Bacterioplankton activity in the surface waters of the Arabian Sea during and after the 1994 SW monsoon

Bacterial biomass and production were measured on two cruises to the northw estern Arabian Sea in 1994; the first cruise took place towards the end of the SW monsoon in September, and the second cruise during the inter-monsoon period in November and December. Although phytoplankton production was sig nificantly higher during the monsoon, bacterial numbers showed little diffe rence. Bacteria were most abundant in the euphotic zone and highest bacteri al numbers were measured during the monsoon period in the Gulf of Oman and the shelf waters off southern Oman; in these regions, numbers ranged from 0 .9 to 1.6 x 10(9) bacterial(-1). On both cruises, bacteria were less abunda nt in the euphotic zone of the central Arabian Sea and typically ca 0.8 x 1 0(9) cells l(-1) were present. The majority of bacteria (80-95%) were small cocci that were larger (median diameter 0.40 mu m) during the monsoon peri od than the inter-monsoon, when the cells had a diameter of 0.36 mu m; ther e was no comparable change in cell dimensions of bacteria present as rods. Bacterial production was measured by the incorporation of H-3-thymidine and H-3-leucine. On both cruises, uptake rates were highest on the Omani shelf and decreased offshore. In the central Arabian Sea, thymidine incorporatio n rates were similar in the monsoon and inter-monsoon periods, but higher r ates of leucine incorporation were measured during the monsoon period. Bact erial production was a relatively small proportion of phytoplankton product ion in both periods sampled; bacterial production was equivalent to between 10 and 30% of the daily primary production in the Arabian Sea. (C) 1999 El sevier Science Ltd. All rights reserved.