Microstratified vertical distribution and migration of phototrophic microorganisms during a diel cycle in Lake Arcas-2 (Spain)

During the study of a diel cycle in Lake Arcas-2 by means of a fine-layer s ampler, dense populations of phototrophic microorganisms have been detected sequentially and sharply layered from the metalimnion-hypolimnion interfac e to the lake bottom. The predominant purple sulphur bacterium was Chromati um weissei which formed a dense plate at the oxic-anoxic boundary. The mean biovolume of C. weissei was statistically different in the cells forming t he plate and those located below it, with the former 22 % larger than the l atter. Two other Chromatiaceae species, Thiocapsa sp., and Amoebobacter sp. were members of this phototrophic community, although their abundances wer e much lower. Green sulphur bacteria (mainly Pelodictyon clathratiforme) we re also present in low numbers below the purple bacteria, Maximum concentra tion of Oscillatoria cf. ornata filaments was observed very close to the po pulations of phototrophic bacteria within the microaerobic and anoxic layer s. Motile cryptomonads (mainly Cryptomonas erosa) were observed at high den sities in the vicinity of phototrophic prokaryotes. Several members of the photosynthetic community moved vertically about 40 cm during the diel cycle . Most microorganisms were concentrated at the oxic-anoxic boundary during the day (showing lower niche breadth) and dispersed through the hypolimnion at night (higher niche breadth). Diel changes in physical and chemical fac tors such as light penetration, oxygen and sulphide profiles are related wi th the complex and structured community of phototrophic microorganisms.