Bacterioplankton production in a water column of Lake Biwa

Seasonal production of bacterioplankton in a water column of Lake Biwa was examined based on cell-specific growth rates at above (surface layer) and below the thermocline (deep layer). The growth rates were estimated by incubating bacterio- plankton in situ with a dilution technique. The rates ranged from 0.05 per day in winter to 0.89 per day in summer, generally with much higher rates in the surface layer than in the deep layer. In an entire water column, bacterial production (in terms of carbon [C]) ranged from 0.217 to 0.811 gC/m² per day with a mean of 0.451 gC/m² per day, which, on average corresponded to 43% of primary production. However, no significant correlation was detected between bacterial production and primary production rates. Although the bacterial production rate correlated positively to water temperature, surface and deep layer rates were comparative for some dates because the deep layer shared a large fraction of the water column. These results suggest that, although specific bacterial activity was low in the deep layer due to the low temperature, bacteria in that layer play substantial roles in consumption of organic matter and material flows in Lake Biwa.