Vertical distribution and activity of bacteria in the Central Arabian Sea

During the RV "Meteor" expedition in the Central Arabian Sea (MINDIK 87) the vertical distribution of particulate organic carbon and nitrogen, bacterial abundance, heterotrophic activity for glucose and the bacterial production determined by [3H-methyl]-thymidine incorporation were surveyed. At the time of observation (April-May) the study area was characterized by a stable pycnocline at 35 m and a nutricline at 50 to 55 m depth. Maxima for all biomass measurements were observed in the nutricline, whereas highest rates were detected in the nutrient-depleted surface layer above the nutricline. Based on these hydrographic conditions, a double vertical zonation was established also for microbiological and planktological events in the water column. The integrated bacterial production in the mixed surface layer (0-30 m) was half that in the layer underneath (30-100 m). This upper zone can be considered as a more or less closed sub-system and a significant amount of primary production (approximately 30 %) was channelled through the bacteria. Corresponding to the higher biomass in the lower zone the turnover of organic material was somewhat slower. This observation together with the presence of nutrients in the chemocline indicated a subsystem of more open character for exchange processes with underlying waters