Water Quality and Trophic Status of Lake Toba, North Sumatra, Indonesia

Lake Toba located in the North Sumatra Province, Indonesia is utilized locally for various interests, such as tourism, transportation, hydroelectric power, drinking water, and fisheries. The industrial and anthropogenic activities result in the degradation of water quality in lake ecosystem. The main problem is due to the accumulation of nutrients in the form of nitrogen and phosphorus in lake water. The purpose of this study was to determine the trophic status of Lake Toba based on nutrient content through survey. The study was carried out from April until July 2018 at three sampling sites, including the waters of Haranggaol (representing the floating net cages for aquaculture), Ajibata-Parapat (representing the sites impacted by household pollution and water transportation), and the central part of the lake as a reference or control site. The obtained results showed that various sampling sites of Lake Toba differed in their water quality as confirmed from the multivariate analysis using a principal component analysis (PCA). The trophic state index (TSI) based on chlorophyll a content, secchi disk depth or water transparency, and total nitrogen and phosphorus content categorized Lake Toba as meso-oligo-eutrophic lake with a consistent mesotrophic characteristics based on abundant nutrient availability at all sampling stations. Compared to the previous study carried out in 2009, the trophic state of Lake Toba has shifted from oligotrophic to mesotrophic which needs a special attention to prevent eutrophication by excessive phosphorus enrichment in future.