The flux of particles from the photic zone was monitored in one open ocean and two shelf stations in the northern Indian Ocean by means of drifting sediment traps in the intermonsoon period, from March to June 1987. Samples were collected over daily intervals and analysed for organic carbon, nitrogen, total phosphorus and silica. Flux rates of all elements differed by up to a factor of 10 between the Oman shelf and the open ocean area. Mean rates of carbon sedimentation were 13.6 and 1.7 mmol C m-2 day-1, respectively. On the Pakistan shelf, however, sedimentation rates were in the same low range as in the open ocean. These differences, particularly between the two shelf regions, were due to the different types of pelagic systems in the respective photic zones. The presence of nitrate in surface water of the Oman shelf permitted ''new'' production, which consequently led to enhanced particle export. In the open ocean and the Pakistan shelf, typical tropical recycling systems retained material by intense regeneration of nutrients in the surface layer. These differences also were reflected in the composition of the sedimenting particles. Changes in production-respiration equilibria in the photic zone lead to rapid shifts in the carbon /silica and carbon /nitrogen ratios of trapped material. Thus short term sedimentation measurements can provide valuable information on structural and functional variations in pelagic productivity.
