Nutrient and bacterial load profile of Lake Mainit, Mindanao, Philippines

Abstract

Lake Mainit, located in northeastern Mindanao, is the fourth largest lake in the Philippines. Its limnology and water quality parameters, however, are not yet thoroughly understood. This study has been conducted to obtain the bathymetry, the vertical temperature-dissolved oxygen profile, and the vertical variations of nutrients in the lake basin including those coming from river tributaries. The bacterial load profile of certain areas has also been studied to determine the levels of coliform bacterial contamination in the lake water. The bathymetric profile of Lake Mainit indicates that the shallowest portions are in the north and south ends of the lake. These portions become progressively deeper towards the center at over 200 m. Maximum depth recorded is 218.75 m. Steep slope dropping off sharply towards the 200-m depth is found at the western coast of the lake along the Malimono Ridge while the eastern coast has a gently sloping profile. The vertical temperature profile of the lake ranges between 26.55°C - 30.00°C with the thermocline layer being observed between 10 m to 40 m. Variations in vertical temperature have been observed at 0 m down to a depth of 30 m. Beyond this depth, variations are minimal ranging between 0.03°C - 0.15°C. The dissolved oxygen concentration at the epilimnion ranges between 7.31 - 7.67 mgOL^-1. Below the thermocline, it rapidly decreases to <2 mgOL-^-1 from 50 m down to the 200-m depth. The oxycline has been observed between 10 m - 40 m. The vertical nutrient profile shows a uniform NH₃-N concentration from 0 m - 60 m which significantly increases at 80 m to 200 m. A decreasing trend in the NO₃-N profile has been observed between 0 m to 30 m which then increases at 60 m to 200 m. Phosphate concentrations are relatively low at 0 m - 40 m. Nutrient measurements from 14 river tributaries show high concentration levels with sources coming from predominantly agricultural lands and residential areas. Bacteriological analyses of water samples coming from 12 sampling stations show high total and fecal coliform (350 to 1600 MPN/100 ml) in the three populated areas around the lake. The study indicates that Lake Mainit is an oligotrophic lake based on its physico-chemical and morphometric characteristics. However, natural productivity and optical properties of the lake waters should also be considered in determining its classification.