Diatom assemblages from different environments of the Acoculco Caldera associated to hydrothermal and anthropogenic activity.

The use of diatoms as indicators of water quality has been studied worldwide; however, the use of diatoms as indicators of thermic anomalies has received less attention. The objective of this study is not only to provide a record on algal communities, but also to investigate the relationship between the diversity in diatom species and the physicochemical conditions of water. Evaluating its temporal variability in a caldera with low permeability and cold acid hydrothermal anomalies. Diatom assemblages were identified at 11 sites. Species composition was compared between seasons (dry and wet) in different environments, as streams, ponds and pools within the caldera. The physicochemical and environmental differences are very specific, which allowed the diatom identification from geothermal and anthropogenic sources without finding mixtures. The results show that the main diatom flora of the Acoculco caldera consists of 15 dominant species. Sulphite-rich acidic hydrothermal waters are characterized by the presence of Eunotia exigua, Eunotia bilunaris, and Pinnularia brauniana. In non-hydrothermal streams, Planothidium, Achnanthidium, and Humidophila species are the most common taxa. Other diatoms from acidic environments were Frustulia saxonica, Surirella, and Stenopterobia. The assemblages are clearly different from those of alkaline environments. Epithemia, Planothidium, and Ulnaria are present in the streams and are not associated with thermalism. Ordination of diatom assemblages showed that pH, conductivity, and nutrient concentrations, some of which are influenced by anthropogenic activities, were the main factors influencing the distribution of diatom composition.