Response of microbial community structure to environmental forcing in the Arabian Sea

The effect of environmental forcing on microbial community structure was investigated in the Arabian Sea during four seasonal cruises during 1995: late Northeast Monsoon (January); Spring Intermonsoon (March-April); late Southwest Monsoon (August-September); and early Northeast Monsoon (December). The distributions of picoplankton populations-heterotrophic bacteria (HBac), Prochlorococcus (Pro), Synechococcus spp. (Syn) and picoeucaryotic algae (Peuc)-were determined by flow cytometric analysis. Seasonal variations in abundance maxima, vertical profiles, integrated abundance (0-200 m), and estimated carbon biomass were contrasted along two transects from the coast of Oman to 1500 km offshore. HBac were numerically dominant in surface waters in all regions (1-3 x 10(6) cells ml(-1)), with higher maximum abundances in coastal waters than at offshore stations. Conversely, Pro were most abundant at the oligotrophic offshore stations, and 100-fold lower, or absent, at coastal stations, except during the Spring Intermonsoon when abundances were extremely high along the entire southern transect. Syn abundances were highly variable, with no consistent trend between coastal and offshore stations. This variability may be explained by the prevalence of mesoscale eddies, but also could be due to overlapping distributions of multiple Syn populations distinguished by pigment type. Syn with a low phycourobilin (PUB) to phycoerythrobilin (PEB) ratio pigment type were more abundant at coastal stations, whereas Syn with a high PUB:PEB ratio increased in abundance offshore. Average depth profiles for Pro, Syn, and HBac displayed uniform abundance in the surface mixed layer, with a rapid decrease below the surface mixed layer depth; however, during the Spring Intermonsoon most profiles had a peak at the base of the surface mixed layer. Distributions of Peuc typically displayed a subsurface maximum near the base of the surface mixed layer, except during the SW Monsoon when abundance peaked near the surface. This report is the first to describe the seasonal and spatial variation in microbial community structure in the Arabian Sea over a complete monsoon cycle. Overall, the eucaryotic component was more important at coastal stations, and the procaryotic components were predominant at offshore stations. Pro abundance was restricted to warm oligotrophic waters and was inversely related to surface nitrate concentrations; thus, an increase in the % Pro as a fraction of total procaryote abundance was also indicative of oligotrophic conditions. The effects of SW Monsoonal forcing on microbial community structure resulted in an increase of Peuc, but this response was limited to coastal stations. Pro and Syn remained dominant at offshore stations. (C) 1998 Elsevier Science Ltd. All rights reserved.