Lake Cadagno is a meromictic alpine lake characterized by a permanent stratification, which creates a permanent anoxic environment that supports the growth of anoxygenic phototrophic sulfur bacteria. The seasonality and inter-seasonality of these microorganisms were examined over a three-year period (2019-2021) through regular monitoring of the water column. A variety of physical-chemical parameters, including temperature, conductivity, light, oxygen and sulfide concentrations and the community composition of anoxygenic phototrophic sulfur bacteria in the chemocline were recorded, to observe potential influence of external weather conditions. Our findings indicate that, despite the lake's consistent physical and chemical stratification, the composition of the anoxygenic phototrophic sulfur bacteria community exhibited notable variations in response to external environmental factors, including changes in rainfall and light irradiance. Specifically, we observed different growth dynamics in the purple (PSB) and green (GSB) sulfur bacteria communities over the three years of monitoring. These variations underscore the complexity of biogeochemical cycles in meromictic lakes and the impact of external environmental factors on this ancestral microbial community dynamics. The results provide valuable insights into the stability of redox-stratified environments, offering a modern analogue for ancient aquatic ecosystems. This research emphasizes the importance of long-term regular monitoring to capture interannual dynamics and assess the implications of climate change on such unique ecosystems.