We investigated the vertical and horizontal distributions of zooplankton in the littoral zone of a pampean shallow lake: Lacombe Lake (Buenos Aires, Argentina) throughout the 24-h day in summer. A new methodology that allows to take multiple samples without disturbance and difficulty was employed for sampling sectors with submerged macrophyte beds. This lake is characterized by the presence of littoral and planktivorous ichthyofauna. Lacombe Lake exhibited high transparency owing to the presence of submerged macrophytes (Myriophyllum quitense). We observed little variation at different depths within the horizontal transects with respect to temperature, dissolved-oxygen concentration, conductivity, and pH and recorded statistically significant (ANOVA) horizontal and vertical zooplankton-abundance gradients. The majority of the small-bodied zooplankton (ciliates and rotifers) evidenced no significant variation in their horizontal or vertical distribution. Littoral cladocerans appear to migrate vertically rather than horizontally, whereas Diaphanosoma birgei exhibited significant changes in its horizontal distribution. The presence of emergent and submerged macrophytes in the littoral zone is not sufficient to promote clear differences in diurnal horizontal zooplankton migration. The differences in the diel fluctuations in particular species may be mostly dependent on specific ecological requirements, environmental conditions, and/or fish predation.

Fil: Claps, Maria Cristina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina

Fil: Benitez, Hernan Hugo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Limnología "Dr. Raúl A. Ringuelet". Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Limnología; Argentina