Mixing regimes in tropical high-mountain Andean lakes

Lowland tropical lakes deeper than 15 m are typically warm monomictic, but whether the regime may switch to polymixis at high elevations remains uncertain. Using a year-long high-frequency dataset from thermistor chains from 10 deep lakes and occasional temperature profiles from 202 lakes in Ecuador's Cajas Massif, this study examines the mixing regimes in tropical high-Andean lakes. We identified monomictic, polymictic, and discontinuous polymictic regimes. Lakes with a mean depth < 7 m are typically polymictic, while deeper lakes are mostly monomictic, resisting wind, and daily temperature oscillations for much of the year. Persistent low stability occurs between July and September, coinciding with intertropical synoptic meteorological fluctuations. During the stable period, the water column is structured into a nearly top-to-bottom progressive stepwise temperature gradient, which, in the deepest (> 40 m maximum depth), the structure occupies only part of the water column, deepening as the stable period advances, occasionally producing a thermal stratification that might resemble that of temperate lakes in summer but with a weaker thermocline. Daily fluctuations, with nighttime convective mixing, dominate thermal variation. Quasi-periodic (biweekly) increases in wind and cloudiness mix the upper layers. The stable period shortens with elevation and transparency for lakes of similar depth and size. Existing generalized scaling of seasonal thermal stratification applies satisfactorily to these lakes. However, fitting the formulation specifically to them downplays the role of water transparency in favor of air-water boundary heat transfer factors. Discontinuous polymixis emerges as a transient regime under a narrow set of mean depth and transparency combinations.