Analysis of main components of Lake Toba’s water quality in different seasons

Lake Toba is one of the largest lakes in North Sumatra Province, Indonesia. Its waters are used for multiple purposes, constituting an important natural and economic resource. Most of the waters of Lake Toba come from the overflow of disposal of agriculture, livestock, fisheries, tourism, households, and other activities. The present study identified water quality based on total nitrogen, total phosphorus, chlorophyll-a and other water quality parameters carried out at 60 sampling sites grouped into 6 observation stations, i.e., control areas (located in the middle and far from direct activities), community floating net cages, company floating net cages, settlement, hospitality, and river mouths. The main water quality components were very dynamic at the monitoring stations in three seasons (rainy, transition, and dry). Total nitrogen concentration tended to be higher in the rainy season than in the transition and dry seasons. However, all stations tended to be classified as mesotrophic or higher in all seasons, with total nitrogen concentrations greater than 12.5 mg L–1. Total phosphorus at the six stations was highly dynamic in all seasons and tended to decrease in the dry season. The concentration of total phosphorus was higher at the settlement and hospitality stations than at the other stations. The total phosphorus of the settlement and hospitality stations reached 0.18 mg L–1 and 0.17 mg L–1, respectively, in the rainy season. In general, total phosphorus concentrations in the waters of Lake Toba were above 0.1 mg L–1, which allowed the lake to be classified as above mesotrophic status.