While floating vegetation at Kannamangala Lake (above) attracts birds like the black-winged stilt (below), it also creates anaerobic conditions leading to extensive death of fish.
Credit: Seshadri K S
Waking up to a groggy Sunday morning, we headed to Sawala Kere, or Saul Kere, a wetland nestled in the south-east part of Bengaluru amid multistorey complexes, in the middle of the IT hub. I last visited this lake a year ago, and it was a paradise for birds. The lake had been drained, creating a marshy habitat with shallow water, and the sewage flow was diverted. We saw a Peregrine Falcon swoop down on a flock of migrant Garganey Ducks. The squelchy marsh with receding water had a large flock of a now-uncommon bird called the snipe. In less than an hour, we spotted nearly 50 bird species without moving more than a few hundred metres along the bund.
Returning on a recent Sunday with around 50 birdwatchers, it felt as if a paradise was lost. The picturesque marsh fields were gone, replaced by a water-covered lake with floating vegetation. The birds did not let us down. Migrant birds like the sandpipers gathered along the water’s edge. A few Indian moorhens were feeding on the submerged plants. What shook us, however, was that raw sewage was being let into the lake. There was no sign of the rejuvenation planned at such a large monetary expense.
The lakes we have around the city are all man-made. They were historically interconnected irrigation tanks with embankments against the flow of water, resulting in a deep end and a shallow end. Most irrigation tanks would also act as marshlands that would hold excess water during monsoons. Today, irrigation tanks have lost their agricultural purpose. They have gradually ceased to be tanks and have been converted into buildings. The remaining few have become lakes to hold the excess rainwater and sewage. Wetlands are metaphorical living, breathing systems. Beyond the water, there are complex dynamics that make the waterbody functional in terms of being a wetland itself and other allied services such as aesthetics, food, or water security.
Bengaluru’s lakes have long struggled, lacking clarity on who is responsible for maintaining them. At various points, lakes were managed by the Forest Department, the Bangalore Development Authority (BDA), the Bruhat Bengaluru Mahanagara Palike (BBMP), the Lake Development Authority (LDA), or, if outside of BBMP limits, by the Panchayat. Irrespective of who is responsible, they seem to follow a template for managing wetlands. They start with draining the lake, dredging, and desilting it; in the process, they make it deeper and of uniform depth, like a soup bowl. The dredged muck is piled along the bund to make it raised and later converted into a walking/jogging track, adorned with lights and benches. All of this is picturesque, and people also lap it up.
Engineering solutions
The soup bowl design is bad for two key reasons: One, the natural shallow foreshore is gone. Birds such as the sandpipers that need shallow, squelchy mud are gone. All the migrant birds we call ‘Waders’ are disappearing from these rejuvenated wetlands simply because the habitat is gone. Two, the wetland is made deeper, and water is retained throughout the year. This alters hydrology and increases sedimentation time. As a result, the sewage that is bringing in organic matter settles down at the bottom. This is a rich source of nutrients. What we were witnessing at Sawala Kere was exactly this.
The availability of nutrients resulted in the profuse growth of pistia, or water lettuce. The plants of this genus are native to tropical regions and are globally known as invasive species or pests. This plant is commonly found in all polluted wetlands and has an incredible ability to absorb nutrients and convert them to biomass. These floating plants, called macrophytes, spread both through sexual and asexual reproduction. Thus, in a relatively short time, the plant uses up the nutrients in water and grows rapidly. Why this plant, and what nutrients, you might ask. The answers lie in some basic high school-level biology and chemistry.
Nitrogen is found in limited quantities in ecosystems. Nitrogen-fixing bacteria can convert atmospheric nitrogen to ammonia or ammonium that can be absorbed by plants. This process is called nitrogen fixation. When living organisms die and decompose, the ammonium gets converted to nitrates and then to nitrates in a process called denitrification. When lakes are made deeper and water is retained without outflow, the organic matter in sewage will start to settle, and there will be a lot of ammonia available for plants to grow. In nature, the nutrients are often unequally distributed. The unavailability of some nutrients holds back the profuse growth of plants. If you imagine each nutrient, such as nitrogen or phosphorus, as the length of stakes that make a drum, the volume of water held in the drum will be limited by the shortest stake. This is known as Leibig’s law of the minimum. Sewage is rich in nutrients, including phosphorus, and plants take this up immediately, leading to profuse growth. In another lake, not far from Sawala Kere, we saw a profuse growth of water lettuce. The presence of the plants made a lot of people uncomfortable, but the wetland was great for birds. We saw hundreds of pond herons walking on this mat and feeding off insects. Birds such as the Bronze-Winged Jacana, which are adapted to walking on floating vegetation, have returned. However, the profuse growth can become a breeding ground for mosquitoes and can prevent oxygen exchange in the water, leading to anaerobic conditions, often seen as the mass death of fish. Citizen groups mount pressure, and administrators jump to fix the issue, often without fixing the underlying causes.
Dealing with water lettuce is an expensive affair. The easiest, but often the most damaging, would be the use of herbicides. Another option is to manually remove these plants using expensive machinery that floats on water and physically drags plants out using a conveyor belt. The use of floating bio booms also helps, but only by preventing the plants from spreading across the lake. But all these ‘tech’ solutions are set up to fail from the start. A pragmatic solution would be to de-nitrify the water as quickly as possible. As a way forward, citizen groups and administrators could explore the possibility of harvesting pistia and using it as compost instead of dumping it near the wetland, into which the nutrients will eventually flow back. Nutrients can be removed at the source through effective sewage treatment. While apartment complexes are required to treat their sewage, there is often no accountability or auditing of their effectiveness. Indeed, every lake cannot have a sewage treatment plant, as they are expensive and difficult to maintain. One should reconcile the idea that wetlands are living things and do not always store water.
Historically, a person from the community using the water would be assigned to regulate the water flow. Technology could now be leveraged to use a gravity siphon to automate this. A continuous flow would create surface flow, and plants like Pistia would get washed down. Ultimately, one would have to make a master plan for the city to regulate urban sprawl. This would help reign in the unsustainable use of water resources and their mismanagement. This should be followed through with tweaking loopholes in existing policies, such as ensuring accountability by enabling public audits of existing treatment plants, especially those within private spaces such as apartment complexes. The final step would be to re-imagine our lakes as functional wetlands instead of the aesthetically pleasing waterfront and to ensure that interventions are drawn from ecological and practical wisdom.
(The writer is an ecologist and teaches at ATREE)