Threats to Urban Wetlands: Pollution, Urbanization, and Climate Change

Urban wetlands are some of the most valuable yet overlooked ecosystems in our cities. They filter water, control floods, recharge groundwater, and support rich biodiversity. Yet, across the world – and especially in rapidly growing Indian cities – these critical habitats are disappearing at an alarming rate. The threats are multi-layered: unchecked urban expansion, rampant pollution, and the growing pressures of climate change. Understanding these threats is the first step toward protecting what remains.

Urbanization and habitat loss 🔗

The most visible threat to urban wetlands is the physical encroachment by expanding cities. As populations surge and the demand for housing, roads, and commercial spaces grows, wetlands are often treated as vacant land available for development. This leads to their draining, filling, and conversion – permanently erasing ecosystems that took centuries to form.

The scale of wetland loss in Indian cities 🔗

India offers some of the starkest examples of urban wetland destruction. According to the Observer Research Foundation, between 1970 and 2015, Chennai lost 90 percent of its wetlands, Bengaluru lost 55 percent, and the Delhi-National Capital Region lost 38 percent . These losses are driven primarily by construction activity and real estate development, often carried out with little regard for the ecological consequences.

According to estimates by Wetlands International South Asia, nearly 30 percent of India’s natural wetlands have been lost over the past three decades, mainly due to illegal construction, unsustainable urbanization, agricultural expansion, and pollution. That is a massive erosion of ecological infrastructure in a relatively short time.

Bengaluru: the “city of lakes” that lost its lakes 🔗

Bengaluru is a particularly telling case. Historically celebrated as the “city of lakes,” Bengaluru’s water bodies declined from 1,452 in the early 1800s to just around 80 today, reducing their water storage capacity from 2,715 million litres per day (MLD) to 388 MLD. This drastic reduction is the result of encroachment, rapid concretization, and the severing of connections between the city’s cascading lake systems.

The built-up area in Bengaluru expanded enormously, while water body coverage shrank by roughly half between the 1990s and 2020. The consequences are now playing out in real time: during summer, the city faces acute water shortages, and during monsoons, it suffers devastating floods – both directly linked to the degradation of its lake and wetland systems.

Delhi’s fragmented wetland governance 🔗

Delhi presents a different kind of challenge. A study found that of 629 water bodies identified in the National Capital Territory of Delhi, as many as 232 could no longer be revived due to large-scale encroachments. The problem is compounded by fragmented governance. Wetlands in Delhi fall under multiple agencies – the Delhi Jal Board, Delhi Urban Shelter Improvement Board, Public Works Department, Archaeological Survey of India, and Forest Department – yet only the Forest Department and Delhi Jal Board have any real expertise in wetland management.

This overlapping jurisdiction with no clear accountability means that wetlands slip through the administrative cracks, with no single agency taking ownership of their conservation.

Why wetland loss triggers urban flooding 🔗

Wetlands act as natural sponges. They absorb stormwater, slow down surface runoff, and reduce the peak intensity of floods. When cities destroy wetlands and replace them with impervious surfaces like concrete and asphalt, they lose this critical buffer. Research by WRI India highlights that natural ecosystems such as lakes, wetlands, and forests function as natural infrastructure that absorbs stormwater and reduces runoff volume and speed, and the destruction and encroachment of these spaces deteriorates the natural flood defence of cities.

The recurring floods in cities like Hyderabad, Chennai, and Bengaluru in recent years are directly linked to the loss of wetland areas that once absorbed excess rainwater.

Pollution and eutrophication 🔗

Even wetlands that have not been physically destroyed often face a slower but equally damaging threat: pollution. When untreated sewage, industrial effluent, and urban runoff flow into wetlands, they overload these systems with nutrients and toxins, fundamentally altering their chemistry and ecology.

