Gupta¹, Shyam M., and Rahul Mohan²

¹National Institute of Oceanography, Goa 403 004, India, Tel. 091 832 2456700 *4320, Fax: 091 832 2456702, E-mail: smgupta@csnio.ren.nic.in and ²Geology Department, Benaras Hindu University, Varanasi, 221005, India

Fluxes of monsoon sensitive radiolarian species in the Bay of Bengal during 1991-93

Radiolarian fluxes from the sediment traps during 5 December 1990 to 5 October 1993 exhibited flux-maxima between January-February-March and May-June-July, corresponding to late northeast and early southwest monsoons in the southern Bay of Bengal. Seasonal variations in the sea-surface temperature (SST), salinity and productivity are the main factors for such distribution. Higher SSTs (>29 ^oC) associated with moderate salinity (>33.5) during April-June characterizing pre-monsoon heat and associated rains, and lower SST (<28.5^oC) and lower salinity (<33.5) during July-September characterizing monsoon rains resulting in cooler SSTs at the trap site. Radiolarian fluxes were higher during January-March, and May-June (early to torrential rains in August-September). Particulate organic carbon and opal contents, a measure of productivity, from the traps varied from 0-18 mg/m²/day and from 0-125 mg/m²/day respectively, exhibiting highest values between July-September 1992. Such high organic carbon and opal fluxes were negatively related to radiolarian flux and could be due to higher monsoon runoff from the floods culminating before the on set of EL-NINO in December 1992 in the eastern Pacific. Distribution (%) of monsoon sensitive Pyloniids radiolarians exhibited maximum between 9 April-5 May, and around 18 July and 6 September 1991; 8 February, 31 March, 26 April and 22 May 1992; and January-March and May–July and early October 1993. Spongaster exhibited maxima during December 1990, April-May and July-August 1991; February-March, May-June, and November 1992; and April and July 1993. Spongodiscids peaked up during February-April 1991, 1992 besides a second maximum during June-July 1992; and exhibited a broad peak from February-June 1993. Euchitoniids were maximum during December 1990, February-April 1991, February-April, and August-September 1992; and January-February and July-September 1993. Anthocyrtidium abundance were higher around 9 April 1991, February to April 1992, and during January, July, and October 1993. Results suggest that radiolarian fluxes are due to oligotrophic conditions and not because of nearby weak upwelling off Sri-Lanka during July-August and off Sumatra during November–January in the southern Bay of Bengal.