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Chlorophyll Profile of the Euphotic Zone in the Lakshadweep Sea During the Southwest Monsoon Season

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Chlorophyll Profile of the Euphotic Zone in the Lakshadweep Sea During the Southwest Monsoon Season Indian J. Fish., 44(1) : 29-43, Jan.-Mar., 1997 Chlorophyll profile of the euphotic zone in the Lakshadweep Sea during the southwest monsoon season V.K. BALACHANDRAN, C.P. GOPINATHAN, V.K. PILLAI, A. NANDAKUMAR AND K.K. VALSALA Central Marine Fisheries Research Institute, Cochin - 682 014, India ABSTRACT The distribution of chlorophyll a, b and c, the inorganic phosphate and nitrite of the euphotic zone at 56 locations in the Lakshadweep Sea was studied during the southwest monsoon period of 1990. The average chlorophyll a concentration varied from 4.62 mg/m^ at the surface to 2.23 mg/m^at 50 m depth with a column mean of 161 mg/m^ in the euphotic zone. The chlorophyll b averaged to 2.28 mg/m^ at 10 m to 2.79 mg/m^ at 50 m depths and 115 mg/ m^ in the column. The chlorophyll c varied from 4.03 mg/m^ at the surface to 4.97 mg/m^ at the 20 m depth and showed an average value of 202 mg/m^for the column. The chlorophyll c was the dominant pigment in the water column throughout the period of study. The average ratios of chlorophyll c/a and b/ a fell in the range of 1.3 - 2.4 and 0.80 - 1.3 respectively and were positively correlated with depths in the euphotic zone. The inorganic phosphate values fluctuated from 0.65 fig at/1 at surface to 1.36 |ig at/1 at 50 m depth with a mean of 49 mg at/m^ in the water column. The nitrite concentration varied widely, very often below the level of detection. Introduction waters also coincide with the same ^ . „ , , ^ , period. Hence the study pertaining to It IS well known that compared to ^^^ availability of photosynthetic pig- other seasons, the southwest monsoon ^^^^^^ ^^^ j^^^.^ components of stand- 1^ highly productive in the Arabian Sea^ . ^^ phytoplankton in the food This IS attributed to the phenomenon of ^^^^^ ^^^. ^^^ monsoon season in upwelling together with the large influx ^-^^^^ ^^^^^^ -^ ^^ ecological of nutnent-rich river water into the sea. imnortance If other factors like, temperature and cloud cover are not hmiting, the abun- While considerable information is dance of phytoplankton increases the available on the physico-chemical basic productivity. The spawning sea- and biological characteristics of the son of many of the commercially impor- Arabian Sea, (Ramamirtham and tant finfishes and shellfishes in these Jayaraman, 1960; Jayaramane^aZ., 1960; V. K. Balachandran et al. 30 Patil et al., 1963, 1964; Sharma, 1966; Radhakrishna, 1969; Qasim, 1977,1978, 1982; Qasim et al., 1972; Bhattathiri and Devassy, 1979; Pant, 1979, 1992) information on the primary producti­ vity during the southwest monsoon period from these waters is very lim­ ited. Subrahmanyan (1959) has re­ ported on the seasonal variation of surface phytoplankton and chlorophyll from the coastal waters of Calicut. Some information on nutrients and chlorophyll a during the southwest monsoon period in the shelf waters of the Arabian Sea have been given by Banse (1968). Krey (1976) has given a general picture of chlorophyll distribu­ tion in this area, and Sumitra Vijayaraghavan and &ishnakumari (1989) have reported on the distribution of chlorophyll a from the southeastern Arabian Sea during June-July 1987. Balachandran et al, (1989) have studied the surface chlorophyll a and pheophytin in the inshore waters off Cochin during 1987-'88. The present account deals Fig. 1. Sampling stations. with a detailed study on the chloro­ inshore stations and from surface, 10, phylls in relation to nutrients such as 20, 30 and 50 m depths for offshore inorganic phosphate and nitrite along stations with Nishkin bottles and the with a comparative study of relevant averages and column values were as­ hydrographic features in the euphotic sessed accordingly for each station. zone for July and August during the southwest monsoon season of 1990. Earlier investigations (Nair et al., 1973) conducted in this area using a Material and methods Tinsley's irradience meter with a deck The data presented in this paper cell and sea cell connected through a was collected during the cruises of ratio meter and galvanometer indicated FORV Sagar Sampada (Nos. 74, 76 & that 1% of surface light penetrates 77) from 56 stations