Algal Bloom

While passing from a water body have you ever noticed a long stretch of green or red colored layer on the surface of the water? This discoloration of the water is mostly caused due to excessive growth of a unicellular microscopic phytoplankton known as algae. Most of have heard of algae, but what we don’t know is the degree to which it can affect us.  Algae are photosynthetic eukaryotic organisms that include a wide range of unicellular phytoplankton to multicellular seaweeds appearing a plant like structure in aquatic environment. With the ability to photosynthesize, algae play a key role in the food chain of marine ecosystem. Usually consumed by zooplanktons, crustacean and small fishes form the base of the food web for aquatic life. So where does the problem lie with the presence of algae?

The problem is accelerated, unintended and uncontrolled growth of one or more algae in fresh water or seawater ecosystems known as algal bloom. The algal blooms can be seen in a variety of colors The algae produce these colorful pigments for the purpose of efficient photosynthesis, the density of pigmentation in the algal cell directly determines the color of the algal bloom. For instance, green color is prominent in green algae due to high chlorophyll content whereas the accessory pigment known as phycoerythrin is prominent in red algae that produces a spectrum of colours ranging from pink to red. Similarly the presence of phycocyanin pigment imparts a bluish color to blue green algae, the carotenoid content in almost all algal cells provides it yellowish orange color and fucoxanthin that gives brownish color to brown algae.

algae in pond

Due to its predominant autotrophic (ability to produce own food) nature, it requires sunlight for its growth; it mostly resides on upper surface of water body. An uncontrolled growth can cover the large surface of water body disturbing the marine ecosystem that resides beneath. How does this happen? Algae respire and consume the dissolved oxygen present in the water body as well as cut the incident light – both these factors are incredibly important to sustain aquatic life.

An algal bloom can have algal count of tens of thousands to millions per milliliter. A small stretch of algal bloom is known as minibloom and a large stretch of algal bloom is known as macrobloom having a stretch in kilometers.

How does the Algal Bloom affect the environment?

Disturbing the food chain of marine ecosystem-

All the phytoplankton depends on sunlight for photosynthesis. When the water surface is covered by algal bloom, most of the photosynthesis will happen at the surface and the penetration of sunlight into the water can be will hindered, there will be limited supply of sunlight for the phytoplankton living in shallow waters affecting the flora of water. Small fishes, zooplanktons and crustaceans depend of various planktons for their food, which are then consumed by larger fishes. Affecting the flora of phytoplankton will disturb the base of the aquatic food chain.

Dissolved oxygen content-

After the process of photosynthesis during day time the algae will require dissolved oxygen for cellular respiration consuming the surface oxygen. Also the dead algal cells will start to decompose by microbial action utilizing dissolved oxygen. Over the period of time the oxygen produced by the phytoplanktons is not able to cope up with the consumption rate of dissolved oxygen be sufficient enough for to sustain all aquatic life. Such suffocating conditions may be fatal of various aquatic lives.

Affecting the coastal economy-

The freshwater or seawater algal bloom can cover several kilometers, creating an unfavorable environment for fishes to breed. The marine life around the algal bloom may also bioaccumulation of toxins released by algal blooms which can lead to health risk for birds and humans consuming them.

Affecting the water supply to cities-

The growth of algae at the source of water supply can lead to problems with treating of water due to its ability to choke treatment systems. There can be depleted level of oxygen in the water, and dead algae could accelerate microbial growth in the water – both unwanted in terms of drinking water supply. The large mat of filamentous algae can choke the water supply, similarly toxins released by the algal blooms can directly enter human body due to drinking and can be lead person falling sick and even fatal in some cases.

Reasons for algal blooms

There are several factors that lead to algal bloom. A combination factors like climatic conditions and man-made activities like excessive nutrient run-offs in contaminated groundwater can trigger the formation of algal blooms

Seasonal changes –

Blue green algal blooms have an affinity for spring season when temperature is warmer and increased duration of light. In tropical regions, warmer water conditions can lead to algal blooms throughout the year. Temperature conditions above 25 C are favorable for growth of blue green algae which gives them an edge over other algae. Low temperatures during winter are not favorable for the growth of blue green algae.

