This work is divided in two parts. In the first part the vertical distribution of planktonic rotifers is described in relation to some physicochemical parameters in a small meromictic lake. Rotifers concentrate their populations at the depths with intense gradients. Two rotifers groups can be differentiated: 1) hypolimnetic and oxyclinal species such as Anuraeopsis miraclei, Filinia hofmanni, Anuraeopsis fissa, Polyarthra dolichoptera and Keratella quadrata; and 2) epi-metalimnetic species such as Hexarthra mira, Trichocerca similis, Synchaeta pectinata, Asplachna girodi, Ascomorpha saltans and Ascomorpha ecaudis. The new, recently described, species A. miraclei is the most highly associated to extreme microaerobic conditions.

The second part is devoted to an experimental study of the rotifer B. plicatilis for determining the relationships between oxygen limitation and the metabolic adaptations of rotifers to it. Previously some methodological problems were solved which proved to be useful for controlling oxygen conditions in other rotifers groups and for maintaining long-term B. plicatilis populations with the newly assayed food source, the alga Tetraselmis sp. thermally treated (65-70 B0C for 90 min.). The population dynamics of Brachionus plicatilis was studied under controlled oxygen conditions. This rotifer is able to maintain relatively high-density populations in oxygen concentrations below 1 mg/l, for more than one month, although its growth and metabolism is extremely reduced. Major features of population growth related to oxygen concentration are discussed. Activities of 10 enzymes and accumulation of two metabolites under experimental hypoxia were also investigated in the rotifer Brachionus plicatilis. Pyruvate kinase, lactate dehydrogenase, malic enzyme, phosphoenolpyruvate carboxykinase, malate dehydrogenase and fumarase activities showed great increase at low oxygen levels. Alcohol dehydrogenase activity was not detected. Alanine concentration showed a increase with low oxygen levels, which is interpreted as enhanced proteolysis. These results show that B. plicatilis use the lactate pathway for anoxic glucose metabolism, and suggest that the glucose- succinate pathway would also function at low oxygen concentrations. This pattern is consistent with the phylogenetic position of rotifers.