GOPALAKRISHNAN: ZOOGEOGRAPHY OF NEMATOSCELIS 



the Arabian Sea (see Wyrtki, 1971:38). Isotherms 

 slope southwestward toward the African coast. 

 During this period no upwelHng is observed off the 

 southwest coast of India, although the Ekman 

 theory would predict upwelling in this area since 

 the surface winds blow equatorward and parallel 

 the coast. Taft (1971) suggested that this may be 

 due to the low speed of the northeasterly wind. 



In the Bay of Bengal upwelling has been re- 

 ported to occur seasonally along the east coast of 

 India and west coast of Burma (La Fond, 1954, 

 1957; Banse, 1960; La Fond and La Fond, 1968). 

 During the early part of the NE monsoon, the 

 northeasterly winds displace subsurface water 

 offshore along the Burmese coast. Wyrtki (1961) 

 also reported upwelling along the coast of Burma 

 and Thailand during the December-February 

 period. The general surface circulation of the Bay 

 of Bengal is clockwise during January through 

 July and it becomes counterclockwise from Au- 

 gust to December (La Fond and La Fond, 1968). 

 The southwesterly winds prevail over the north- 

 ern Bay of Bengal in March and over the entire 

 region in April. During these months an intense 

 upwelling is noticed along the east coast of India 

 (La Fond and La Fond, 1968). In this region, along 

 with the nearshore occurrence of dense water, the 



average sea level also reaches a minimum height 

 during this period (La Fond, 1954). 



A remarkable feature of the Indian Ocean is the 

 rapid attenuation of dissolved oxygen with depth; 

 at 200 m values become 0.2 ml/1 or less in the 

 Arabian Sea and Bay of Bengal (Wyrtki, 1971). 

 Vinogradov and Voronina (1961) and Vinogradov 

 (1968) have discussed the association of low zoo- 

 plankton biomass within the oxygen minimum 

 layer in the Arabian Sea. North-south and east- 

 west transects in the Arabian Sea indicate that 

 oxygen values as low as 0. 1 ml/1 occur below 200 m 

 during both seasons (Figure 2A, B). According to 

 Wyrtki ( 1971) the concentrations of oxygen in the 

 minumum layer do not vary seasonally, although 

 the depth of this layer changes. The vertical 

 profile at four north-south transects across the 

 equatorial Indian Ocean show the occurrence of 

 very low oxygen concentrations below 100 m (Fig- 

 ure 3). Since these profiles were taken during dif- 

 ferent months, it appears that this feature persists 

 throughout the year. 



The vertical distribution of temperature along 

 four north-south transects (same transects as 

 shown in Figure 3) indicates that the thermocline 

 is situated between 75 and 100 m in the regions 

 north of lat. 10°S. The South Equatorial Current 



4000 



Figure 2. — Vertical distribution of oxygen in the 

 Arabian Sea: A. East-west transect along lat. 

 11°N, long. 45°E to lat. 16°S, long. 72°E (July- 

 August); B. North-south transect along lat. 24°N, 

 long. 60°E to lat. 20°S, long. 67°E (November- 

 May). (Charts reproduced from Wyrtki, 1971.) 



1045 



