Diatom assemblages of thermal and mineral waters from volcanic environments in Sao Miguel Island, Azores

Volcanic areas are hostile environments where the release of toxic gases is added to the low pH and high temperature, subjecting living organisms to extreme conditions. High-temperature environments, particularly those associated with volcanic activity have been evolving on earth throughout time. The aim of this study is to contribute to increasing knowledge of these high-temperature events and describe their associated diatom assemblages. The fumarolic fields of SAo Miguel Island in the Azores Archipelago were selected as a case study due to its particular characteristics and were studied seasonally during 2007. Temperature, pH and conductivity variables were measured in situ. Other environmental data were available in the literature. Diatom diversity, assessed using the Shannon-Wiener Index (H), ranged between 0.02 and 4. Statistical analyses revealed that temperature is a significant driver for the distribution of diatom species. Achnanthidium exiguum Grunow, Nitzschia cf. thermalis var. minor (Hilse) Grunow, Pinnularia joculata (Manguin) Krammer, P. trivialiformis Lange-Bertalot & Metzeltin and P. subcapitata Gregory were prominent with relative abundance above 40% of diatom assemblages. Some diatom taxa with high relative abundance, such as N. cf. thermalis and P. joculata may be considered representative of thermal springs and may be considered important biological indicators of geochemical changes induced by volcanic activity.