Bacterioplankton density and activity in benthic nepheloid layers of Lake Michigan and Lake Superior

A particularly well-defined benthic nepheloid layer was present in Whitefish Bay, Lake Superior, where particle concentrations increased 10 times over the bottom 30 m. Bacterial abundance, however, increased only 1.5 times in the same depth interval and a maximum of only 16% 1 m above the sediment in both lakes when the sediment boundary layer was deliberately resuspended. Bacterial abundance was greater in Lake Michigan than in Lake Superior. Bacterioplankton abundance and the frequency of dividing cell volumes (0.024-0.042 μm³) was similar in both lakes, regardless of depth. Less than 5% of the heterotrophic bacteria were actively dividing and only 2.0-3.4% of the bacterioplankton in the epilimnion and hypolimnion were synthetically active in both lakes. Bacterioplankton doubling times increased from 29 h at 5 m to >230oh at 310 m in the Caribou Basin of Lake Superior. -from Authors