Red Algal Bloom

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Red Algal Bloom

Red Algal Blooms

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Context

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

Red Algal Blooms

Caused by microscopic organisms, mainly a group called Euglenophytes.
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.

Euglena

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.

Why does the water turn 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 euglenoids their bright red colour.

Causes

Eutrophication: Excess nutrients (sewage, agricultural runoff, industrial effluents).
- In recent years, more and more red blooms have been reported across eutrophic water bodies—where nutrient levels are unnaturally high

Additional Information

Causes of Harmful Algal Blooms

Both physical and chemical factors contribute to the formation and persistence of HABs in freshwater and marine systems, including:

nutrient loading (both nitrogen and phosphorus, including organic forms);
light availability;
water temperature;
alteration of water flow;
salinity
stratification/vertical mixing;
pH changes; and
trace metals.

Source:US EPA

Negative impact on water quality

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

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

FAQs

1. What causes red algal blooms in Indian water bodies?

Red algal blooms are primarily caused by euglenoid organisms like Euglena, Phacus, and Trachelomonas. These thrive in nutrient-rich, stagnant, or slow-moving freshwater bodies.

2. What pigment gives red algal blooms their characteristic color?

Astaxanthin, a fat-soluble red xanthophyll pigment produced by euglenoids, gives the blooms their bright red or pink color.

3. Are Euglenoids algae or something else?

Euglenoids are protists, not true algae. They show both plant-like (photosynthesis) and animal-like (heterotrophic feeding) characteristics.

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