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Pakistan Journal of Marine Sciences, Vol.3(1):25-30, 1994

SEASONAL AND SPATIAL DISTRIBUTIONS OF CHLOROPHYLL a IN THE NORTH ARABIAN SEA BORDERING

S.M. Saifullah Department of Botany, University of , Karachi-75270, Pakistan.

ABSTRACT: The average integrated chlorophyll a values for a 30-m deep surface layer in the North Arabian Sea 1 2 bordering Pakistan ranged from negligible amounts to as high as 0.53 p.g chl. a r (15.9 mg m" ) during the period January 20, 1977 to June 4, 1977. The values, in general, decreased offshore except for the westernmost part of the shelf, where unexpectedly high values were recorded over deep water. Seasonal distribution showed very high values in January (northeast monsoon season) which, with a few exceptions, gradually decreased to very low values in May, and then increased in June. The January peak may be related to winter cooling of surface waters resulting in convection and the June peak to the onset of southwest monsoon season in May. Coastal waters shallower than 30 m showed no seasonality and were often sites of intense phytoplankton blooms.

KEY WORDS: Chlorophyll a- distribution- seasonal- spatial - Pakistan.

INTRODUCTION

While there is significant information on chlorophyll a distribution from different parts of the Arabian Sea, the northern part bordering Pakistan still remains neglected (Banse, 1987). The area is well known for its upwelling and high productivity (Banse, 1984). Banse and McClain (1986) and Banse (1987) studied seasonality in chlorophyll a distribution in the Arabian Sea and reviewed all previous works, but again Pakistan's shelf area was neglected. Saifullah (1979) had intensively surveyed, for the first time, the entire Pakistan shelf and deep sea vicinity, but presented only maps showing chlorophyll a distribution at different depths without commenting upon the results or discussing its spatial and seasonal distribution patterns. The present paper also attempts to interpret the results on a water column basis rather than at individual depths. MATERIALS AND METHODS

A number of cruises were carried out on board the Norwegian ship "Dr. Fridtjof Nansen" in the entire Pakistan shelf area and vicinity in 1977. The area of study, shown in Fig.1, has been described elsewhere (Anonymous, 1978; Chaghtai and Saifullah, 1988). As many as 75 fixed locations, about 10 nautical miles apart, were repeatedly sampled during the several cruises from January 19, 1977 to June 4, 1977. In al1,576 water samples were collected from three depths: 1, 10 and 30m; the number of stations occupied during a single day varied between one and seven. In general, 2 litres of seawater were filtered through a millipore filter (0 .45 J.lm pore size) on board the ship immediately after collection. The filters were stored in the dark and deep frozen until the time of extraction and estimation (Saifullah, 1979). No estimation or correction for phaeophytin was made. The chlorophyll a values were integrated for the entire 30 m water column and then averaged per litre. This was necessary for comparison with other works. If more than one ~tation was occupied during a single day, the values at these stations were 26 Pakistan Journal of Marine Sciences, Vol.3(1), 1994

26

MAKRAN COAST

25 ~ ~ • (/) ~ ~ 24 .• • ,. 0 Ill o• • • • N • 23 • 1 N. ARABIAN SEA • • 22~--,------r-----~------~------~------r-----~~ 62 63 64 65 66 67 68% Fig. 1. Map showing 75 fixed positions which were repeatedly occupied during the cruises on Pakistan shelf and vicinity (dashed line indicates the boundary of shelf). pooled and divided by their number to give the average value for the day. Simultaneously recorded surface temperature and salinity values have already been described (Anonymous, 1978).

