Productivity of the Arabian Sea along the Southwest Coast of India

Monsoon plays a critical role in triggering environmental features such as seawater temperature, salinity, dissolved oxygen content and nutrient generation which in turn become responsible for production of phytoplankton and zooplankton. Along the southwest coast of India the intensity of southwest monsoon as evidenced by annual rainfall showed a declining trend during the years 1983 to 1988. An attempt is made here to correlate certain environmental features with the abundance and fluctuations in phytoplankton and zooplankton in the inshore waters off Cochin, Vizhinjam and Miiucoy. At Cochin, the annual total rainfall for the years 1986 to 1988 were respectively 2495,2410 and 2895 mm as against expected rainfall of around 323 cm per annum. During the monsoon months the rainfall was 1295,1490 and 1665 mm respectively. Generally, June received the maximum rainfall. At Vizhinjam the rainfall did not show any definite pattern. During 1984-85 the premonsoon period and during 1987-88 the postmonsoon period recorded more rainfall of about 490 mm. At Cochin, the levels of gross primary production showed increase from 0.830 to 1.624 g C/m'/day corresponding with increasing rainfall during April to June 1986. Again in 1987 similar increase of production from 0.597 to 0.975 g C/m'/day was observed to coincide with increasing monthly rainfall from May to June and in August. However, Chlorophyll a concentration in surface waters showed a decreasing trend during premonsoon, monsoon and postmonsoon months. This has been attributed to physiological state and productive potential of phytoplankters during the sampling period. The studies also revealed that productivity of 10 m station was around 50% of the productivity in the 20 m depth zone. Gross primary production also showed positive correlation with abundance of nutrients sudi as phosphates and nitrates in inshore waters which in turn is attributed to coastal upwelhng. The annual net primary production in the euphotic waters off Cochin was estimated as 731.43 tonnes Carbon/ km' indicating high productivity. Zooplankton biomass indicated higher volumes during monsoon months as also most of the constituent groups. Peaks of secondary production were observed in September of 1984 and 1986. While primary peaks of biomass abundance coincided with copepod maximum in July '85, '86 and August '88, the abundance of other groups contributed to the peak in July '84 and August '87 including blooms of Fragilaria oceanica or swarms of cladocerans or salps. The fluctuation in the abundance of various groups are discussed in detail. In terms of Carbon, the mean production in the area was worked out as 6.652 t C/kmVyear. The average displacement volumes at Vizhinjam recorded higher values during postmonsoon season and low values during premonsoon period. High salinity was observed to be a characteristic of premonsoon period. Within the overall range in salinity values, peaks of different plankton groups coincided with higher values of salinity. It was observed that landings of pelagic fishes were maximum during monsoon months, followed by postmonsoon months. In the Lakshadweep, zooplankton was observed to be maximum during premonsoon period and lower in other months. Zooplankton volumes were higher in the open sea than in the lagoons. INTRODUCTION production, initially at the primary and subsequently at the secondary and tertiary levels. It is well known that the production of phyto Among these, southwest monsoon in India is of and zooplankton in the sea has a great bearing on critical importance in the production of phyto and the fish yield. Environmental features such as monzooplankton especially in the inshore upwelling soon, upwelling, temperature, salinity and disareas. It has been known that an intense monsoon solved oxygen and nutrients play vital role in this triggers of strong upwelling along the southwest 10 M. S. RAJAGOPALAN et al coast of India. The studies carried out by Subrahmanyan (1959), Qasim and Reddy (1967), Nair et al. (1968), Radhakrishna (1969), Subrahnnanyan et al. (1975) have revealed that the phytoplankton production is at its peak during the southwest monsoon (June-September) all along the west coast. According to Silas (1972), the peak in the zooplankton production in the shelf area of the west coast of India occurs during June-October period. The later studies (Anon., 1976; Devidas Menon and George, 1977) have confirmed these findings. Mathew et al. (1989) have extended the period of high abundance for zooplankton in the the shelf waters upto December. Since 1983, the southwest monsoon has been a failure along the west coast. The year 1987-88 was specially considered as a period of weak monsoon. Consequently there has been marked decrease in the catch of pelagic fish especially the oilsardine. In this situation, an understanding of the effect of monsoon on the fish production along the west coast has become an essential prerequisite. For this purpose a comparative study of the various environmental and productivity parameters in the inshore areas at selected centres viz. Cochin, Vizhinjam and Lakshadweep has been made and the present paper embodies the results of these studies.