spread over the between 60 and 50 m over the continen­ area 08° 30' N to 12° 30' N and 74° 25' tal shelf region on a normally bright E to 76° 55' E in the Lakshadweep Sea day. In the clear blue waters of the open during July-August 1990 (Fig. 1). The ocean it may even extend upto 90 m. number of observations were 15 and 41 Towards the coast where there is tur­ respectively for July and August. The bidity it shrinks to about 30 m on cloudy water samples were collected from days. So the compensation depth for the surface, 10, 20, and 30 m depths for shelf waters for most part of the year Chlorophyll profile of the euphotic zone 31 has been assumed to be 50 m. So the Strickland and Parsons (1968). The term 'euphotic zone' has been used for methods given by Murphy and Riley the productive zone over the shelf In (1962) and Bendschneider and Robinson the text Zs = denotes sampling depth in (1952) were followed for the determina­ the euphotic zone. tion of inorganic phosphate and nitrite respectively by colorimetric method The method of Parsons et al. (1984) using the spectrophotometer (Spectronic was followed for the extraction and 1001, Milton Roy). estimation of chlorophyll. Five hundred ml of the sample was filtered through 47 Results mm HA Millipore membrane filters of 0.45 |J, pore size. Ten ml of 90% acetone The results presented in this paper was used for extraction. Extinction are based on the pooled data for July values were measured at the wave­ and August unless otherwise stated. lengths of 630, 647, 664 and 750 nm The observed values are given in using a digital UVATIS spectrophoto­ Table 1. meter (Spectronic 1001) and the chloro­ Chlorophyll a phyll a, b and c were calculated using the formulae given by Parsons et al. The general distribution pattern of (1984). The column chlorophylls were chlorophyll a from surface down to Zs = assessed using the formula of Dyson et 50 m in the study area is given in Fig. al. (1965). Dissolved oxygen was deter­ 2a - 2e. Distribution of surface chloro­ mined by winkler method and salinity phyll a from south to north in the by titrimetric method as mentioned in Lakshadweep Sea showed (Fig. 2a) TABLE 1. The variations, average, ratio and the column values of chlorophyll a, b and c and phosphate in different depths of euphotic zone (0-50 m) in the Lakshadweep Sea during southwest monsoon season of 1990. (Average values are given in brackets) Parameters ! Surface 10m 20m 30m 50m Column values mg/m'' Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Min. Max. Chi. a 0.80 19.40 0.67 17.32 0.86 22.41 0.76 22.23 0.41 4.37 45 906 (mg/m^) (4.62) (3.43) ( 3.98) (2.76) ( 2.23) (161) Chi. b 0.81 11.06 0.06 5.05 0.57 9.55 0.32 7.26 0.52 7.27 36 258 (mg/m') (2.59) (2.28) (2.70) (2.60) (2.71) (115) Chi. c 0.73 15.0 1.23 12.66 1.39 16.22 0.90 12.60 0.62 11.63 71 447 (mg/m^) (4.03) (4.30) (4.97) (4.67) (4.79) (202) Ratios: Chi. c/a 0.18 3.84 0.30 3.59 0.20 3.49 0.26 5.44 0.86 3.43 - (1.29) (1.60) (1.75) (2.01) (2.39) Chi. b/a 0.07 1.94 0,03 1.69 0.07 2.06 0.04 2.33 0.42 2.31 - (0.82) (0.87) (0.97) (1.18) (1.28) mg at/m^ Phosphate 0.15 1.51 0.15 2.25 0.42 2.51 0.21 2.46 0.57 2.46 18 86 i\ig at P/1) (0.65) (0.83) (1.17) (1.25) (1.36) (49) V. K. Balachandran et al. 32 CHLOROPHYLL o mg/m' CHLOROPHYLL o mg/m SURFACE lOm dtpth \t' •• T4« SO' 7»* 10' T«' 30' 77' Ti" so' TV so' fe^ 15' f?« Fig. 2 a. Distribution of chlorophyll a at Fig. 2 b. Distribution of chlorophyll a at surface (July-August). 10 m photic depth (July-August). pockets of rich phytoplankton biomass 10) within the continental shelf, north with varjdng intensity at different loca­ of Quilon. tions such as north of Trivandrum The chlorophyll a vaired from 0.41 where the depth to bottom was within mg/m^at Zs = 50 m in St. 1 to 22.4 mg/ 40 m, north of Quilon (40 and 100 m m^ at Zs = 20 m in St. 10 in the euphotic depth zone), off Cochin (50 and 200 m zone. Though the maximum chlorophyll depth zone) and off Calicut (150 and 200 a recorded, was at Zs = 20 m, moderately m depth zone). South of Kasaragod, it high values of surface chlorophyll a was found progressively increasing to­ (19.4, 16.1, 14.5 and 13.9 mg/m^) were wards stations in deeper zones. More or noticed in stations 10, 19, 24 and 38 less similar trend was observed with respectively. The maximum average lesser magnitudes at the subsequent chlorophyll a (4.62 mg/m') was at lower depths in the euphotic zone.
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