AbstrAct. The nature of the riparian and surrounding landscape has been modified by anthropogenic activities, which may subsequently alter the composition and functional structure of macroinvertebrate assemblages. The effect of these changes on function of benthic fauna is difficult to assess due to the scarce knowledge on functional structures in tropical streams. In this study we evaluate whether sites impacted and unimpacted by anthropogenic alterations differed in assemblage composition and density, richness and diversity of each functional feeding group. The selection of the sites was related to their distinct riparian characteristics, following the QBRy riparian quality index. Collector-gatherer was the dominant functional feeding group, comprising 91% of total density, whereas the proportion of shredders was very low, representing less of 0.5% of total density. Asemblage composition of macroinvertebrates differed between impacted and unimpacted sites. Predators were dominant in taxa number, representing about 60% of total taxa richness. In addition, the diversity and richness of collector-gatherers differed significantly between degraded and unimpacted sites, reflecting the sensitivity of this group to environmental changes and the utility to be used in the assessment of anthropogenic modifications. The results of this study reinforce the idea that riparian corridor management is critical for the distribution of macroinvertebrate assemblages as well as functional organization of lotic streams.KEywords. Anthropogenic changes, Argentina, lotic streams, mountain basin, riparian quality index. rEsumEn. Efecto de los impactos antrópicos sobre los ensambles de macroinvertebrados bentónicos en ríos subtropicales de montaña. Las actividades antrópicas han producido modificaciones en el paisaje ripario y el sistema terrestre adyacente, las cuales causan alteraciones en la composición y estructura funcional de los ensambles de macroinvertebrados. Asimismo, el efecto de estos cambios sobre la estructura funcional de macroinvertebrados es difícil de evaluar debido al escaso conocimiento de este parámetro en ríos tropicales. En este estudio se evaluó si sitios degradados y no degradados por impactos antrópicos diferían en la composición de los ensambles y en la densidad, riqueza y diversidad de cada grupo funcional. La selección de los sitios se relacionó con su calidad riparia, de acuerdo con el índice QBRy. Colectores-recolectores fue el grupo funcional dominante, comprendiendo el 91% de la densidad total, mientras que la proporción de trituradores fue muy baja, representando menos del 0.5% de la densidad total. La composición de los ensambles de macroinvertebrados difirió significativamente entre sitios degradados y de buena calidad riparia. Los predadores fueron dominantes en número de taxones, representando alrededor del 60% de la riqueza de taxones. La diversidad y riqueza de collectores-recolectores fue menor en sitios degradados, reflejando la sensibilidad de este grupo frente a cambios antrópicos. Los resulta...

The solubility equilibrium of calcite is influenced by physicochemical, climatic and biological factors. Annual cycles of exceptionally prolonged drought, in conjunction with naturally occurring diffuse organic pollution, generate the unique conditions for the precipitation of lithified carbonate structures (microbialites). The aim of this article is to analyze the possible implications of calcite precipitation produced in mats of Cladophora sp. in an Andean subtropical basin, considering it is the first time this phenomenon is described for the region. We collected samples from selected sites at the Lules River Basin, in four sampling dates between the years 2003 and 2004, within a monitoring work of 15 years. Samples were analyzed using an electron microscope and X-ray diffraction analysis. We found that Gomphonema sp. attached to Cladophora sp. contributes to precipitation of calcite and formation of microbialite like structures, in the studied area. This work presents an initial discussion of the discovery of microbialites-like structures attached to Cladophora sp. mats in a subtropical Andean stream and the environmental conditions that lead to their production, as well as the possible ecological implications of these microbialites.