Hydrography, bacteria and protist communities across the continental shelf and shelf slope of the Andaman Sea (NE Indian Ocean)

Abstract

The hydrography and plankton community structure was investigated in the Andaman Sea off Phuket, Thailand. Two cruises were conducted in 1996, one representing the calm dry NE monsoon season (March) and the other representing the stormy and rainy SW monsoon season (August). Sampling was performed along 3 transects perpendicular to the shelf break, from the coast across the shelf into deep water. The water column at the nearshore stations was vertically mixed, while the water column at off shore stations was strongly stratified, hence a frontal zone was estab- lished at the mid shelf. A prominent feature of the area was the pronounced internal wave centred around the pycnocline. The wave was observed from the outermost stations to the mid-shelf front. The height of the wave reached peak values of approximately 60 m in areas of approximately 300 m bottom depth. At all stations in stratified waters the vertical distribution of the phytoplankton showed a pronounced subsurface chl a peak in association with the pycnocline. The highest chl a values and primary production was observed at the front established at the mid shelf where the pycnocline meets the bottom, and salt nutrient-rich water is mixed up in the surface layer. We did not find any relationships between hydrography and the other key components of the microbial food web. No difference in productivity or food web structure was observed between the 2 seasons despite a significant difference in climatic forcing. Pico- and nanoplankton dominated the biomass in both seasons and Synechococcus contributed 72 to 74% of the biomass. Analysis of the microbial food web and establishment of carbon-flow budgets illustrates the importance of the microbial food web for making the primary producers available to the higher trophic levels.