RESULTS AND DISCUSSION

The spatial distribution of chlorophyll a in a 30-m water column in the area of study is shown in figures 2-4. It is evident that the values, in general, decreased offshore with high values towards the coast and low ones towards the open sea. The same trend was shown at each three depths by Saifullah (1979). However, there was one exception, the westernmost part'ofthe area adjacent to the Makran shelf. There, values as high as 1.28 J.Lg r 1 and 0.7 J.Lg r 1 were recorded at the surface at two deep stations (23°50'N, 62°15'Eand24°20'N, 62°20'E) onFebruary 7, andMarch31, 1977 respectively (Saifullah, 1979), bringing the average integrated values to more than 0.4 J.Lg r 1 (Figs.2 and 3). Since the two stations were as deep as 3250 m and 2200 m, respectively, it is strange to find such high values in an otherwise relatively poor area. However, other observations support this. Saifullah and Chaghtai (1991) found Noctiluca scintillans (Macartney) Ehrenb. blooms in exactly the same position as the forementioned stations on March 4, 1977 suggesting that it was feeding on organic matter produced by the phytoplankton bloom. Haq eta!. (1973) also reported large concentrations of some species of zooplankton in the same area. All this suggests that Saifullah: Distribution of Chlorophyll a 27

KARACHI •

24 ~03~ ~·) .. . O·()/ __J 23 1 N.ARABIANSEA 0~ 22~--~----~------~------r----\~·~------r----~ 62 63 64 65 66 67 68°E' Fig.2. Distribution of average integrated chlorophyll a values in Jlg r 1 for the water column in the area during the period January 19, to February 9, 1977.

25

24

N 23 1

N. ARABIAN SEA

62 63· Fig.3. Distribution of average integrated chlorophyll a values in Jlg r 1 for the water column on the Makran shelf and vicinity during March 28, to Aprill, 1977. 28 Pakistan Journal of Marine Sciences, Vo1.3(1), 1994

MAKRAN COAST (A) (B)

25 N 1

24

-0·1

0·2 N 23 f\ ----... O·l. 0.05 1

65 66 6 8 65 66 67 68°E Fig.4. Distribution of average integrated chlorophyll a values in Jlg r 1 for the water column on Indus Delta shelf, during (A) April13 to 27, 1977 and (B) May 18 to June 4, 1977.

I INDUS DELTA : MAKRAN COAST INDUS DELTA l 0.7 I I 0.6 ... 2 I IV 0.5 4 5 2 2 J: 0.4 (.) J m 5 J :1 0.3 4 0.2 2 3 0.1 0.0 20 21 22 23 24 26 29 30 31 01 02 03 04 OS 06 07 08 0917 28:29 31 011416171819 20 23 24 25 262719 23 24 25 30 31 01 02 04

I I I I I JAN 1 FEB ! MAR; APR : MAY rJUN

Fig.5. Seasonal distribution of average integrated chlorophyll a values for a 30 m water column (shallower stations neglected) on Pakistan shelf and vicinity during the period January 20, to June 4, 1977. Numbers on bars indicate number of stations occupied on that day. Saifullah: Distribution of Chlorophyll a 29

3.0

1- ~ 2.5 (.) I lz INDUS MAKRAN I INDUS DELTA I ~~ DELTA COAST .I 2.0 ·~:::1:

'7 1.5 ::c~ (J C) :I 1.0

0.5

0.0 19 20 22 23 26 30 01 02 00 08 29 31 01 03 14 16 18 20 23 26 27 23 24 28 31 03 . ' JAN I FEB ~MAR! APR I MAY ! JUN Fig.6. Seasonal distribution of average integrated cholorphyll a values for the water column shallower than 30m, during the period of study. there is a pocket of high chlorophyll and, hence, of high primary production, in the deep waters of the Makran shelf at 24°N and 62°E. Detailed studies on the hydro­ graphy and nutrient chemistry of the area are, however, required to establish this. In general, chlorophyll a values in the area were higher than those in the central Arabian Sea (Krey, 1973; Bimse and McClain, 1986). This may be mainly due to the upwelling phenomenon prevailing there (Banse, 1984). The distribution is also more heterogeneous on the Indus Delta shelf than on the Makran shelf, most probably because of the discharge that disturbs the temperature and salinity regimes (Anonymous, 1978; Saifullah, 1979). The seasonal distribution of chlorophyll a showed high values in January with a generally decreasing trend towards May, with the exception of a two intermittent short 1 duration peaks in between. The January values were mostly above 0.4 Jig r , later 1 decreasing gradually to very low values (0.05 Jig r ) in May (Fig.5). At times the values were negligible and undetected at certain depths and stations. The high values in January may be attributed to winter cooling of surface waters, which leads to convective overturning (Banse, 1987). April and May mark the transition period between the nqrtheast and southwest monsoon seasons and are characterized by changing winds and increasing temperatures, which account for low production during the period (Banse, 1968). The peak observed on April27, 1977 was probably a result of the shallowness of the site as it occurred very close to shore. The other peak on March 31, 1977 occurred in the productive area of the Makran shelf (Fig.2). 30 Pakistan Journal ofMarine Sciences, Vol.3(1), 1994

