The Thermal Regime of Lake Lanao (Philippines) and Its Theoretical Implications for Tropical Lakes1

A study of temperature profiles, climatic records, and chemical data for Lake Lanao, a low‐altitude tropical lake, over 14 months shows that the annual pattern of heat distribution depends partly on a seasonal air temperature minimum and partly on nonseasonal climatic changes. The lake circulated during January and February at the time of seasonal cooling, was intermittently stable during March and April, and was stratified during all other months. During stratification the principal thermocline achieved equilibrium with storm winds at 40–50 m. Secondary and tertiary thermoclines repeatedly split the epilimnion into an upper turbulent layer and a lower stagnant layer for periods up to 2 months, but were displaced or dissipated at irregular intervals by storms. The term atelomixis is proposed to denote the mixing of chemically divergent layers during stratification. Changes in the shape of thermal profiles in Lake Lanao include sharpening of thermodines by wind and convection, smearing of thermoclines by internal water movements, unstable thermal inversions due to cooling at the surface, and stable thermal inversions on the bottom that result from heat retention during cool weather.