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Marine Ecology Progress Series

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MEPS 274:69-86 (2004)  -  doi:10.3354/meps274069

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

Torkel Gissel Nielsen1,*, Peter Koefoed Bjørnsen1, Pensri Boonruang2, Michael Fryd3, Per Juel Hansen4, Vudhichai Janekarn2, Vitthaya Limtrakulvong2, Peter Munk5, Ole Schou Hansen1, Suree Satapoomin2, Suchat Sawangarreruks2, Helge Abilhauge Thomsen3, Jette Buch Østergaard3

1Department of Marine Ecology, National Environmental Research Institute, Frederiksborgvej 399, PO Box 358, 4000 Roskilde, Denmark
2Phuket Marine Biological Center, PO Box 60, 83000 Phuket, Thailand
3Botanical Institute, University of Copenhagen, Øster Farimagsgade 2D, 1353 Copenhagen, Denmark
4Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark
5Department of Marine Ecology and Aquaculture, Danish Institute for Fisheries Research, Kavalergaarden 6, 2920 Charlottenlund, Denmark

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 established 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.

KEY WORDS: Tropical ecology · Internal wave · Biological oceanography · Bacteria · Microbial foodweb

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