A resurgence of very high values was noted in the beginning of June, which may be related to the onset of southwest monsoon season in late May (Currie et al., 1973). The cruise ended after this period and, therefore, information during the southwest monsoon season is lacking, which is supposed to be the most productive period in Northern Arabian Sea (Banse, 1987). Coastal areas shallower than 3 0 m were often sites of intense phytoplankton bloom as depicted by very high chlorophyll a values at several locations and occasions (Fig.6; see also Saifullah, 1979). Such areas do not show any seasonality in values because they are more affected by their proximity to land (land mass effect) than seasonal changes in climatic factors.

ACKNOWLEDGEMENT

The author is indebted to Professor Dr. K. Banse, of University of Seattle, Washington for his suggestions on the interpretation of the results.

REFERENCES

Anonymous, 1978. Survey results of''Dr. FridtjofNansen", January-June 1977, Joint Norad/Pakistan Project, Fish Assessment Survey ofPakistan Waters, Bergen. Pp.1-12. Banse, K. I 968. Hydrography ofthe Arabian Sea shelf ofindia and Pakistan and effects on demersal fishes. Deep Sea Research 15: 45-79. Banse, K. 1984. Overview ofthe hydrography and associated biological phenomena in the Arabian Sea, offPakistan. In: Marine Geology and Oceanography of Arabian Sea and Coastal Pakistan (Eds. Haq, B.U. and J.D. Milliman). Van Nostrand Reinhold, N.Y. Pp.271-303. Banse, K. 1987: Seasonality of phytoplankton chlorophyll in the central and northern Arabian Sea. Deep Sea Research 34: 713-723. Banse, K. and C.R. McClain. 1986. Winter blooms of phytoplankton in the Arabian Sea as observed by the coastal zone colour scanner. Marine Ecology-Progressive Series 34: 201-211. Chaghtai, F. and S.M. Saifullah.1988. An illustrated account ofspecies ofCeratium Schrank found inN. Arabian Sea bordering Pakistan. C.E.M.B. , Publication No.5. Shamim Printing Press, Karachi. Pp.l-50. Currie, R.I., A.E. Fisher and P.M. Hargraves. 1973. Arabian Sea upwelling. In: The Biology ofthe (Eds. B. Zeitzschel and S.S. Gerlach). Springer-Verlag, N.Y. Pp.37-53. Haq, S.M., J. Alikhan and S. Chaghtai. 1973. :The distribution and abundance of zooplankton along the coast of Pakistan during postmonsoon and premonsoon periods. In: The Biology ofthe Indian Ocean (Eds. B. Zeitzschel and S.S. Gerlach). Springer-Verlag, N.Y. Pp.257-272. Krey, J. 1973. Primary production in the Indian Ocean. I. In: The Biology ofthe Indian Ocean (Eds. B. Zeitzschel and S.S. Gerlach). Springer-Verlag, N.Y. Pp.115-126. Saifullah, S.M. 1979. Occurrence of dinoflagellates and distribution of chlorophyll a on Pakistan shelf. In: Toxic Dinoflagellate Blooms (Eds. D.L. Taylor and H.H. Seliger). Elsevier/North Holland, N.Y. Pp.203-208. Saifullah, S.M. and F. Chaghtai. 1991. Incidence of Noctiluca scintillans (Macartney) Ehrenb., blooms along Pakistani shelf Pakistan Journal ofBotany 22: 94-99.

(Received: 4 October 1993)