BACTERIOPLANKTON DENSITY AND ACTIVITY IN BENTHIC NEPHELOID LAYERS OF LAKE-MICHIGAN AND LAKE-SUPERIOR

Benthic nepheloid layers were observed in Lake Michigan and Lake Superior during July 1988 using the Johnson-Sea-Link II submersible. 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 cells were both related to temperature and organic carbon concentrations. The range of individual cell volumes (0.024-0.042-mu-m3) 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, estimated from field measurements of the frequency of dividing cells, increased from 29 h at 5 m to > 230 h at 310 m in the Caribou Basin of Lake Superior. The abundance, cell size, and activity of heterotrophic bacterioplankton in the deep hypolimnia of Lake Michigan and Lake Superior were unaffected by the presence of a benthic nepheloid layer.