Microbial microstratification, inorganic carbon photoassimilation and dark carbon fixation at the chemocline of the meromictic Lake Cadagno (Switzerland) and its relevance to the food web.

The microstratification of the microbial community at the chemocline of Lake Cadagno and the associated inorganic carbon fixation activity was studied by fine layer sampling. A deep chlorophyll maximum caused by diatoms overlying Cryptomonas was found at the upper edge of the chemocline. A high population density of phototrophic sulphur bacteria, mainly Amoebobacter cf. purpureus, occurred closely below the oxic-anoxic boundary. Despite the small fraction of total lake volume represented by the chemocline, half of the total carbon photoassimilation of the lake occurred within the chemocline with approximately equal contributions by oxygenic and anoxygenic phototrophs. Rates of dark carbon fixation in the chemocline were even higher than rates of photoassimilation, especially at the depths where oxygen and sulphide coexisted during part of the day. These results indicate a substantial contribution by chemolithotrophic organisms to the carbon cycle in Lake Cadagno. Analysis of stable carbon isotopes suggests that zooplankton may obtain as much as half of its carbon at the chemocline, indicating a strong link between production in anoxic waters and the food web in the oxic part of the lake. [ABSTRACT FROM AUTHOR]