Warming and low oxygen shrink rohu habitat of Loktak lake by 2100 (Image: Zehawk/Flickr)
India’s largest freshwater lake is known for its floating phumdis, rich fisheries and biodiversity. But beneath Loktak Lake’s tranquil surface, changes in temperature, rainfall and dissolved oxygen are beginning to alter the conditions that support aquatic life.
For thousands of fishing households in Manipur, the health of the lake is closely tied to livelihoods and food security. Loktak, a Ramsar wetland, also supports endemic wildlife and performs important ecological functions across the Imphal valley. One species whose future is closely linked to these changing conditions is the Indian major carp, Labeo rohita, commonly known as 'rohu'.
A study published in Ecological Indicators examines how climate change could alter the habitat available to juvenile Rohu in Loktak through the end of this century. Researchers Vicky Anand, Bakimchandra Oinam, Sebastian Schwindt, Stefan Haun and Silke Wieprecht combined hydrological modelling, three dimensional hydrodynamic modelling, water quality simulations and an expert driven fuzzy logic habitat suitability model to assess how changes in rainfall, temperature, streamflow, dissolved oxygen and water depth could affect fish habitat.
The projections show a gradual decline in habitat quality under both low and high emission scenarios. Under the higher emission pathway, much of the habitat currently considered suitable could become less favourable by the end of the century.
The findings place Loktak within a wider challenge facing freshwater ecosystems: climate change is altering not only the amount of water available but also the physical and chemical conditions that determine whether aquatic species can survive.
Fish respond closely to changes in their surroundings. For juvenile Rohu, the study identifies water depth, temperature and dissolved oxygen as particularly important factors determining habitat suitability.
The researchers first assessed present-day conditions in Loktak before modelling two future climate pathways: SSP1 2.6, representing a low-emission future, and SSP5 8.5, representing a high-emission future. They examined two periods, 2041 to 2060 and 2081 to 2100.
The models project an increase in annual temperatures across the catchment of about 0.5°C to 3.7°C, depending on the scenario. Rainfall is also projected to increase, but the change is expected to come through heavier rainfall concentrated over fewer rainy days. The onset of the south-west monsoon is also projected to become delayed, extending the summer period and exposing the lake to longer periods of warming.
These changes can affect the lake in several ways. Warmer air temperatures increase surface water temperatures, while warmer water has a lower capacity to retain dissolved oxygen. Changes in streamflow can also alter water depth and circulation.
The shallow shoreline areas are particularly vulnerable because they warm more quickly than deeper parts of the lake. Under the highest emission scenario, these areas are projected to develop broadly low habitat suitability as prolonged warming reduces the lake's ability to buffer temperature changes.
The study also indicates that stronger thermal stratification could develop in deeper areas. As warmer surface water becomes increasingly separated from cooler water below, oxygen exchange can decline. This could leave deeper waters oxygen deficient, while aquatic organisms near the surface face increasing thermal stress.
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The current assessment shows that only a limited portion of Loktak provides highly suitable conditions for juvenile Rohu. Around 9 square kilometres are classified as having very high suitability, while 51.2 square kilometres have high suitability, 80.7 square kilometres have medium suitability and 14.9 square kilometres have low suitability.
The projections show these favourable areas gradually contracting. Under SSP1 2.6, areas currently classified as medium suitability increasingly shift towards low suitability over time. Some highly suitable areas remain, but the overall trend is towards declining habitat quality.
Under SSP5 8.5, the change is much greater. From 2041 to 2060, very highly suitable habitat virtually disappears from the model projections, while areas of high suitability contract and low suitability expands. By 2081 to 2100, most of the lake is projected to fall within low and medium suitability categories.
The study estimates that nearly 80 square kilometres of habitat currently classified as medium suitability could become low suitability under the high emission scenario by the end of the century. For Rohu, the implications extend beyond habitat. The species is an important commercial carp in India and contributes to inland fisheries and aquaculture. A reduction in suitable natural habitat could therefore affect fish production, fisher livelihoods and food security.
The study also demonstrates the value of combining expert knowledge with fuzzy logic modelling in regions where long term ecological observations may be limited. Such approaches could support habitat assessments in other Indian lakes where comprehensive field data are not available.
