Vertical models of phototrophic bacterial distribution in the metalimnetic microbial communities of several freshwater North-American kettle lakes

The composition and the structure of the metalimnetic communities of phototrophic microorganisms were studied in 24 lakes of Wisconsin and Michigan (USA), during the period of summer stratification, and related to environmental parameters. The presence of phototrophic bacteria was reported for the first time in some lakes. Different types of phototrophic microorganisms were separated in three different clusters, by decreasing values of photosynthetic available radiation (PAR) and redox potential (Eh): (1) cyanobacteria and eukaryotic phytoplankton, (2) Chromatiaceae and multicellular filamentous green bacteria, (3) green sulfur bacteria. Each cluster conformed one different layer of the vertical distribution of the community. At each layer, usually one type of microorganisms outcompeted the others, but at times they were found together in competition. The three layers were mixed at the bottom of the lake at the beginning of the stratification period, before widening and colonising the water column. At the end of summer, the contraction of the metalimnion resulted in overlapping or suppressing some layers. (C) 1998 Federation of European Microbiological Societies. Published by Elsevier Science B.V.