Holocene changes in the composition and structure of diatoms assemblages from the Frontino Páramo (Occidental Andes Cordillera, Colombia)

Abstract

This paper presents the analysis of temporal variation in composition and structure of Holocene diatom assemblages from the Llano Grande Basin, Frontino Páramo, located at 3,460 msnm in the northern portion of the Occidental Andes Cordillera Occidental, Colombia. The interval analyzed is constituted of 8.5 m of a core drilled in the central portion of the basin, corresponding to the Holocene. 57 levels were randomly sampled and permanent slides were prepared, from which the taxonomic composition of the assemblages and respective densities were performed. A high variability in the density, composition and structure of the diatoms assemblages was established. 292 species were recognized, including 149 with open taxonomic nomenclature, recognized at genus level. These last taxa should correspond to intraspecific variations of known species or new ones, demonstrating the biological richness of these environments, subject to particular stress conditions at high altitude. Along the core, 169 taxa disappeared, representing a loss of 40.9 % in taxonomic composition. These disappearances are attributed to environmental changes related to temperature, foreign input of volcanic ashes, demineralization and decrease in depth of the water column. In the interval correspondent to the early Holocene, strong environmental disturbances, as well as physical and chemical variations were recorded, with effect on the diatoms assemblage, causing drastic density reduction and change in the compositions towards the levels corresponding to the middle and late Holocene. Changes in the middle Holocene are interpreted as fluctuations in the volume and demineralization of the water body and the external addition of volcanic ashes. In the late Holocene, changes are related to the progressive decrease of the water level in the ecosystem and colonization by macrophytes.