Influence of cyanobacterial hyperscum on heterotrophic activity of planktonic bacteria in a hypertrophic lake.

The response of the planktonic heterotrophic bacterial community to the buildup and breakdown of a semipermanent, crusted, floating cyanobacterial mat, or hyperscum, that covered 1 to 2 ha was studied in a hypertrophic lake (Hartbeespoort Dam, South Africa). The initial response of bacteria in the main basin to the release of dissolved organic carbon (DOC) from the hyperscum 1 km away was an increase in activity per cell from 35 x 10 to 153 x 10 mug of C cell h for total cell counts, while activity per cell for metabolically active cells increased from 19 x 10 to 85 x 10 mug of C cell h. No major population growth occurred at this stage. Later, with the continuous supply of DOC from the hyperscum, total bacterial numbers increased from 6.6 x 10 to 20 x 10 cells ml, while the activity per cell declined. Metabolically active bacteria followed the same trend. Shorter-term DOC increases caused only increases in bacterial activity per cell. The data from Hartbeespoort Dam demonstrate an interesting and little-documented mechanism by which aquatic bacteria respond to increased DOC concentration and which may be universal for aquatic systems.