Algal blooms choke a wetland, threatening biodiversity and livelihoods

Harmful algal blooms, driven by untreated sewage, urban runoff, and solid waste, are appearing in Deepor Beel, a wetland in Assam.
Algal blooms threaten biodiversity in the wetland and its ability to support local communities.
Researchers stress holistic conservation, reducing nutrient inflows, conserving surrounding habitats, and long-term microbial monitoring.

As Guwahati’s major stormwater storage basin, the Deepor wetland is under mounting stress from unregulated urban runoff. Combined with land-use change and solid waste dumping, these pressures have triggered eutrophication or accumulation of excessive nutrients, leading to harmful algal blooms that threaten the wetland’s biodiversity and its ability to support local communities.

Home to 68 fish species, more than 234 native and migratory bird species, including 18 globally threatened vertebrates as well as diverse amphibians, reptiles and more, Deepor Beel as it is known in Assam, is a biodiversity hotspot and a Ramsar wetland site. It functions as a corridor and a feeding ground for elephants, and is a groundwater recharge site. For the 800 fishing households dependent on it, the wetland contributes an estimated ₹11.6 crore (₹116 million) annually. But fishers now report declining catches and the disappearance of several species.

A team from the Indian Institute of Science Education and Research (IISER), Kolkata monitored the wetland from November 2024 to February 2025, studying blooms of two common toxin-producing cyanobacteria (blue-green algae), Planktothrix and Microcystis, and the environmental conditions driving their formation.

“Organisms in lakes and ponds often encounter algal blooms during their lifespan. While algal blooms are natural events, increasing human-driven eutrophication and global climate change have made them more frequent, abundant, and persistent worldwide,” explained Rajkumari Nikita, who has been conducting on-ground data collection at Deepor Beel and closely observing its dynamic ecosystem. “It causes hypoxia (low levels of oxygen) which can cause oxygen depletion in the aquatic ecosystem, block sunlight, and release toxins that can accumulate in the food web.”

The researchers collected samples from six stations of the monitoring programme, the Deepor Beel Ecological Time Series (DBETS). Established by the Integrative Taxonomy and Microbial Ecology Research Group of IISER-Kolkata in 2022, the DBETS programme that studies the health of the wetland has transitioned from seasonal surveys to a monthly monitoring program in 2025. Key water quality parameters were measured, including surface temperature, pH, dissolved oxygen, total dissolved solids, electrical conductivity, and transparency, and correlated with satellite observations.

Microcystis algae, as seen under a microscope. Researchers from Kolkata monitored Deepor Beel from November 2024 to February 2025, studying blooms of two common toxin-producing blue-green algae, Planktothrix and Microcystis. While algal blooms are natural, increasing human pressures on wetlands are making them more frequent and abundant, say the researchers. Representative image by NOAA Great Lakes Environmental Research Laboratory via Wikimedia Commons (CC BY-SA 2.0). — Microcystis algae, as seen under a microscope. Researchers from Kolkata studied blooms of two common toxin-producing blue-green algae at Deepor Beel, Planktothrix and Microcystis. While algal blooms are natural, increasing human pressures on wetlands are making them more frequent and abundant, say the researchers. Representative image by NOAA Great Lakes Environmental Research Laboratory via Wikimedia Commons (CC BY-SA 2.0).

Pollution drives algae blooms

“One of the clearest findings of the study is that nutrient pollution, such as nitrate, phosphate, ammonium and silicate, plays a decisive role in bloom formation. These nutrients are largely sourced from untreated sewage, agricultural runoff, and solid waste carried into the wetland by the Basistha and Kalmani rivers, the main sources of water to the wetland, and local monsoon runoff, ” stated Punyasloke Bhadury of the Centre for Climate and Environmental Studies, IISER Kolkata.

It observed that surface water temperature decreased by approximately 2-3 °C between November (23-25 °C) and February (20-22 °C). The water was more alkaline in November, while dissolved oxygen levels increased in February, reflecting the higher oxygen-holding capacity of cooler water. The quantities of total dissolved solids and electrical conductivity were significantly higher in February, indicating an increase in dissolved salts and pollutants.

The findings reveal a rapid shift in harmful algal blooms, which dominated and reshaped the wetland’s aquatic communities within a span of just four months. In November, Planktothrix dominated the bloom, thriving in warm alkaline waters, exhibiting high cyanobacterial dominance and low taxonomic diversity.

By February, as temperatures dropped and nutrient levels increased, Planktothrix declined, giving way to a more diverse assemblage of green algae (Chlorella), diatoms (Navicula and Aulacoseira), and euglenoids (Euglena and Phacus).

“Fish assemblages such as small indigenous fish species are dependent on algal communities for their food and shelter. Any change in the communities will alter the food availability for these species which are essential for local fisherfolk. The change of bloom to the above organisms can rapidly change the dissolved oxygen, which can cause death of fish and other organisms, ” observed Bhadury.

