Lake Zirahuén is one of the ecologically better preserved and small-sized lakes in Mexico. Observations revealed that Lake Zirahuén is subjected to a consistent diurnal wind because of the presence of a valley–mountain breeze that triggers semidiurnal and 3–8 h variability oscillations, with the latter among the natural frequencies of the lake, whereas the former and the principal forcing are in near resonance with the second vertical mode. The thermodynamic variability was greater in the metalimnion where the analysis of eigenfunctions shows that higher modes (>2) are important at depths below the thermocline. The numerical modeling adequately described the water temperature evolution and surface layer currents in an averaged manner. The daily observations showed drifts in surface currents, acquiring velocities of up to 0.1 m/s, owing to wind stress, which caused an increase in temperature at the northern section of the lake. Data averaged over three months revealed that the surface currents flow northward with an anticyclonic return to the east part and a pair of cyclonic returns to the northwestern and southwestern parts of the lake; whereas at the vertical, the structure showed two circular regions divided by the thermocline located at a depth of 15–20 m.

In this study, optimal diffusion coefficients for Lake Zirahuén, Mexico, were found under particular conditions based on images taken with a drone of a dye release experiment. First, the dye patch concentration was discretized using image processing tools, and it was then approximated by an ellipse, finding the optimal major and minor axes. The inverse problem was implemented by comparing these observational data with the concentration obtained numerically from the 2D advection–diffusion equation, varying the diffusion tensor. When the tensor was isotropic, values of $K_{11}=K_{22}\approx 0.003{ \mathrm{m}}^2/\mathrm{s}$ were found; when nonequal coefficients were considered, it was found that $K_{11}\approx 0.005{ \mathrm{m}}^2/\mathrm{s}$ and $K_{22}\approx 0.002{ \mathrm{m}}^2/\mathrm{s}$, and the cross-term $K_{12}$ influenced the results of the orientation of the ellipse. It is important to mention that, with this simple technique, the parameter estimation had consequences of great importance as the value for the diffusion coefficient was bounded significantly under particular conditions for this site of study.

The condition of Lake Zirahuén was evaluated before and after a significant algal bloom took place. The state of the lake was tracked using measurements of chlorophyll‐a concentration from a fluorometer attached to a conductivity, temperature, and depth (CTD) profiler and complemented with the use of satellite images from Sentinel‐2. It was found that the current state of the lake is in transition from an oligotrophic to a mesotrophic level with some evidence of deterioration, but still being considered as a clear lake. With the use of empirical algorithms to retrieve chlorophyll‐a from satellite data it was found that the images presented an acceptable feasibility (R² = 0.64). The observed algal bloom was detected one month before the algae reached its major extent, being the reflectiveness of the green band a good indicator of the onset of the algal bloom event. Favorable conditions for the algal bloom were also recorded two months before its major extend through a chain of thermistors.Practitioner Points: The trophic state of Lake Zirahuén was evaluated before and after a significant algal bloom took place. The lake is classified as being in transition from oligotrophic to mesotrophic. Nonetheless, it continues to have good clarity. Satellite images improve in the description of the spatial‐temporal variability of the lake. In particular, the green band reflectiveness.

The hydrodynamical connectivity of Lake Zirahuén was analyzed for the stratification (summer) and isothermal (winter) periods, in which the dispersion of passive particles was investigated for two hypothetical cases: i) a surface dispersion and ii) a deep resuspension, processes. The Lagrangian probability density functions were used to estimate the connectivity at a time scale of up to 30 days, and real drifting buoys were used to adjust the horizontal dispersion coefficients in the numerical model. In case i), the horizontal dispersion was similar between both seasons; only at the beginning during the first week was slightly different, but after day 10 and forward, they were alike due to the lake's small size. With respect to the vertical dispersion, the particles remain above 10 m deep during summer and travel along the total water column during winter, mainly from 10-20 m. For case ii), the horizontal dispersion was similar between both seasons. However, the vertical dispersion was inhibited by the thermal stratification during summer, where the particles do not reach depths above 20 m. In winter, the dispersion was more intense, and around day 3 they reached the total water column.