The heat budget of a large tropical lake, Lake Titicaca (Peru-Bolivia)

The annual cycle of a lake's chemical and biological regime is driven by the pattern of stratification and circulation. Factors that control the extent and timing of mixing are best understood through studying the heat budget sources and sinks. Though thermal stratification has been studied in several tropical lakes (e. g., TALLING 1966; LEWIS 1973) work lags behind that in temperate lakes and analytical heat budgets have been neglected. In addition, year-to-year and long-term variability in the frequency and depth of circulation is an important environmental parameter that has received little attention in either tropical or temperate lakes. This paper presents the analytical budget for Lake Titicaca for 1973 and a preliminary estimate of the budget's variability based on 10 years of weather records available from the lake basin. Lake Titicaca is a large (8,100 km2, 281m max. depth), high altitude (elev. 3803 m) lake that lies at 16° S latitude. Lake Titicaca's surface heat fluxes are strongly influenced by the region's cool, semi-arid, and equable climate. There is a prominent though variable annual cycle of a cloudy rainy summer, December to March, and a dry sunny winter, May to August. A survey of physical, biological, and chemical parameters was undertaken in 1973 and has been reported by RICHERSON et a!. (1975, 1977) and WIDMER et a!. (1975). The pattern of mixing in 1973 resembled the warm monomictic type of HuTCHINSoN's (1957) thermal classification. However, the lake was never quite isothermal in 1973 and chemistry data indicate that mixing to 150m did not occur. Thermal profiles in Lake Titicaca resemble those of other tropical lakes, having a thick epilimnion only a few degrees warmer than the hypolimnion.