Protozooplankton and bacterioplankton in a large oligotrophic lake--Loch Ness, Scotland

The seasonal changes in the abundance of protozoan and bacterialplankton in a large, coloured, oligotrophic lake. Loch Ness(Scotland), were investigated between August 1991 and January1993. The coloured water supported only low concentrations ofchlorophyll a (<1.6 µgl^–1). with the highestvalues occurring in summer. Mean bacterial abundance rangedbetween 2.3 x 10⁸ and 7.1 x 10⁸ l^–1 in the 100 m watercolumn. Maximum abundance did not correlate with maximum chlorophylla concentrations, but appeared to be related to the input ofallochthonous carbon from the catchment, which in turn was influencedby rainfall levels. Consequently, the highest bacterioplanktonconcentrations occurred in autumn and winter. The pattern ofheterotrophic nanoflagellate abundance tended to follow thatfor bacteria, with mean concentrations in the top 100 m of thewater column of between 12 x 10³ and 273 x 10³ l^–1. Ciliateabundance showed no seasonal trends over the study period andprobably mirrored the fluctuating availability of various foodresources. Oligotrichs, particularly mixotrophic taxa, werea prominent element of the community throughout the year. Aggregatesof detrital material were a regular feature in the plankton.When these occurred, they formed foci for bacteria and nanoflagellates.The evidence suggests that the dynamics of the microbial planktonin Loch Ness may be driven by allochthonous carbon inputs ratherthan by the more usual dominance of carbon fixed within thesystem. ¹Present address: School of Zoology, La Trobe University, Bundoora,Melbourne, Victoria 3083, Australia ²Present address: Loch Ness & Morar Project, Loch Ness Centre,Drumnadrochit, Invernesshire, UK