AME 43:193-207 (2006)  -  doi:10.3354/ame043193

ABSTRACT: We followed the changes in the protist assemblage over an annual cycle at 3 sites and different depths of Traunsee in the Austrian Alps and quantified the variability of the ciliate assemblage along successive depth and time intervals, respectively. More than 60 ciliate species were identified alive and after quantitative protargol staining (QPS). The ciliate diversity was high, and is described in detail for 50 taxa in time–depth intervals. Rimostrombidium brachykinetum/Rimostrombidium hyalinum and Balanion planctonicum were the most frequent species, accounting for 44% of the annual mean abundance and 15% of the annual mean biovolume, respectively. Our results suggested a stronger variability in the ciliate assemblage structure within seasons than along the depth gradient. Gradual changes in the assemblage structure with depths: (1) were accompanied by a decrease of algivorous and mixotrophic and an increase of bacterivorous ciliates from surface to deeper layers; (2) were highly significantly altered with steep depth gradients of their potential food resources, i.e. the biomass of heterotrophic flagellates, bacteria and algae; and (3) were related with lower significance to environmental parameters. High similarity (>80%) between successive months was reached only when net changes in the total ciliate abundance were negligible, while a strong increase or decrease in the ciliate abundance was associated with pronounced changes in the species composition. These seasonal changes in the ciliate assemblage structure (4) were linked to shifts of algivorous and mixotrophic, but not of bacterivorous ciliates and (5) were less predictable with food resources compared to the depth gradient. The phagotrophic flagellates generally followed the seasonal and vertical patterns described for ciliates, and were shown to be important members of the planktonic food web in a cold, deep, oligotrophic lake.

KEY WORDS: Microbial assemblage structure · Functional analysis · Microbial food web · Aquatic protists · Seasonal succession · Ciliate species list