This paper presents seasonal variation of planktonic bacterial density along the transect from Nha Phu lagoon to Binh . Cang and Nha Trang bays in 20092010 with the application of flow cytometry and fluorescent microscopy technique. Yearly average bacterial density were from 5.0 - 103.0 x 10 exponent 5 cells/m2 and showed clear different among stations, seasons and sampling moths. Bacterial density reached the highest peak in December (57.0 +/- 18.0 x 10 exponent 5 cells/m2) and another smaller peak in July. Bacterial density were high during wet season with northeast (NE) monsoon and dry season with southwest (SW) monsoon, 31.7 and 24.7 x 10 exponent 5 cells/m2 respectively; and low during the dry with northeast monsoon and dry with transitional monsoon (from NE to SW), 15.8 and 18.5 x 10 exponent 5 cells/m2 respectively. There is also different in density between surface and bottom layes of water column where stronger variation at surface (13.4 +/- 10.0 - 32.9 +/- 28,7 x 10 exponent 5 cell/m2) than at bottom layer (13.7 +/- 4.4 - 24.2 +/- 12.1 x 10 exponent 5 cells/m2). There were negative correlation of bacteria density with salinity and temperature while positive with suspended matter concentration, especially at stations in Nha Phu lagoon. Higher bacterial densities were associated with sa/ini(v from 31-32 percent0 alld temperaturefj'om 25.5-26.5°C. The HNA bacteria densi(v decreased from wet to dry seasons and always contributed larger proportion (50 percent) comparing to LNA bacteria. Investigation on fluorescent microscopy showed there were mainly 4 types of bacteria morphology including cocoid, rode, commar and spiral shapes. Ratio ( percent) among shese groups showed clear different between seasons. The cocoid bacteria contributed the largest proportion (61.0 -76.7 percent) among others.
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