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Although climate change emerges as the dominant long-term driver in this study, the researchers are careful not to portray it as the lake's only problem. Loktak has experienced decades of ecological degradation arising from human activities. Floating fishing enclosures known as athaphums interfere with water circulation. Excessive nutrient loading from agriculture promotes eutrophication. Pesticides used in illegal fishing practices further degrade water quality. Altered hydrological regimes associated with infrastructure development have already transformed many ecological processes within the lake.
Climate change acts as a threat multiplier rather than an isolated driver. Warmer water naturally holds less dissolved oxygen. If nutrient pollution simultaneously increases algal blooms, oxygen depletion becomes even more severe. If water circulation is already obstructed by floating biomass, thermal stratification intensifies. Habitat fragmentation caused by shoreline encroachment reduces the ability of fish populations to relocate in response to changing environmental conditions.
The study also acknowledges several limitations that should guide future research. Species interactions, invasive fishes, ecological competition, behavioural adaptation and genetic responses were beyond the scope of the modelling. These factors may either worsen or partly offset projected impacts, underscoring the need for long-term ecological monitoring rather than one-time assessments.
Nevertheless, the central conclusion remains robust: climate change is amplifying existing ecological stress in Loktak Lake and progressively reducing habitat quality.
The study's findings point towards a need to look beyond the lake itself. Loktak's future ecological condition is closely connected to the wider watershed that supplies it with water.
Catchment restoration is one area where action could strengthen resilience. Afforestation, soil conservation, erosion control and sustainable land management upstream could moderate runoff, reduce sediment inflow and improve the stability of streamflows.
Water quality management is equally important. Reducing nutrient inflows from agriculture, preventing untreated wastewater discharge and eliminating destructive fishing practices could improve dissolved oxygen conditions and reduce additional ecological stress.
The lake's shoreline also requires attention. Shallow areas are particularly exposed to warming, making littoral vegetation, wetlands and habitat complexity important components of restoration. Protecting and restoring these areas could provide thermal refuges for juvenile fish.
Hydrological management will also need to account for ecological requirements alongside water supply and hydropower considerations. Environmental flow assessments, seasonal water level management and better coordination among agencies could help maintain suitable conditions during critical breeding periods.
For fisheries, the study points towards an ecosystem-based approach. Climate-resilient stocking strategies, protection of spawning grounds, seasonal fishing regulations and conservation of native fish diversity could become increasingly important as environmental conditions change.
Loktak is not an isolated case. Across India, freshwater ecosystems are experiencing changes in hydrology, water quality, biodiversity and climate conditions, yet inland wetlands have received less attention in climate adaptation planning than some other ecosystems. The study suggests that lakes such as Loktak, Chilika, Wular and Vembanad could benefit from integrated monitoring systems that bring together hydrology, water quality, biodiversity and climate observations.
The modelling framework used in the Loktak assessment could also serve as a decision support tool. By combining climate projections with hydrological and ecological processes, it can help identify vulnerable habitats before changes become difficult to reverse. Such information could support wetland restoration, fisheries management and climate adaptation planning.
The research also points to the importance of distinguishing between local adaptation and wider climate action. The projections show that lower emission pathways result in less ecological damage than high-emission futures. Restoring and managing Loktak can reduce local pressures, but it cannot substitute for efforts to limit climate change.
For generations, Loktak Lake has supported fisheries, biodiversity and communities across Manipur. The study suggests that climate change could alter the environmental conditions that make this possible, particularly for juvenile Rohu.
The projected changes are not confined to temperature. They involve rainfall patterns, monsoon timing, streamflow, water depth and dissolved oxygen, all of which interact to shape the habitat available to aquatic species. At the same time, existing pressures from pollution, altered hydrology and fishing practices can compound the effects of a warming climate.
Protecting Loktak will therefore require attention to both the lake and the watershed that sustains it. Better catchment management, improved water quality, restoration of shoreline habitats, ecological consideration in hydrological planning and climate-responsive fisheries management can help reduce pressure on the ecosystem.
The study provides a way to anticipate where these pressures could become most severe. Whether that knowledge translates into better outcomes for Loktak's fisheries, biodiversity and communities will depend on how effectively scientific evidence is incorporated into freshwater management and climate adaptation.