COMPARATIVE-STUDY OF 2 MEROMICTIC BASINS OF LAKE-BANYOLES (SPAIN) WITH SULFUR PHOTOTROPHIC BACTERIA

The annual limnological dynamics of two meromictic basins of Lake Bany oles (C-III and C-IV) have been studied and compared on the basis of t heir physical, chemical and biological characters. Stability values ca lculated for both basins gave 865 g cm cm(-2) and 495 g cm cm(-2) for C-III and C-IV respectively. These values are in agreement with the fa ct that C-IV was almost completely mixed during winter. In this basin, during stratification, the monimolimnion increased in thickness as th e stability increased. Isolation of the respective monimolimnia result ed in the development of anoxic conditions and the accumulation of sul phide in both C-III and C-IV, which favoured the development of dense populations of sulfur phototrophic bacteria. The purple sulphur bacter ium Chromatium minus and the green sulphur bacterium Chlorobium phaeob acteroides were identified as the main components ol these photosynthe tic populations. The different depths at which the O-2/H2S boundary wa s situated in both basins (and consequently the different light intens ity reaching this zone) determined tile growth of these bacteria. Ligh t intensities at the chemocline of C-IV reached values up to 5% of sur face incident light. In contrast, in C-III this variable was sensibly lower, with values depending on season and seldom reaching 1%. Phototr ophic bacteria were consequently found earlier in C-IV than in C-III, where no significant concentrations were found until August. Finally s tability is discussed as an important factor controlling chemical and biological dynamics in meromictic lakes.