Analysis of the relationship between phytoplankton biomass and...
Transcript of Analysis of the relationship between phytoplankton biomass and...
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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