Trophic state in a tropical lake based on Chlorophyll-a profiler data and Sentinel-2 images: The onset of an algal bloom event

The condition of Lake Zirahuén was evaluated before and after a significant algal bloom took place. The state of the lake was tracked using measurements of chlorophyll-a concentration from a fluorometer attached to a conductivity, temperature, and depth (CTD) profiler and complemented with the use of satellite images from Sentinel-2. It was found that the current state of the lake is in transition from an oligotrophic to a mesotrophic level with some evidence of deterioration, but still being considered as a clear lake. With the use of empirical algorithms to retrieve chlorophyll-a from satellite data it was found that the images presented an acceptable feasibility (R² = 0.64). The observed algal bloom was detected one month before the algae reached its major extent, being the reflectiveness of the green band a good indicator of the onset of the algal bloom event. Favorable conditions for the algal bloom were also recorded two months before its major extend through a chain of thermistors. PRACTITIONER POINTS: The trophic state of Lake Zirahuén was evaluated before and after a significant algal bloom took place. The lake is classified as being in transition from oligotrophic to mesotrophic. Nonetheless, it continues to have good clarity. Satellite images improve in the description of the spatial-temporal variability of the lake. In particular, the green band reflectiveness.