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however, peaks in DMS (9.7 nM of DMS at 41 m) and DMSPt (DMSPt maximum of 31.3 nM) were found around a depth of 40 m (Fig. 4). Further, DMS and DMSPt levels were detectable up to 150-175 m. This occurrence of DMS at deeper depths is unique to the central regions of the Indian Ocean (because of deep mixed layers) and contrasts with that in the Arabian Sea and the Bay of Bengal.

Table 1 lists DMS and DMSPt variations as averages in the surface mixed layer. In the coastal

(southwest coast of India) Arabian Sea the highest values occurred during the SWM and the least were during the intermonsoons. In the Bay of Bengal the concentrations of DMS and DMSPt near the coast are almost same as the open ocean. The DMS and DMSPt concentrations in the Central Indian Ocean are nearly similar to those in the Bay of Bengal. On the whole, the Arabian Sea (coastal + open ocean) surface mixed layer contained more than double DMS and DMSPt (7.8 nM and 43.6 nM,

Fig. 3 Profiles of temperature, nitrate and chlorophyll (at 9.94°N, 72.44°E, SK158) showing subsurface chlorophyll maximum

Fig. 3 Profiles of temperature, nitrate and chlorophyll (at 9.94°N, 72.44°E, SK158) showing subsurface chlorophyll maximum respectively) than the Bay of Bengal (2.8 nM and 12.6 nM, respectively) and central Indian ocean (2.7 nM and 10.1 nM, respectively) and followed the same trend as that of the primary production (Behrenfeld and Falkowski, 1997).

Spatial variations

Figure 5 depicts the variation of DMS in the Arabian Sea, along a section near 15°N (off Goa), during January, July, October and December of 1998. Very clear coastal to open ocean trends were observed. In January (winter), the DMS concentrations varied between 6 and 12 nM within 50 m of the water column near the coast but between 200 and 400 km away from the shore the concentrations ranged between 2 and 4 nM before increasing to 10 nM further offshore. During summer (July 1998), DMS values were higher near the bottom in coastal waters (29 nM), and decreased towards the open ocean. Features similar to those in summer were seen in October 1998 when DMS concentration levels near the coast varied between 15 and 40 nM while in offshore waters these were low. In December 1998 coastal DMS concentrations were around 30 nM but decreased offshore. Thus in the eastern Arabian Sea high DMS concentrations were found closer to the coast.

Data obtained through periodic sampling along a small transect (-13 km, Fig 1b) off the Candolim

Fig. 4 Typical vertical profiles of DMS and DMSPt in different regions of the Indian Ocean

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