The government’s attempts to turn Lake Toba, the largest permanent body of water in Southeast Asia, into a first-rate tourist destination is facing a major hurdle, as a recent study found that pollution in the lake led to the deaths of millions of fish last month.

The study, conducted by the Environment and Forestry Ministry following the sudden death of millions of fish, revealed that the amount of phosphorus in the lake was 300 percent greater than it was in 2012.

Phosphorus is a common constituent of agricultural fertilizers, manure and organic waste in sewage and industrial effluent. It is an essential element for plant life, but when there is too much of it in water, it can speed up eutrophication (a reduction in dissolved oxygen in water bodies caused by an increase of mineral and organic nutrients) in rivers and lakes.

The mass death of fish in Lake Toba was...