RESUMO
In general, lakes provide various ecosystem services that support biotic habitats and human life. Lake Toba, the largest caldera volcanic lake in the world, has been functioning as a tourism destination and for fish farming, a fresh water source, and for power supplies. The lake has maximum depth of about 505 m. Stratification in the lake water column especially in a tropical region like Indonesia is a common phenomenon. Stratification of the lake is one important factor which will influence the next stage in the biological processes and water quality of the lake. The objective of the current study was to analyze and elucidate the stratification of Lake Toba through the variations of physical, chemical, and isotopic parameters. Water temperature, dissolved oxygen concentrations, water chemical contents, and isotopic parameters were observed periodically from 2016 until 2019. Fourteen sampling points evenly distributed across the surface of the lake were predetermined to represent the North, South, East, and West of the lake. A conductivity, temperature, and depth (CTD) device and Baro-divers were used to measure temperature and conductivity at various depths in the water column for each sampling point. The water samples for isotopic and chemical parameter measurements were collected from water depths of 0, 20, 40, 60, 80, and 100 m using a horizontal transparent acrylic water sampler at each sampling point. Some results from isotope analysis showed that all the water at all levels along the water column had undergone evaporation. Although there were some slight fluctuations, the chemical composition of the lake water was relatively homogenous up to 100 m of depth. The chemical pattern suggested that there was no other secondary process that altered the chemical composition of the lake water, thus the lake water and river water had the same facies. The stratification of Lake Toba was found to be permanent. The depth of the hypolimnion layer was consistently at about 80 m below the surface. However, the depth of the upper layer, the epilimnion, was strongly affected by the climate at the surface of the lake.
