Red algal blooms are a biological red flag, a signal that our water bodies are in distress

When we talk about building self-sustaining cities, water conservation is at the heart of it. Yet, many of our urban and peri-urban water bodies-lakes, ponds, temple tanks—are under serious ecological stress. Increasing eutrophication, high levels of organic pollution, and poor oxygen conditions are signs of a crisis that often goes unnoticed.

You know what to do when you see a red traffic light—you stop. But what should you do when you see red-coloured water in your local lake or pond? We’re used to seeing green algal mats or floating scum on polluted water surfaces. But many water bodies today are turning red or pink, with dense layers of floating biofilms. These aren’t always the result of spilled paint or industrial dyes. Many such cases are caused by microscopic organisms—specifically, a group called Euglenophytes. Euglena is a unicellular, flagellated microorganism commonly found in stagnant or slow-moving freshwater ponds, ditches, and urban lakes. Belonging to the group Euglenophyta, these protists are highly adaptable, capable of both photosynthesis (like plants) and feeding on organic matter (like animals). This flexibility helps them thrive in nutrient-rich, oxygen-poor environments. Three of the most common euglenoid genera observed in Indian water bodies are: Euglena, Phacus, Trachelomonas sp. These species often dominate surface waters, especially during warmer months.

Why the water turns red

The striking red or pink colour seen during blooms is due to a pigment called astaxanthin, produced by many euglenoids. Astaxanthin is a fat-soluble xanthophyll that gives these organisms their bright red colour. In recent years, more and more red blooms have been reported across eutrophic water bodies—where nutrient levels are unnaturally high due to sewage discharge, agricultural runoff, or industrial effluents.

Negative impact on water quality

While Euglena might seem like just another algae, its blooms can be harmful to water quality, biodiversity, and even public health.

1.  Oxygen depletion

During the day, Euglena photosynthesises, but at night it consumes oxygen, leading to diurnal oxygen fluctuations. This can result in hypoxia (low oxygen levels), which is dangerous for fish and other aquatic life.

2.  Dense surface scums

Euglenoid blooms form thick mats on the water surface, which block sunlight from reaching submerged plants. This reduces the lake’s productivity and alters its ecological balance.

3.  Fish mortality

Euglenophyte blooms can stick to fish gills, making it hard for them to breathe. Some species also produce toxins like euglenophycin, which directly kill fish or indirectly affect them by disrupting the food chain and oxygen levels.

4.  Aesthetic and cultural degradation

Red or green scums, foul smells, and murky water reduce the recreational, cultural, and religious value of water bodies—especially in urban India, where such spaces are already limited.

5.  Water treatment challenges

Blooms clog filtration systems, adds unwanted odours and tastes, and increases the cost of treating water for domestic use.

A threat to aquaculture

For fish farms and aquaculture systems, these blooms spell economic trouble. In recent years, dense euglenophyte blooms have caused tangible revenue losses, as toxins and poor water quality reduce fish health and growth. Some blooms in the 21st century have even been linked to toxic alkaloids that devastate aquaculture productivity.

Understanding the ecology to manage the crisis

Despite their increasing prevalence, euglenophytes are still understudied. We need to better understand:

· Their seasonal abundance

· Toxin production patterns

· Responses to environmental variables like pH, temperature, and nutrient load

Only with this knowledge can we develop effective monitoring and management strategies, restoring ecological health and ensuring water bodies can support biodiversity, recreation, and livelihoods.

The Indian urban context

In India, Euglena and its relatives are now widespread indicators of deteriorating water quality. Urban and peri-urban water bodies, already stressed by population pressure, are becoming hotspots for these biological pollutants.

Negative impacts in Indian water bodies include:

·  Signalling organic pollution and sewage contamination

· Reducing oxygen levels, harming fish and invertebrates

· Forming unsightly surface mats and blocking light

· Compromising community use of water bodies

· Clogging filters in decentralised drinking water systems

A call to action

The presence of red water is a biological red flag—a signal that our water bodies are in distress. These blooms are not just an eyesore; they are symptoms of a deeper ecological problem tied to urbanisation, poor wastewater management, and nutrient overload. Recognising Euglena not just as a microorganism but as an indicator of crisis can help policymakers, researchers, and citizens come together to safeguard our precious water bodies. Tackling this issue requires: improved sewage and waste management, regular monitoring of algal communities and also ecosystem-based restoration approaches. Unfortunately, detailed ecological studies are no longer prioritised in academic institutions, leading to a widening knowledge gap. Without microscopic analysis, it is impossible to fully understand the toxic microorganisms inhabiting aquatic ecosystems. In addition to comprehensive ecological assessments, strict policies and effective implementation are essential for the conservation and protection of our freshwater bodies.

Moumita Karmakar (Ph.D) is Senior Programme Associate, Water and Environment vertical, NIUA, Delhi

Views expressed are the author’s own and don’t necessarily reflect those of Down To Earth