CONICET | Buscador de Institutos y Recursos Humanos

In water bodies where cladocerans are typically small or present in low biomass, such as in the tropics andsubtropics of South America, rotifers may play a larger role in ecosystem functioning than in temperate lakes. Theaim of this study was to analyze the horizontal distribution of richness and abundance of rotifers, their diel distributionpatterns, and the relative importance of physical and biotic characteristics as their shaping factors, in a subtropicalshallow vegetated lake of the Paraná River floodplain (Argentina). Three sampling stations were located alongthe major axis of the lake, one in open waters and two in littoral area dominated by the emergent Panicum elephantipes,Cyperus alternifolius and floating mats of the filamentous algae Cladophora spp. Two further stations werelocated on the transverse axis. Rotifer samples were taken on four occasions (at noon and midnight) during a fiveweek period in the summer to avoid possible changes in the pattern of distribution of macrophytes and hydrologicalvariability of the lake. Rotifer richness was higher in the littoral zone whereas rotifer abundance was higher in thelimnetic zone. Some rotifers, such as Brachionus havanaensis and Keratella tropica exhibited changes in their dielhorizontal distribution with higher concentrations of organisms at vegetated zones during the day time. In contrast,Polyarthra sp. were more abundant at night-time in both limnetic and littoral areas. The heterogeneous horizontaldistribution of rotifers cannot be explained by the rather homogeneous distribution of both physical environmentalfactors and phytoplankton abundance. Rotifer distribution may be related to a complex of factors including the presenceor absence of vegetation, the relatively uniform distribution of visual predators (fishes) and the heterogeneousdistribution of non-visual predators (insects and shrimps).

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