The detection of annual hypoxia in a low latitude freshwater reservoir in Kerala, India, using the small AUV Maya

Abstract

The Idukki Reservoir at an altitude of 748 m covering an area of 53 km sup(2) is surrounded by tropical forests in the Western Ghats in the southwestern Indian state of Kerala. We used the small autonomous underwater vehicle (AUV) Maya with onboard sensors of dissolved oxygen, chlorophyll, turbidity, temperature, and depth to monitor the water quality environment of the Idukki Reservoir in May 2006. The use of AUVs in confined spaces like small lakes and reservoirs is new and uncommon, requiring extra safety to be implemented. As this is a prototype AUV, we shall describe in brief key aspects of the vehicle attributes, namely, its novel mechanical design, the autopilots that control the heading and cruising depth, and the safety and endurance of the platform. The data acquired by Maya revealed an acute oxygen deficiency at 21 m, a mid-water low turbidity layer between 10 m and 15 m, and a prominent chlorophyll maximum in the thermocline region of Idukki waters at 6 m. These experiments were repeated a year later in May 2007 and were in unambiguous agreement with the 2006 findings of hypoxia. These are the first observations of hypoxic processes using a small AUV in any Indian lake. The relevance of these results in freshwater systems shows similarities to hypoxia in saline coastal waters of the west coast of India and is discussed briefly.