Structure of the protozooplankton community in a tropical shallow and eutrophic lake in Brazil

Abstract

Monte Alegre Lake is a shallow, eutrophic and polymictic reservoir situated in Southeastern Brazil. Seasonal distributions of planktonic protozoans (HgCl2 fixed samples) and bacteria (AODC) were sampled at the surface and bottom, fortnightly from April 1988 to March 1989. Species composition and biomass (wet weight) of protozooplankton were analysed. Protozoan densities ranged from 3.60 × 103 to 3.89 × 105 ind l–1 at the surface and from 7.11 × 103 to 9.75 × 104 ind l–1 at the bottom, and the biomass ranged from 37 to 1170 μg l–1 and from 63 to 4958 μg l–1. The highest densities and biomass of both protozoans and bacteria were detected during the water column stratification period with anoxia at the bottom (warm-wet season). The smallest protozoan species prevailed at the surface and the largest at the bottom. Chlamydophrys sp., Arcella hemisphaerica, Mesodinium pulex and Halteria grandinella were numerically more important. On the other hand, Holophrya cf. discolor, Arcella hemisphaerica, Paramecium spp. and Spirostomum teres were dominant in biomass. Coccoid bacterial cells were abundant, except during the wet season when a phototrophic elongated ovoid bacterium, Thiopedia rosea, accounted for 61% of the total bacterial biomass at the bottom. The controlling factor of the protozooplankton is probably the food availability (bacteria) associated with dissolved oxygen and high temperatures.