How untreated sewage degrades wetlands 🔗

In many Indian cities, wetlands essentially function as sewage dumps. The River Yamuna, which passes through six Indian states, receives approximately 1,789 million litres per day (MLD) of untreated wastewater from Delhi alone – about 78 percent of the river’s total pollution load. This has caused dissolved oxygen levels to plummet and biological oxygen demand to spike, making the Delhi segment of the Yamuna one of the most polluted river stretches in the country.

When similar volumes of partially treated or raw sewage enter wetlands, the results are predictable. Monitoring of Indian aquatic resources shows that water bodies near urban centres are becoming increasingly saprobic and eutrophicated due to the discharge of partly treated or untreated wastewater.

Understanding eutrophication 🔗

Eutrophication occurs when excess nutrients – primarily nitrogen and phosphorus – enter a water body and trigger explosive algal growth. As the UN Office for Disaster Risk Reduction explains, untreated or inadequately treated sewage remains a major source of nitrogen and phosphorus, often originating from municipal wastewater or agricultural runoff.

Elevated levels of nitrogen and phosphorus lead to eutrophication, which can result in toxic algal blooms that deplete oxygen levels and create dead zones. When these algae die and decompose, they consume large quantities of dissolved oxygen in the water, creating hypoxic (low-oxygen) conditions. Fish and other aquatic organisms that cannot survive in such conditions either migrate or die.

Industrial pollutants and heavy metals 🔗

Beyond nutrient pollution, industrial discharges introduce heavy metals and persistent chemical pollutants into wetland sediments, where they accumulate over time. These chemicals can be toxic to aquatic life, reducing biodiversity and disrupting ecosystem functions. Heavy metals like lead, mercury, and cadmium can enter the food chain, posing long-term health risks not just to wildlife but also to communities that depend on wetland resources for food and water.

Biodiversity loss from pollution 🔗

The combined effect of sewage, industrial waste, and nutrient overloading is a dramatic decline in wetland biodiversity. By 1996 in Bengaluru, sewage and eutrophication had affected roughly 28 percent of the city’s lakes, effectively turning them into sewage dumps and threatening local ecosystems. Native plant species are replaced by invasive weeds like water hyacinth, which choke waterways and further reduce oxygen levels. Birds, fish, amphibians, and invertebrates that depend on clean, oxygen-rich water are pushed out.

The US Environmental Protection Agency identifies pollutants such as sediment, fertilizers, human sewage, pesticides, and heavy metals as key threats, noting that these inputs can exceed a wetland’s natural capacity to absorb them, causing irreversible degradation.

Climate change and wetland sustainability 🔗

Pollution and urbanization are immediate, visible threats. Climate change, however, is a more pervasive force that is altering the very conditions wetlands need to exist. Changes in temperature, precipitation patterns, and the frequency of extreme weather events are reshaping wetland hydrology in ways that are difficult to reverse.

Altered precipitation patterns 🔗

According to the Washington State Department of Ecology, climate change is expected to impact wetlands due to changes in temperature and the timing and amount of precipitation, which can alter wetland conditions and processes, including the types of habitat they provide and their ability to manage water quality and flooding.

For wetlands, water is everything. They depend on predictable cycles of rainfall and dry periods. When climate change disrupts these cycles – delivering too much rain in short bursts or prolonged droughts – wetlands cannot maintain their normal functions. In some regions, wetlands that rarely dry out may shift to more frequent drying, while in others, increased rainfall may cause prolonged waterlogging that changes plant composition entirely.

Rising temperatures and evapotranspiration 🔗

Warmer temperatures, drought, and changing precipitation patterns can increase evapotranspiration and lead to water losses in wetlands. Higher temperatures accelerate the rate at which water evaporates from wetland surfaces and transpires through vegetation, reducing the amount of water available to sustain the ecosystem.

A 2024 study published in Nature Communications modelled the future of North American wetlands and found that under high emission scenarios, annual wetland area could decrease by approximately 10 percent at the continental scale, with regional changes varying up to 50 percent. Critically, the dominant driver of wetland changes shifts from precipitation to temperature in higher emission scenarios, causing substantial drying during summer when biological activity peaks.

