Remote Sensing of Water Quality in Lake Toba (Indonesia)

Lake Toba, on the island of Sumatra, is the largest volcano-tectonic lake in the world and is the location of a massive super-eruption which occurred approximately 75,000 years ago. The caldera is presently home to over a half-million inhabitants and has broad economic significance for the region. Since the 1990s, Lake Toba has suffered from a continual decline in water quality due to anthropogenic, climatic and geological pressures, all of which threaten the long term value and sustainability of ecosystem-services provided by the lake. An assessment of available remote sensing platforms for monitoring water quality of Lake Toba was made to assist the government of Indonesia in strengthening capabilities for water quality monitoring in this region. We surveyed available earth observation imagery, including Sentinel-2A, Landsat-8, MERIS, Sentinel-3A and Himawari-8 to generate synoptic visualizations of water quality and land use and evaluate feasibility of water quality monitoring using satellite remote sensing. We constructed time series focusing particularly on turbidity and chlorophyll at the lake surface as indicators of eutrophication. Additionally, we discuss a particularly interesting water quality event in the Southeastern portion of the lake near the town of Bakara, which is visible in high contrast from multiple satellite sensors.

A water-quality event on January 9th 2017 in the SE of part of Lake Toba (Baktiraja region) is clearly visible in remote sensing imagery from multiple platforms including Landsat-8, Sentinel-3A and Himaware-8. The reported in-situ water-quality reached severe hypoxic conditions and resulted in significant loss of local aquaculture. Although the clohrophyll-a levels were elevated, reaching 10mg/m ³ to 30 mg/m ³ locally, satellite imagery shows evidence of benthic sediment re-suspension, likely driven by discharge from the Aek Silang watershed. This evidence supports a hypothesis that re-suspension of the benthic-microbial mat could also influence vertical mixing of Hydrogen-sulfide (H2S), additionally impairing the water-quality following significant meteorological events. Work is ongoing to design operational approaches for water quality monitoring in Lake Toba using remote sensing imagery and scarce ground measurements