PROTOZOOPLANKTON AND BACTERIOPLANKTON IN A LARGE OLIGOTROPHIC LAKE - LOCH-NESS, SCOTLAND

The seasonal changes in the abundance of protozoan and bacterial plank ton in a large, coloured, oligotrophic lake, Loch Ness (Scotland), wer e investigated between August 1991 and January 1993. The coloured wate r supported only low concentrations of chlorophyll a (<1.6 mu g l(-1)) , with the highest values occurring in summer. Mean bacterial abundanc e ranged between 2.3 x 10(8) and 7.1 x 10(8) l(-1) in the 100 m water column. Maximum abundance did not correlate with maximum chlorophyll a concentrations. but appeared to be related to the input of allochthon ous carbon from the catchment, which in turn was influenced by rainfal l levels. Consequently, the highest bacterioplankton concentrations oc curred in autumn and winter. The pattern of heterotrophic nanoflagella te abundance tended to follow that for bacteria, with mean concentrati ons in the top 100 m of the water column of between 12 x 10(3) and 273 x 10(3) l(-1). Ciliate abundance showed no seasonal trends over the s tudy period and probably mirrored the fluctuating availability of vari ous food resources. Oligotrichs, particularly mixotrophic taxa, were a prominent element of the community throughout the year. Aggregates of detrital material were a regular feature in the plankton. When these occurred, they formed foci for bacteria and nanoflagellates. The evide nce suggests that the dynamics of the microbial plankton in Loch Ness may be driven by allochthonous carbon inputs rather than by the more u sual dominance of carbon fixed within the system.