Abstract

1. Large mixotrophic ciliates (Stentor araucanus, S. amethystinus and Ophrydium naumanni) were a characteristic component of a temperate, oligotrophic lake in North Patagonia. During a 1-year study, the abundance, biomass and primary production of these large Chlorella-bearing ciliates were compared with those of the total plankton community. 2. Mixotrophic ciliates peaked in spring and from late summer to autumn, accounting for 1.6-43% (annual average: 16.3%) and 67-99% (annual average: 92%) of total ciliate abundance and biomass, respectively. Their contribution to total zooplankton biomass, including flagellates, rotifers, ciliates and crustaceans, was 14-76%, or 47% as an annual average. Endosymbiotic algae accounted for up to 25% of total autotrophic biomass (annual mean: 3.9%). 3. Maximum cell-specific photosynthetic rates of S. araucanus and S. amethystinus at light saturation varied between 80 and 4400 pg C ciliate-1 h-1 with high values during autumn and winter, and low values during summer. The depth-integrated rates of photosynthesis (0-40 m) of algal endosymbionts contributed 1-25% to total photosynthesis (annual mean: 6.5%). 4. A comparison of calculated ingestion rates with photosynthetic rates of Stentor indicates that photosynthate produced by endosymbionts generally exceeded heterotrophic food supply of Stentor during autumn and winter, but was much lower during summer, when food supply was high. 5. The mixotrophic ciliates represent an important 'link' between nanoplankton and higher trophic levels within the plankton community because of their high heterotrophic biomass and considerable contribution to total photosynthesis.