The lake city Udaipur, Rajasthan is known for its picturesque, serene lakes with clear waters appearing bright white and deep blue. Unusual pink/red colour waterbodies looks almost unreal. Here we reporta unique crimson pink coloured water body near Nagda (24°43’51.29″N and 73°43’41.59″E) observed during recent field trip (Mid-January) to Udaipur while on the way to Eklingji. It is nearly 300m southwest of man-made lake Baghela which is about 23 km North of Udaipur.

The colour of any water body is result of how water interacts with light i.e, which wavelengths are absorbed, scattered, or reflected by the water itself plus the materials suspended or dissolved in it. It is indicator of ongoing physical and biochemical processes and the bluer once generally indicates clearer water and a healthier ecological state. Though change in colour of the lakes had been observed earlier in past at Udaipur, however, the extreme tone of red colour was witnessed this year. In 2016 thick green mats covered the blue water of Lake Pichola engulfing large portions of Kumaharia and Rang Sagar water bodies. Blooms of salt-loving halophilic bacteria or algae are widespread in saline environments (e.g., Halo bacterium, Halococcus, Dunaliella salina) that are responsible for pink colour of Lake Tyrrell, Australia and Lonar Lake, Maharashtra. However, the visual observation of Nagda water body appears to be attributed primarily to freely floating red fern, Azolla (Mosquito fern). While green Azolla represents healthy growth in shaded conditions red Azolla is generally caused by abiotic stresses (change in solar irradiance, salinity, temperature, nutrient runoff) due to presence of algal pigments (e.g., anthocyanin) that impart pink/red coloration to water. The preliminary analysis of satellite images Sentinel-2 when observed for the period of January and previous months indicates that reddening was not observed in November and has started during December (Figure 2). Spectral profile from Sentinel-2 image shows high reflectance after 690nm. In VNIR spectroradiometer profile of samples shows low reflectance in 500-600 nm (green) increasing 590nm onwards with prominent peak near 640nm (red), typical of stressed vegetation indicated by degradation of chlorophyll and shift in colour from green to red.

Figure 1. (a) Field view of the water body near Nagda, Udaipur and spectral profiles from (b) Sentinel-2 and (c) spectroradiometer.

Azolla under optimal environmental conditions is considered the one of fastest growing plants on Earth with exponential doubling of biomass in water. It is capable of fixing large amounts of carbon and producing vast amounts of organic nitrogen. This extant species is also key to the past and has important role as climate indicator and glaciation events of the Earth. Approximately 49 million years ago, Earth witnessed one of its hot phase (PETM; Palaeocene Eocene Thermal Maxima) when its global average temperature reached 12-15°C warmer than the present-day averages. This geologically brief time interval ended with Azolla event that triggered a global cooling event and pulled the Earth into the ice age by its rapid proliferation in the Arctic Ocean (and thereby removal of CO₂ from the atmosphere). Azolla is a great potential aquatic plant as biofertilizer, bioethanol source, phytoremediator and source rock for oil/gas exploration. While untamed Azolla can have harmful impact on ecosystem their regular monitoring is essential to evaluate the cyclicity using multi-temporal satellite datasets. Further field based assessment of turbidity, chlorophyll, nutrients, dissolved organic carbon, and depth/sediment characteristics are significant for understanding its role and impact on ecosystem and climate.

Figure 2.  Natural colour composites of the water body near Nagda, Udaipur from Sentinel-2 satellite images acquired for (a) November 09, 2025 (b) December 27, 2025 (c) January 16, 2026.

Acknowledgement: Sincere gratitude to Dr. N.K. Chauhan, Retired Professor (Geology) Mohanlal Sukhadia University Udaipur for his help and support during field visit to this site.  

References:
Barke, J et al. 2012. Coeval Eocene blooms of the freshwater fern Azolla in and around Arctic and Nordic seas. Palaeogeography, Palaeoclimatology, Palaeoecology, 337-338, 108-119, https://doi.org/10.1016/j.palaeo.2012.04.00
Mishra, A., Hakeem, K.A., Rao, V.V., Rao, P.V.N. and Chowdhury, S. 2021.

Assessment of colour changes in Lonar lake, Buldhana district, Maharashtra, India using remote sensing data. Current Science, 120 (1) doi: 10.18520/cs/v120/i1/220-226.

Shen, X., Ke, C.Q., Duan, Z., Cai, Y., Li, H., and Xiao, Y. 2025. Satellite observations reveal widespread color variations in global lakes since the 1980s. Water Resources Research, 61, e2023WR036926. https://doi.org/10.1029/2023WR036926

Times of India, Jaipur. “Harmful Azolla envelops water bodies in Udaipur.” A report by Geetha Sunil Pillai, Feb 08, 2016.

Yang, X., O’Reilly, C. M., Gardner, J. R., Ross, M. R. V., Topp, S. N., Wang, J. and Pavelsky, T. M. (2022). The color of Earth’s lakes. Geophysical Research Letters, 49, e2022GL098925. https://doi.org/10.1029/2022GL09892

Wagner, G.M. (1997) Azolla: a review of its biology and utilization. The Botanica Review, 63, 1-26.