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Investigation of Thermal Stratification of a Lake Based on Low-cost Observational Data Thermal stratification can be established in deep mid-latitude lakes, being related to surface heating during the warm part of a year. Due to separation of lake into distinct vertical layers and inhibition of vertical mixing, stratification may affect hydrogeochemical and biological conditions in a lake. Here, we investigate thermal stratification of the 46 m deep lake (Kozjak, Croatia) at fine temporal and vertical resolution. The lake is the twelfth and the largest one (area 0.82 km2, volume 0.01271 km3) in the chain of sixteen karstic Plitvice Lakes interconnected by cascades and waterfalls. Fifteen waterproof temperature sensors with data loggers (HOBO TidBit 400, measurement accuracy of ±0.20°C for temperatures between 0° and 70°C) were fixed to a string at depths ranging from 0.2 m to 43 m. The string was attached to a buoy which was moored to ensure its steady position in the deepest part of the lake (φ = 44.89°, λ = 15.60°). Analysis of temperatures observed at 2-min temporal resolution during 6 July – 5 November 2018 revealed thermocline deepening from about 8 m (July) to about 15 m (end of October). Furthermore, diurnal variation of water temperature, density and Brunt–Väisälä frequency was found in the uppermost, approximately 3 m deep layer and in concurrent meteorological data. Power spectrum densities computed for lake temperatures suggest periods of 8.3 min and 5-6 h at depths ranging from 9 to 17 m and from 17 to 23 m, respectively. |
