A submersible fluorometer for measuring a fluorescence of a photosynthetic material in a liquid, the fluorometer includes a set of light emitting diodes (LEDs) for supplying light to direct towards the liquid and a photodiode for measuring a detected light emitted from the liquid. The light is used for measuring the fluorescence of the photosynthetic material in the liquid. The fluorometer also has a water-tight volume for protecting the photodiode from the liquid.