[show abstract][hide abstract] ABSTRACT: We demonstrate for the first time a dynamic seasonality within thermophilic mat communities in a tropical geothermal spring. Biomass fluctuated such that it is greatest in the dry season, before falling drastically as the summer rains arrive, and then re-colonization culminates in a new climax in the following dry season. Species richness estimates based upon 16S rRNA gene environmental phylotypes mirrored this pattern, where those unique to the dry season disappear during the wet season only to reappear the following year, and vice versa. Relative abundance of some phototrophic phylotypes was also shown to vary seasonally. Environmental variables within the thermal environment that were most closely correlated to these variations were temperature and phosphate, with the latter a covariable to heavy seasonal tropical monsoon rainfall. Stochastic disturbance caused by a strong typhoon caused significant although temporary effects and both diversity and standing biomass recovered within a few months. Tropical hot spring communities clearly function under a fundamentally different set of abiotic variables from those in temperate locations which do not display seasonality. This is of particular relevance to bioprospecting efforts where targeting the most biodiverse niche is desired, because future sampling strategies for tropical thermal environments should consider diversity on temporal as well as spatial scales.