Light –

Due to the autotrophic nature of algae, light plays an important role in algal growth. Intermittent high and low intense light are favorable for growth of blue green algae. Such light conditions can be seen just below the water surface or turbid water. Also very high intense light can lead to death of algal cells.

Stable water flow-

Most cyanobacteria prefer stable water conditions with less mixing and higher retention time. Human activities like building dams, irrigation and other sources of water consumption reduces the flow rate the river providing favorable environment for algal growth.


Phytoplanktons are capable of generating carbon containing sugars from photosynthesis, but they also require other nutrients for growth and reproduction. These nutrients include nitrogen, phosphorus, iron, calcium etc. Nitrogen and phosphorus are necessary for all the algae while other nutrients are required for specific marine plankton. In deep oceans, the depleted nutrient from surface is replaced by nutrients from deep nutrient rich water due to upwelling or from the nutrient rich coastal runoffs. Depending on the seasonal conditions and nutrient availability there are algal blooms which last for short duration. But in areas with favorable environment and continuous high concentrations of nutrients, algal bloom can sustain all along the year. Presence of large concentration nutrient in the water ecosystem is known as eutrophication. Agricultural runoff, storm water, discharge of sewage water and other human activities leads to eutrophication which is biggest contributor to algal blooms.

Types of algal bloom

Algal blooms are integral part of marine ecosystem. They are oxygen producers, almost half of the oxygen dissolved in the water ecosystem and atmosphere is available as by a product of photosynthesis by phytoplanktons. But algal bloom due eutrophication is most problematic.

Filamentous algal blooms

These are slimy green covering mat of microscopic algae attached to each other. Though such algae are not known to produce toxins, they do possess other danger to aquatic life like oxygen depletion, blocking the sunlight and affecting the photosynthesis for other phytoplanktons and impair the aesthetics of water body due to odor and unpleasant sight.

Harmful Algal bloom (HAB)

An algal bloom which poses a potential hazard to health of humans, and marine life and birds due to its toxin production or oxygen depletion causing threat to marine life due high concentration of algal bloom can be classified as harmful algal bloom.Some algal species produces toxins that can cause severe harm to human health or even death in some instances. The toxins produced by algae under normal conditions are broken down due to microbial action, but during an algal bloom the concentration of this toxin produced is very high. Cyanotoxin is produced by variety of cyanobacteria while domoic acid is produced by diatoms which is a known neurotoxin. Cyanotoxin are further classified into to category based on their biological effect. Like hepatotoxin affecting liver, microcystin which affects kidneys, liver and is a known carcinogen, neurotoxin that affects respiratory muscles and brain activity, dermatotoxins affecting the skin and endotoxins causing inflammation of gastrointestinal epithelium. Ingestion of such toxins through drinking water or swimming in such area can pose health of a person.

The toxins can also be passed indirectly to humans or birds through food chain. The concentration of toxin in increased as the toxins are passed from smaller organisms to next organism in food chain, The shellfish feed by filtering the particles along with phytoplanktons, due to this a concentrated level of toxin is accumulated in it. When finally the human consumes the fish as food, the toxins are released in the body and impacts human health.In case of large algal bloom covering hundered of kilometers, the water beneath the bloom becomes anoxic due to decaying process of algal cells by microbial action reducing the dissolved oxygen content also the high concentration of algal cells clog the gills of fishes and causes irritation . Such conditions in an ecosystem lead to suffocation causing death of large population of marine life.

Red Tide

Red tide is a natural phenomenon of marine ecosystem where algal species of diatoms or dinoflagellates forms algal bloom during favorable environment conditions, warm water and high nutrient availability. The water is discolored to red as the concentration of diatoms or dinoflagellates reached 1000 cells per milliliter and the color intensifies with increased concentration of algal cells. Diatoms are known to produce a neurotoxin known as domoic acid which can affect higher vertebrates, birds and humans. Even these toxins can reach human and birds via food chain leading affecting human health.

