GUWAHATI, ASSAM — Harmful algal blooms driven by extreme eutrophication from untreated sewage, urban runoff, and solid waste, are causing severe stress in Deepor Beel, a Ramsar wetland site and crucial biodiversity hotspot in Assam. The blooms are threatening the wetland rich ecosystem, including its 68 fish species and over 234 native and migratory bird species, and jeopardizing the livelihoods of approximately 800 local fishing households.
A study conducted by the Indian Institute of Science Education and Research Kolkata, monitored the wetland from November 2024 to February 2025, focusing on two toxin-producing cyanobacteria: Planktothrix and Microcystis.
Nutrient Pollution Drives Ecological Shifts
The research clearly established that nutrient pollution sourced from untreated sewage, agricultural runoff, and solid waste carried by the Basistha and Kalmani rivers plays a decisive role in bloom formation. These blooms cause hypoxia (low oxygen levels), block sunlight, and release toxins that accumulate in the food web.
The study recorded a rapid shift in the dominant algal communities over four months:
November (Warmer, Alkaline Water) bloom was dominated by Planktothrix, which thrives in warm, alkaline conditions.
February (Cooler Water, Higher Nutrients) as temperatures dropped and nutrient levels increased, Planktothrix declined, giving way to a more diverse assemblage including green algae (Chlorella), diatoms, and euglenoids.
Researchers warned that this rapid shift in algal communities will alter the food availability for fish, which are essential for local fisherfolk who already report declining catches. Statistical analysis confirmed that subtle changes in pollution and water chemistry determine which harmful cyanobacteria take over the wetland. Furthermore, microscopic analysis found that polluted zones were enriched with opportunistic and potentially pathogenic bacteria (such as Aeromonadaceae), reflecting the degree of pollution.
Conservation and Master Plan Recommendations
The findings coincide with the Ministry of Environment, Forest and Climate Change’s draft notification to declare a 5 km Eco-Sensitive Zone around the Deepor Beel Wildlife Sanctuary. The notification mandates the Assam government to prepare a Zonal Master Plan to integrate ecological priorities and prohibit the discharge of untreated effluents.
To ensure effective conservation and safeguard the wetland’s Ramsar status, the \text{IISER} study recommended three key strategies:
• Reduce Nutrient Inflow: Implement better management of sewage, agricultural runoff, and solid waste.
• Conserve Habitats: Protect and restore surrounding habitats to maintain ecological resilience.
• Long-term Monitoring: Continue monthly monitoring of phytoplankton and microbial communities, ideally involving local fisherfolk with traditional knowledge.
However, urban planners warn that the proposed boundary may be inadequate, as it fails to include the primary sources and conduits of pollution, specifically the sewage-laden Basistha–Bahini/Bharalu system and the leachate pathways from the legacy waste site at Boragaon. They recommend designating the waste footprint as a ‘Restoration & Hazard-Mitigation Sub-zone’ to effectively curb nutrient inflows.