Heterotrophic bacterioplankton in the Arabian Sea: Basinwide response to year-round high primary productivity

Heterotrophic bacterial abundance and productivity were measured during fiv e and four cruises, respectively, in the northwest Arabian Sea as part of t he US JGOFS Process Study, which provided a new view of seasonal bacterial dynamics in that part of the basin influenced by monsoonal forcing. In this paper, surface layer data are used to address two questions concerning the influence of the monsoon cycle on bacterial dynamics: (1) Is there a bacte rial bloom in the SW Monsoon? and (2) Is bacterial production low during th e oligotrophic Spring Intermonsoon? An extensive comparison of epifluoresce nce microscopy and flow cytometry, unprecedented at this scale, detected es sentially the same heterotrophic bacterial populations and distributions, w ith some between-cruise differences. Use of the two methods allowed us to e xtend our observations in space and time. Bacterial productivity, both in t he surface layer and integrated over the euphotic zone, was elevated less t han 2-fold during the Southwest Monsoon. Levels of bacterial abundance and production were low during the Northeast Monsoon, then increased in March d uring the Spring Intermonsoon. There was some stimulation of abundance or p roduction inshore in response to coastal upwelling. In general, the basin w as enriched in bacterial biomass > 5 x 10(8) cells 1(-1) throughout the yea r, relative to other tropical regimes, presumably in response to overall hi gh PP and DOC levels. Seasonally uniform DOC levels may be regulated in par t by intense bacterial utilization rates, but also reflect seasonal consist ency in PP. (C) 2001 Elsevier Science Ltd. All rights reserved.