“Statistical analysis showed Planktothrix growth was strongly linked to high pH and light penetration, while Microcystis blooms thrived with high nitrate and silicate concentrations. Meaning that even subtle changes in pollution and water chemistry can decide which harmful cyanobacteria take over the wetland,” he added.

Beyond phytoplankton, the researchers studied bacteria, fungi, and other microscopic life in the lake. Using DNA sequencing techniques, they found bacterial communities differed from one part of the wetland to another. Cleaner upstream sites were dominated by bacteria like Burkholderiaceae and Comamonadaceae, while opportunistic and potentially pathogenic (disease-causing) bacteria such as Aeromonadaceae and Enterobacteriaceae enriched the more polluted zones. Fungi, particularly Ascomycota and Basidiomycota, were dominant across all sites, indicating their role in breaking down organic matter. Other groups, such as rotifers, arthropods, and protists, also fluctuated with bloom conditions, highlighting how algal blooms ripple through different layers of the food web.

These microbial shifts act like ecological fingerprints, reflecting the degree of pollution in different areas of the wetland.

“Points where harmful algal blooms were detected indicate declining water quality, primarily driven by excess nutrients flowing in from surrounding rivers and rivulets. Bioremediating these sewage-fed rivers can help restore water quality in Deepor Beel. From a conservation perspective, this is crucial for sustaining endemic biodiversity and, ultimately, for safeguarding the wetland’s Ramsar status,” Bhadury stated.

The study recommended three key strategies. First, reducing nutrient inflow by managing sewage, agricultural runoff, and solid waste more effectively; second, conserving habitats within and around the wetland to maintain ecological resilience; and third, continuing long-term monitoring of phytoplankton and microbial communities to anticipate and mitigate harmful blooms.

“Field-based monitoring of the wetland, along with the use of environmental DNA (eDNA) as a biomonitoring tool, could be highly beneficial. Additionally, the involvement of local communities, particularly through sensitising fisherfolk who already possess traditional knowledge, is equally important, ” added Nikita.

A waste dumping ground on the banks of Deepor Beel. Researchers say that nutrient pollution plays a decisive role in bloom formation. These nutrients enter the wetland from untreated sewage, agricultural runoff, and solid waste carried into the wetland by the Basistha and Kalmani rivers. Image by India Water Portal via Flickr (CC BY-NC-SA 2.0).

Zonal master plan for conservation

IISER Kolkata’s findings come at a crucial time, coinciding with the Ministry of Environment, Forest and Climate Change’s draft notification to declare a 5 km Eco-Sensitive Zone (ESZ) around the Deepor Beel Wildlife Sanctuary, covering 38.84 sq. km. The notification points to the very stressors that the study identify as drivers of harmful algal blooms and ecological instability; and mandates the Assam government to prepare a Zonal Master Plan within two years that integrates ecological priorities into development decisions.

The proposed measures include shielding forests, agricultural land, and open spaces from industrial or commercial conversion, restoring degraded areas, protecting catchments feeding the wetland, and prohibiting discharge of untreated effluents. Stricter rules for waste management, pollution control, and tourism are also outlined, while positive interventions such as rainwater harvesting, organic farming, renewable energy use, and environmental awareness are encouraged to strengthen ecological resilience.

However, urban planner and placemaking practitioner Urmi Buragohain argues that the current draft boundary falls short of addressing the real sources of pollution. “The study highlights that stations proximal to sewage/storm inflows and waste influence zones show significantly higher cyanobacterial abundance and taxonomic shifts to bloom-prone genera, while stations farther from direct inflows show lower counts and more stable assemblages. That spatial pattern indicates the ESZ must include the sources and conduits, not just the waterbody,” she explained.

Buragohain points out that the existing ESZ outline fails to capture the hydrological catchments and stormwater corridors feeding into Deepor Beel. “It leaves out the Basistha–Bahini/Bharalu system, which channels sewage-laden stormwater through the Pamohi canal and Mora Bharalu into the wetland. And it excludes the legacy waste site at Boragaon from where leachate pathways continue to threaten it,” she said.

“The Boragaon/East-Boragaon waste footprint, leachate ponds, conveyors, and drains should be brought within the ESZ and designated as a ‘Restoration & Hazard-Mitigation Sub-zone’. Creating minimum buffers around the wetland’s open waters and marshes would help reduce nutrient inflow from nearby habitations. Controlling the Khanajaan- the outflow channel to Brahmaputra, and low-lying backwaters that regulate the water residence time of the beel would further strengthen protection,” she added.

“Integrating scientific evidence into the Zonal Master Plan could ensure that conservation measures for Deepor Beel are not only well-intentioned but also effective, safeguarding one of Assam’s most critical wetlands while balancing urban development pressures,” opined Bhadury. “Lessons from Deepor Beel can guide the management of riparian wetlands both regionally and globally, and may even contribute to developing a global framework for monitoring harmful algal blooms across Ramsar sites.”

Banner image: Deepor Beel is Guwahati’s major stormwater storage basin, and is facing stress from unregulated urban runoff. Image by Arnie via Flickr (CC BY 2.0).