East Kolkata Wetlands – fast turning death traps for aquatic ecosystems?

Fishermen use wastewater from Kolkata to rear fish (Image Source: India Water Portal)
Fishermen use wastewater from Kolkata to rear fish (Image Source: India Water Portal)

While freshwater ecosystems in India are known to harbour rich biodiversity, their health is being increasingly challenged in recent years. And the East Kolkata Wetlands, one of the important Ramsar sites in India, and the largest wastewater fed aquaculture systems in the world that provide fish and support paddy and vegetable cultivation, are no exception.

The East Kolkata Wetlands, often referred to as the ‘Kidneys of East Kolkata’, are located on the eastern periphery of Kolkata Metropolitan area and cover 12,500 hectares of area. The entire geographical region of EKW is dominated by water bodies along with 254 sewage-based fishing grounds (5852.14 hectares), agricultural land (4718.56 hectares), productive farming zone (602.78 hectares) and urban and rural settlement areas (1326.52 hectares) respectively.

What is so unique about EKWs?

Chakraborty and Das Gupta in their report inform that the city of Kolkata generates an estimated 750 mililitres of sewage every day. This wastewater is converted into food and used in fisheries and agriculture across the sprawling 12,500 hectares of wetland area! And the process through which the waste gets converted into food is unique as well!

The wastewater from the city is led by underground sewers to pumping stations in the eastern limit of the city and then pumped into open channels. This sewage is then drawn by the local fishery owners into the fish ponds or bheris directly from the tributary wastewater canals. Bheris are a unique feature of the Kolkata wetlands, and are shallow fishponds fed by naturally treated wastewater rich in algae, which allows for low-cost fish cultivation. Organic waste gets loaded in the bheris at the rate of 20 to 70 kg per hectare per day. It is allowed to stay in the sun and these sewage filled fishery ponds act as solar reactors.

The heat generated by the sun is adequate for photosynthesis to take place and helps in the growth of a dense plankton population in the bheri, which in turn grows on the organic matter in the wastewater. This high growing plankton serves as food for the fish population who thrive on this nutrient rich plankton. The fish play a two fold role – they maintain the balance of the plankton population in the pond and convert the available nutrients in the wastewater into readily consumable form (fish) for humans.

Organic pollution in the wastewater is thus reduced by 80 percent and the coliform bacteria in the wastewater are reduced by 99.9 percent in these ponds. Channels drain out the effluents and slurry from the treated wastewater, that is then used to grow rice and vegetables. Around a quarter of the city’s fish and vegetables are grown from the bheris.

Not only do the wetlands help in providing cheap food and vegetables for the city, but also support the livelihoods of about 1,18,000 people.

Urbanisation takes its toll

However, urbanisation and anthropogenic changes have taken their toll. Along with illegal land acquisition and wetland conversion, loads of untreated organic and inorganic wastes from industries, agriculture and  sewage from the city are being dumped into the shrinking wetlands at a rapid pace.

This has led to a gradual deterioration and eutrophication of the wetlands, threatening the aquatic life that is crucial for the survival and sustenance of the wetlands and its unique water filtering system, warns the paper titled ‘Anthropogenic stress on a Ramsar site, India: Study towards rapid transformation of the health of aquatic environment’ published in the journal Environmental Challenges.

What is eutrophication

Eutrophication is characterised by excessive plant and algal growth in water bodies due to the increased availability of one or more factors needed for photosynthesis such as sunlight, carbon dioxide and nutrient fertilisers. And human activities are accelerating the rate and extent of eutrophication through both point-source discharges and non-point discharge of nutrients such as nitrogen and phosphorus (cultural eutrophication) from agricultural, industrial pollutants and discharge of untreated sewage.

These have been found to be very harmful for drinking water sources, fisheries, and recreational water bodies. This is because nutrient enrichment due to anthropogenic activities can trigger algal blooms in the water bodies. Algal blooms limit light penetration, reduce growth and cause death of plants in littoral zones while also affecting the ability of organisms living in the water to access food. When algal blooms die, the accompanying microbial decomposition severely depletes dissolved oxygen, creating a hypoxic or anoxic ‘dead zone', which serves as a death knell for a number of organisms living in the water.

Some algal blooms also produce toxins and such harmful algal blooms (HABs) are known to have a negative impact on water quality, fisheries and increase public health risks. Within freshwater ecosystems, cyanobacteria are the most important phytoplankton associated with HABs and blooms of toxic cyanobacteria can lead to poisoning of domestic animals, wildlife and even humans. Cyanobacteria have also been found to affect the safety and potability of  municipal drinking water systems as well as aquaculture-raised fish, resulting in large financial and increase in health risks.

Eutrophication thus leads to disruption in the natural ecological balance of freshwater ecosystems and can be extremely harmful for freshwater biodiversity.

And the EKWs are choking!

The paper discusses the findings of a study that explores the extent of seasonal variations in water quality of the EKW  and eutrophication in the context of increasing impact of anthropogenic changes on the EKW.

The study reveals that the surface temperature of the wetlands and the total dissolved solids are higher during pre-monsoon period than the post-monsoon season and the concentrations of inorganic nitrogen, total phosphorus, potassium and ammonium-nitrogen in the water are very high in the pre-monsoon than the post monsoon season, which serves to accelerate the process of eutrophication in the wetland ecosystem.

The dissolved oxygen (DO) levels in the water are low while the biological oxygen demand (BOD) levels and chlorophyll concentrations have increased. About 40 percent of the wetlands come under eutrophic and hyper-eutrophic stages due to higher accumulation of nitrogen and phosphorus from various non-point sources.  

Every day about 600 million litres of sewage and 2500 t of solid wastes are generated from the Kolkata Municipal Corporation (KMC) area and these are dumped near the EKW site. A crisscross of canals over the city of Kolkata drain out monsoonal rainwater from the city into the wetlands while  various city effluents are released into these outfall canals through the six pumping stations.

The sewage is distributed among the fisheries of EKW region with the help of this 61 km long canal system for sewage-based pisciculture.  In this region, 980 million litres of effluents are recycled every- day whereas approximately 3000 tons of metropolitan wastes are treated in the dumping ground of Dhapa per day.

However, the effluents in large quantities that are transported through canals from urban and periurban areas are not always treated properly before being released into the canals thus leading to deterioration of the wetland ecosystem. The effluent carrying canals have been found to be contaminated due to the disposal of several non-degradable wastes. Also as majority of water bodies are not protected by embankments and the harmful nutrient rich effluents are discharged into the wetlands and trigger the undesirable phytoplankton growth.

Toxic chemical fertilisers such as ammonium sulfate, calcium nitrate, super phosphate, ammonium nitrate, urea etc. are now used in the paddy fields and vegetable farms near the EKW region and these inorganic compounds are easily transported into the water bodies through surface run-off during monsoon season. At the same time, various non-biodegradable wastes including domestic wastes, medical wastes, industrial wastes etc. also get deposited into the wetlands through surface run- off. This has triggered the process of eutrophication in the wetlands.

The study argues that measures such as:

  •  Promoting sustainable agricultural practices; discouraging use of chemical fertilisers and encouraging use of organic products in agriculture and proper treatment of sewage discharged from the city before being discharged into the wetlands can be greatly useful in preventing further deterioration of the wetlands.
  • Construction of embankments to prevent flow of contaminated sewage into the wetlands; regular dredging of sediments from the bottom of the water bodies and flushing out of high turbid water with the help of fresh rainwater and techniques such as phyto-remediation to clean out the nutrient-rich water can greatly help in restoring the health and functionality of this unique wetland system in Kolkata.