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Abstract
Continuous temperature monitoring for two adjacent tropical crater lakes in Mexico at 4200 m amsl shows that the lakes have rhythmic episodes of heating and cooling with a duration of ~ 30 days during the warmest months. The episodes were caused by rise and decline of solar irradiance reaching the lake surface. One lake, El Sol, showed over each heating and cooling episode a stable mixed layer (~ 20 days) and a deeper layer with a weak thermal gradient. Temperatures below the mixed layer warmed progressively by eddy diffusion after the mixed layer formed. Stratification was followed by full mixing of the water column. Within the same crater, an adjacent second lake, La Luna, showed the same cycles of heating and cooling; it stratified daily but not over multiple days. The difference between the lakes (discontinuous polymictic, continuous polymictic) is explained by the lower transparency of El Sol, which led to greater heat uptake near the surface than the more transparent La Luna. Lower transparency of El Sol was caused by modest anthropogenic effects on total suspended solids and nutrient loading, i.e., small deviations from the natural condition of El Sol caused it to differ qualitatively from La Luna. Events observed in these lakes would not have been evident from weekly temperature records.
Highlights
Lakes at high elevation in the tropics were first studied extensively by Löffler (1964, 1972), who designated lakes above 3000 m amsl as “high mountain tropical,” which in the American tropics are typically referred to as either “paramo” or “puna”
Löffler observed that tropical high mountain lakes typically lack a seasonal, stable ice cover, but may develop full or partial ice cover for brief intervals
Montane lakes below the high mountain zone are more likely to be warm monomictic than higher lakes (Gunkel and Casellas 2002; Salas De Leon et al 2016)
Summary
Lakes at high elevation in the tropics were first studied extensively by Löffler (1964, 1972), who designated lakes above 3000 m amsl as “high mountain tropical,” which in the American tropics are typically referred to as either “paramo” (wet, narrow temperature range) or “puna” (dry, broad temperature range). Hutchison and Löffler (1956) concluded that many tropical high mountain lakes are polymictic but, if deep, can be warm monomictic, i.e., show complete mixing of the water column on a seasonal basis alternating with sea‐ sonal stratification of the water column, as does Lake Titi‐ caca (Andes, 3800 m amsl: Vincent et al 1984; Richerson et al 1986). The present study uses continuous water column tempera‐ ture measurements as a basis for analysis of heat flux and vertical heat distribution in two tropical high mountain crater lakes, El Sol and La Luna, that are located in the Mexican
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