Worldwide, the increasing use of fertilisers rises the risk of eutrophication, a sudden growth of algae in presence of nitrates and phosphates that leads to oxygen depletion in water with critical economic and environmental consequences. Continuous monitoring of oxygen in environmental waters could improve the early detection of eutrophication events and prevent anoxic conditions. However, online and in situ dissolved oxygen sensors are yet to be implemented due to cost, portability and power limitations. Here we propose to use a self-powered ceramic soil microbial fuel cell to detect algal growth via monitoring of dissolved oxygen in water. When immersed in an algal solution, the sensor signal follows the characteristic photosynthetic cycle with maximum day current of 0.18 ± 0.2 mA and a minimum night current of 0.06 ± 0.34 mA, which correlates with dissolved oxygen (R²=0.85 (day); R²=0.5 (night)) and algal concentration (R²=0.63). A saturated design of experiments on seven factors suggests that temperature, dissolved oxygen, nitrates and pH are most influential operational factors on the voltage output; while operating the system at maximum power point (R_ext= 2 kΩ) improves the sensor sensitivity, in the range of study. This is, to our knowledge, the first proposed MFC based biosensor for in field early detection of eutrophication events.