Seasonal heterotrophic flagellate and bacterial plankton dynamics in a large oligotrophic lake - Loch Ness, Scotland

1. Bacterial production in the 0-30 m water column of Loch Ness was measured using a dual labelling procedure with [3H] thymidine and [14C] leucine between May 1993 and June 1994. In most cases the uptake of the two labels did not covary, suggesting unbalanced growth. Rates of bacterial production varied from undetectable to 46.2 μg C l-1 day-1. Highest production coincided with the period of highest primary production, but carbon derived from this source was insufficient to meet the bacterial carbon demand, which was met by allochthonous humic inputs to the system. 2. Heterotrophic flagellate (HNAN) grazing rates, measured using fluorescently labelled bacteria, ranged between 10.3 and 24.5 bacteria cell-1 day-1 at temperatures between 5 and 15 °C. They removed up to 27% of the bacterial production per day. 3. Heterotrophic flagellate specific growth rates ranged from 0.043 to 0.093 h-1 between 5 and 15 °C, giving generation times of 7.4-16.1 h. 4. bacterial and HNAN abundances were not coupled, but the highest HNAN grazing impact related to a time of high bacterial productivity.