Pune sewage reshapes microbial life in Mula-Mutha, and monsoon flows fail to restore it

Sewage and pollution from Pune city are altering the natural makeup of the microorganisms living in the Mula-Mutha river. Even after heavy flows during the monsoon, the condition does not return to what it was. This emerged from a study by the Agharkar Research Institute in Pune, whose researchers reached the finding while studying the river's microalgae. The study made it clear that natural flow may wash pollutants away, but the effect of urban pollution on the river's biodiversity persists.

Earlier research had shown that in tropical rivers, seasonal floods flush out pollutants and the natural mix of microbial species is re-established. In the case of the Mula-Mutha, this study showed that the effect of heavy monsoon flow is limited. During floods the makeup of the microorganisms does change for a short time, but the level of pollution decides which species survive.

Researchers Pratyasha Nayak and Dr Balasubramanian Karthick collected samples in 2022 and 2023 from 17 locations, from the river's headwaters to urban and industrial areas. The samples included water as well as the biological film growing on stones. Using them, they studied diatoms, which are single-celled microalgae.

Each diatom cell is surrounded by a shell of silica. These organisms are sensitive to environmental changes and are therefore used to assess water quality. For this reason, the study of diatoms is considered important for understanding how river pollution is affecting the ecosystem.

The study found that the overall diversity of diatom species at any one point on the river stays more or less stable across seasons. But as the river moves from its headwaters towards the urban stretch, large changes appear in the composition of species. Near the source, where the water is clean and rich in dissolved oxygen, native species of the Western Ghats were found. In the urban stretch, rising sewage and nutrients reduced the numbers of these sensitive native species. Pollution-tolerant species were seen increasing in their place.

The research also showed that the main chemical factors behind the change in microbial makeup are the water's electrical conductivity and its dissolved oxygen. If dissolved salts in the water increase, its electrical conductivity can rise. Sewage also affects the amount of oxygen in the water. As a result, the survival of native species that are sensitive to environmental changes can be affected.

Maintaining natural flow alone is not enough to save the river. Urban sewage must be properly treated and the pollutants discharged into the river kept in check. Strict enforcement of water quality standards is also important.