Wetlands shifting from carbon sinks to carbon sources 🔗

One of the most concerning climate-related impacts is the potential for wetlands to switch from being carbon sinks to carbon sources. Healthy, undisturbed wetlands store enormous amounts of carbon in their soils – the organic soil in peatlands can take thousands of years to develop, with up to 250 years required for just one inch of peat to accumulate.

Research published in the Journal of Cleaner Production warns that higher temperatures resulting in drought might shift the role of both constructed wetlands and peatlands from a carbon sink to a carbon source. When wetland soils dry out, the stored organic matter begins to decompose aerobically, releasing carbon dioxide and methane into the atmosphere. This creates a dangerous feedback loop: climate change degrades wetlands, which then release more greenhouse gases, further accelerating climate change.

Extreme weather events and cascading impacts 🔗

Climate change also increases the frequency and intensity of extreme weather events. Climate-change-driven rainfall variability is increasing, with cities receiving rainfall equivalent to monthly or seasonal averages within just a few days or even hours. For urban wetlands already weakened by encroachment and pollution, these sudden deluges can cause erosion, flush in additional pollutants, and overwhelm the wetland’s capacity to store and filter water.

The UN Office for Disaster Risk Reduction notes that climate change contributes to wetland loss through rising temperatures, altered precipitation patterns, and increased frequency of extreme weather events such as storms and droughts, which disrupt hydrological regimes and damage wetland habitats.

The compounding effect of multiple threats 🔗

What makes the situation particularly dangerous is that urbanization, pollution, and climate change do not act in isolation. They compound each other. A wetland that has been reduced in size by encroachment has less capacity to absorb polluted runoff. A polluted wetland is less resilient to the stresses of changing temperatures and erratic rainfall. The combination of wetland disturbance from human activities and changes in climate may have greater impacts on wetland functions than either stressor would alone.

This means that conservation efforts must address all three threats simultaneously. Protecting a wetland from encroachment but ignoring the sewage flowing into it, or cleaning up pollution without accounting for climate-driven hydrological changes, will not be sufficient.

What is being done? 🔗

There are some positive signals. India’s National Mission for Clean Ganga formulated a toolkit in January 2021 for the management of wetlands and water bodies in urban areas. Delhi’s Master Plan 2041 includes provisions for protecting and developing an integrated network of water bodies and green spaces. Bengaluru has seen community-led initiatives and researchers from the Indian Institute of Science have developed portals to visualise data and track the damage done to the city’s lakes.

At the international level, the Ramsar Convention provides a framework for wetland conservation, though as many experts point out, declaring a wetland as a Ramsar site does not automatically guarantee its conservation, as there is no binding legal obligation on governing agencies for wetland management.

The real challenge lies in translating policy into consistent, on-the-ground action – which requires political will, inter-agency coordination, community participation, and sustained funding.

What do you think? Given that urban wetlands face threats from all directions – development, pollution, and climate change – should cities be legally required to maintain a minimum percentage of wetland area within their boundaries? And what role can local communities play in holding governments accountable for wetland protection?

References

1. https://www.orfonline.org/research/governing-urban-wetlands-in-india-a-pathway-to-sustainable-urbanisation
2. https://wri-india.org/perspectives/urban-india-going-underwater-again
3. https://www.undrr.org/understanding-disaster-risk/terminology/hips/en0403
4. https://iwaponline.com/wpt/article/19/11/4355/104875/A-review-of-the-wetland-s-restoration-mechanisms
5. https://www.epa.gov/sites/default/files/2021-01/documents/threats_to_wetlands.pdf
6. https://ecology.wa.gov/water-shorelines/wetlands/tools-resources/wetlands-climate-change
7. https://www.nature.com/articles/s41467-024-45286-z
8. https://www.sciencedirect.com/science/article/pii/S030147972100222X
9. https://www.undrr.org/understanding-disaster-risk/terminology/hips/en0304
10. https://www.ramsar.org/