Water quality trends of the Rinconada lake system and the implications in lake management

Abstract

Monitoring of important water quality parameters were conducted in the Rinconada Lake System in Camarines Sur composed of Lake Bato (3,800 ha), Lake Buhi (1,800 ha), and Lake Baao-Bula (400 ha). Monthly samplings were conducted in 14 stations in Lakes Bato and Buhi and in ten stations in Lake Baao-Bula. For Lake Bato, dissolved oxygen at the surface is within the optimum range of 6.3 mg/L-10.47 mg/L for the wet seasons and 5.7 mg/L - 9 mg/L for the dry seasons but drops drastically at the bottom during the dry season. Critically low mean average of 0.71 mg/L prevailed in the diurnal oxygen trend. High levels of ammonia were recorded in particularly in Station 1 (0.81 mg/l), Station 2 (0.8125 mg/l), and Station 10 (0.8825 mg/l). The mean values of the water quality parameters of Lake Buhi from the surface to a depth of 4 m are within the optimum level. However, critically low dissolved oxygen and high levels of ammonia were recorded at the bottom, indicating the build up of anoxic layer. The Lake is stratified in most months of the year with a gradient difference of 1.74°C during the wet season and 2.15°C in the dry season. Dissolved oxygen, temperature and pH of Lake Baao-Bula is generally within the optimum levels with slight fluctuation in alkalinity. Nitrite (0.7 mg/l) and ammonia (1.5 mg/l) were critically high during the wet season. Results of the water quality monitoring provided management recommendations in establishing aquaculture zones, redesigning of cages and the use of the lakes for aquaculture and fisheries.