SHORT-TERM SEDIMENTATION PATTERNS IN THE NORTHERN INDIAN-OCEAN

The flux of particles from the photic zone was monitored in one open o cean and two shelf stations in the northern Indian Ocean by means of d rifting sediment traps in the intermonsoon period, from March to June 1987. Samples were collected over daily intervals and analysed for org anic carbon, nitrogen, total phosphorus and silica. Flux rates of all elements differed by up to a factor of 10 between the Oman shelf and t he 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, sed imentation rates were in the same low range as in the open ocean. Thes e 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 expor t. In the open ocean and the Pakistan shelf, typical tropical recyclin g systems retained material by intense regeneration of nutrients in th e surface layer. These differences also were reflected in the composit ion of the sedimenting particles. Changes in production-respiration eq uilibria in the photic zone lead to rapid shifts in the carbon /silica and carbon /nitrogen ratios of trapped material. Thus short term sedi mentation measurements can provide valuable information on structural and functional variations in pelagic productivity.