Pune: Monsoon might change the flow and chemistry of Mula-Mutha river but can’t erase the impact of pollution on its aquatic life because urban sewage and other human-induced changes had a stronger influence as it passed through Pune, a study by Agharkar Research Institute revealed.Microscopic algae in the river changed more significantly from one stretch to another than between seasons, researchers found. Pollution-tolerant species dominated the impacted midstream and downstream areas, it found.The researchers studied diatoms or microscopic algae that lived on submerged surfaces and were sensitive to changes in their surroundings at 17 locations across the Mula-Mutha river basin during pre-monsoon, monsoon and post-monsoon periods in 2022 and 2023. They identified 170 diatom species.The river basin showed a clear change from forest-dominated upstream areas in the Western Ghats to urbanised and industrialised midstream stretches, and agricultural areas downstream. The study found that the overall number and diversity of species remained relatively stable, but the types of species changed substantially along the river.In the upstream stretches, where the water is better oxygenated, several sensitive diatom species were found. In the midstream sections affected by urban inputs, the community was more strongly shaped by environmental conditions and pollution-tolerant species were more common. “The midstream stretch, characterised by urban inputs and altered water quality, likely imposes selective pressures that favour tolerant taxa,” a researcher said.Pratyasha Nayak and Balasubramanian Karthick, the study’s authors, identified dissolved oxygen and electrical conductivity as the two main environmental factors linked to changes in diatom communities. Higher dissolved oxygen was associated with upstream sites, while increasing conductivity was seen towards midstream and downstream stretches, reflecting the influence of human-induced inputs.“The study found that monsoon flows did influence the river’s microscopic communities, particularly by increasing movement and mixing of organisms. During monsoon and post-monsoon periods, community assembly was predominantly influenced by stochastic processes, suggesting greater dispersal during high-flow conditions,” said Nayak.The seasonal changes were weaker than the differences seen between the river stretches. The researchers said the findings showed “stronger spatial than seasonal differentiation” in diatom community structure.Nayak and Karthick found that the polluted middle and lower reaches had largely lost the endemic diatom species associated with the Western Ghats. These were replaced by widespread species that can tolerate pollution. Karthick said, “This pattern indicated a major loss of ecological uniqueness linked to nutrient enrichment and urban sewage loading.”The study identified dissolved oxygen and electrical conductivity as significant drivers of the diatom communities, while sulphate, magnesium and fluoride were not individually significant. The authors noted that the shift along the river represented a transition from oxygen-dominated conditions upstream to ion- and pollution-related conditions downstream.The researchers concluded that strong seasonal changes in water chemistry did not result in equally strong seasonal changes in the diatom communities, particularly in the midstream and downstream sections. They attributed this to the influence of chronic urban pollution and sewage. “Without adequate water quality, hydrological flow alone may not sustain native and endemic biodiversity,” the authors said.The study, therefore, pointed to water quality as an important factor alongside maintaining river flows. The authors stated that long-term nutrient loading and pollution could override natural seasonal dynamics in monsoon-fed tropical rivers.