What can we do to treat algal blooms.

To avoid further expansion of algal bloom and release of biotoxins into water bodies, it is necessary to control the bloom before it damages the ecosystem and poses health risk for humans and marine life. There are various methods that can help to control algal bloom.

Mechanical control-

With this method, the filamentous algal blooms can be removed manually or with the help of machines. The pumping of surface water that contains most of the algal cells can also help get rid of algal blooms to a great extent. But this method for controlling algal bloom is temporary as the remaining cells in the water body can lead to algal bloom over the period of time. Filtrations unit are used to separate algal cells are and purify drinking water bodies. Aeration system help destratify the thermal and light layers in the water body creating an unfavorable condition for algal bloom of toxic cyanobacteria. The greatest disadvantage of this technique is it is not cost effective, highly time consuming and cannot be used to curb algal blooms which are larger in size.

Chemical control –

This method to control algal blooms consist of adding chemical additives to water that can precipitate the phosphate from the water. Many clay and chemical additives such as alum, copper compounds and chloramines are added in freshwater systems to flocculate and remove algal cells. Algaecides usually derived from aquatic herbicides are used to treat algae, but they are costly and need frequent dosage to check the algal population . Apart from controlling the algal growth the chemical additives and algaecides can have ill effects on water ecosystem. The added chemicals can rupture the algal cells releases harmful toxins into the water body that can be fatalities of fishes and marine life. These chemicals may also accumulate overtime and lead to other unwanted problems in the aquatic ecosystem.

Biological controls-

Various biological agents like bacteria, viruses and parasite have been used to treat algal blooms. Gymnodinium mikimotoi have been used as algaecide to treat a dinoflagellate species. Viruses are highly host specific algaecide which can effectively target a single species of algae. But such a process of biological control one organism with other organism can have deleterious effect on natural fauna and can replace the indigenous species with non-indigenous species

Why prevention is the best cure when it comes to algal blooms

Most of the algal blooms that occur in fresh water or near the sea coast are due to eutrophication by human activities. To avoid the occurrence of algal bloom it is most preferable to take measures that do not encourage the creation of algal blooms.


The nuisance of HAB’S disrupting the water body over the years have led to monitoring of water quality that can help to forecast the possible algal bloom development and help strategize plans to tackle them. Such proactive strategies can help in minimizing health risk and economic impacts due to algal blooms. Detection of nutrient levels or identifying the presence HAB’s from various locations of water body can help in forecasting the possible algal bloom.

Regulatory policies-

The local and state authorities can work together and design programs for monitoring and containing measures for water management. Policies should be strictly followed on usage of fertilizers for agriculture and discharge of sewage water into the water body.

Barrier creation –

The landscape modification of region across fresh water and coastal ocean allows easy nutrient input due to agricultural runoff into the water body causing eutrophication. Creation of barriers around the water body can avoid the entry of nutrient rich water source entering the water ecosystem.

Reducing Chemical run-off –

Methods to contain the sources of nutrient runoff like nitrogen and phosphorus from fertilizers should be implemented. Methods like drip irrigation can help in focused application of fertilizers avoiding any traces of nutrients for run-offs. Implementing waste water treatment strategies in industries for biological nutrient removal before its discharge into water body can reduce the nutrient concentration in to water body.

Usage of bioremediation strategies –

Beneficial bacteria can be used as part of an eco-friendly and natural pond bioremediation strategy for effective lake cleaning product. The logic behind using these bacteria is simple. Bacteria feed on suspended nutrients and organic sludge in water bodies. They also utilise nitrates and phosphates for growth, making it unavailable for algae & aquatic plants and naturally curbing eutrophication. Once the nutrients in the water are reduced, the bacteria become dormant and stop activity. They naturally activate and multiply once there is an influx of nutrients and the cycle restarts itself. The bacteria naturally adapt themselves to the pond ecosystem and reduce the frequency of treatment. This is why bioremediation strategies remain the one of the best options to curb the menace of algal blooms.


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