Journal of Oceanography, Vol. 53 (No. 5), pp. 435-442, 1997
Takeshi Naganuma and Shoichi Miura
Faculty of Applied Biological Science, Hiroshima University, 1-4-4 Kagamiyama, Higashi-hiroshima 739, Japan
(Received 25 October 1996; in revised form 18 April 1997; accepted 20 April 1997)
Abstract: The abundance, specific growth rate, estimated production rate and viability of bacterioplankton were determined in surface waters of the Seto Inland Sea, Japan. Bacterioplankton were counted by epifluorescence microscopy, stained with either acridine orange (AO) or two-color dye mixture to differentiate live/dead cells based on membrane integrity. The AO-based abundance showed area-to-area variation corresponding to eutrophication levels of the areas. The abundance varied from 4.0 ×108 to 2.2 ×109 cells l-1, and the overall average was 9.0 ×108 cells l-1. Specific growth rate varied from 0.037 to 0.174 h-1, with an overall average of 0.125 h-1. Bacterial production rate, estimated by multiplying the abundance and growth rate, was within the range of 24 to 172 mg C l-1 day-1, with an overall average of 67 mg C l-1 day-1, which may correspond to or exceed phytoplankton primary production. A correlation of chlorophyll-a concentration with bacterioplankton abundance was shown, but not to bacterioplankton growth rate. The viability of bacterioplankton based on membrane integrity varied slightly from 61.8 to 75.1%, with an average of 69.7%. This viability estimate is in good agreement with those measured in coastal and inland waters by other protocols. The technique employed can serve as an alternative for estimating bacterial viability in natural waters. It is concluded that a large fraction of bacterioplankton in the Seto Inland Sea is viable and involved in bacterial production which may exceed the primary production of the area.