality. Although the combined area of coral reefs off Kuwait'7 is about 4 km·2

, grazing pressure by zoo­plankters emerging at night may be extensive and the energy flow through this community is high3x. Impact of microzooplankton and zooplankton grazing in regulating the phytoplankton standing crop remains to be evaluated.

For development of mariculture in bays and inlets several physical and numerical models are used to predict the dispersal of nutrients and pollutants. Ad­ditionally, models utilizing biological criteria to

SUBBA RAO & AL-Y AMANI: PHYTOPLANKTON EUPHOTIC LAYER 423

assess the quantity of algal biomass that can be sus­tained and factors that govern its production are es­sential. In the Gulf, this can be accomplished through studies based on improved systematic sampling to resolve the role of nutrients on algal growth and short-term dynamics of phytoplankton production. A multidisciplinary ecosystem approach utilizing robust field data is required. Comparative studies on the physiological response of natural assemblages of in­shore and offshore phytoplankton to a gradient of pollutants are crucial to our understanding of the photosynthetic functioning of this unique environ­ment, and are recommended.

Acknowledgement We thank our colleagues who have contributed to

the collection of the data. We thank Dr. Sulaiman Almatar, Manager, MFD for encouragement. Grateful thanks are due to Mrs. Bala T. Durvasula for expert assistance with graphics and data processing and to Mr. S. J. Smith, Bedford Institute of Oceanography, Dartmouth, Canada and Dr. Yimin Ye, MFD, for sta­tistical advice. We thank Mr. P. G. Jacob for com­ments on an earlier version.

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Reports submitted to the Government of Kuwait, Ministry of Electricity and Water. (Dames & Moore, Mew consultant: Chas T. Main International Inc. Kuwait) 1983.

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3 Jacob P, Zarba M A & Anderlini V, Mahasagar.- Bull Natn Inst Oceanogr.13 (1980) 325.

4 Jacob P, Zarba M A & Anderlini V. Indian 1 Mar Sci. 8 (1979) 150.

5 Valiela I, in Fundamentals of aquatic ecology. edited by R S K Barnes & K H Mann, (Blackwell Scientific Publications, London) 1991 , 26.

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