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 by excessive growth of unicellular microscopic phytoplankton known as algae.
Most of us have heard of algae, but what we don’t know is the degree to which they can affect us.
Algae are photosynthetic eukaryotic organisms that include a wide range of unicellular phytoplankton to multicellular seaweeds, appearing as plant-like structures in the aquatic environment.
With the ability to photosynthesize, algae play a key role in the food chain of marine ecosystems.
Usually consumed by zooplanktons, crustaceans, and small fishes, it forms the base of the food web for aquatic life.
So, where does the problem lie with the presence of algae?
The problem is the accelerated, unintended, and uncontrolled growth of one or more algae in freshwater or seawater ecosystems, known as an 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 and 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 a yellowish-orange color, and fucoxanthin gives a brownish color to brown algae.
Due to its predominantly autotrophic nature (the ability to produce its own food), it requires sunlight for growth and mostly resides on the upper surface of the water body.
Uncontrolled growth can cover a large surface of the water body, disturbing the marine ecosystem that resides beneath.
Recent data indicate that as of May 2025, UNESCO has recorded nearly 10,000 global marine harmful algal bloom (HAB) events, representing a 1.8% annual increase since 2003.
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 an algal count of tens of thousands to millions per milliliter.
In fact, some HAB events in U.S. waters have recorded cell densities exceeding 1 million cells/mL in summer 2024.
A small stretch of algal bloom is known as a mini-bloom, and a large stretch of algal bloom is known as a macro-bloom, measured in kilometers.
All the phytoplankton depend on sunlight for photosynthesis.
When the water surface is covered by an algal bloom, most of the photosynthesis will happen at the surface, and the penetration of sunlight into the water can be hindered.
There will be a limited supply of sunlight for the phytoplankton living in shallow waters, affecting the flora of the water.
Small fishes, zooplankton, and crustaceans depend on 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.
Recent satellite monitoring (2023–2025) indicates that harmful algal blooms (HABs) in Lake Erie have reduced underwater light levels by up to 70% during peak bloom weeks.
After the process of photosynthesis during the daytime, 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 a 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 to various aquatic life.
In 2024, Gulf of Mexico HABs created a “dead zone” spanning over 20,000 km², with dissolved oxygen levels falling below 2 mg/L in many areas, resulting in massive fish kills.
The freshwater or seawater algal bloom can cover several kilometers, creating an unfavorable environment for fish to breed.
The marine life surrounding the algal bloom may also bioaccumulate toxins released by the algal bloom, which can pose health risks to birds and humans consuming it.
U.S. economic losses due to HABs average $10–100 million annually; Lake Erie alone lost ~$82 million in 2024 via fisheries and tourism impacts.
The growth of algae at the source of the water supply can lead to problems with treating water due to its ability to choke treatment systems.
There can be depleted levels of oxygen in the water, and dead algae could accelerate microbial growth in the water, both of which are unwanted in terms of the drinking water supply.
The large mat of filamentous algae can choke the water supply.
Similarly, toxins released by the algal blooms can directly enter the human body due to drinking and can lead to a person falling sick, and even be fatal in some cases.
For example, Toledo, Ohio’s 2014 HAB caused microcystin levels to spike above 2 µg/L, prompting a citywide “do not drink” advisory for over 400,000 residents. Similar events in 2023–24 led to upgraded microfiltration systems in 60% of U.S. municipal plants.
Several factors contribute to an algal bloom.
A combination of factors, such as climatic conditions and human-made activities like excessive nutrient runoff in contaminated groundwater, can trigger the formation of algal blooms.
Blue-green algal blooms have an affinity for the spring season when the temperature is warmer and the duration of light is increased.
In tropical regions, warmer water conditions can lead to algal blooms throughout the year.
Temperature conditions above 25 °C are favorable for the 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.
Global surface water temperatures rose by ~0.2 °C per decade (2010–2024), extending bloom seasons into late fall in many regions.
Due to the autotrophic nature of algae, light plays an important role in algal growth. Intermittent exposure to high- and low-intensity light is favorable for the growth of blue-green algae.
Such light conditions can be seen just below the water surface or in turbid water.
Also, very high, intense light can lead to the death of algal cells.
Most cyanobacteria prefer stable water conditions with minimal mixing and longer retention times.
Human activities, such as building dams and irrigation, as well as other sources of water consumption, reduce the river’s flow rate, creating a favorable environment for algal growth.
In May 2025, South Australia’s Murray River experienced a prolonged low‐flow event leading to a 150 km HAB, confirmed by satellite imagery.
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 algae, while other nutrients are required for specific marine plankton.
In deep oceans, depleted nutrients from the surface are replaced by nutrients from deep, nutrient-rich water due to upwelling or from nutrient-rich coastal runoffs.
Depending on seasonal conditions and nutrient availability, algal blooms can occur that last for a short duration.
But in areas with favorable environments and continuous high concentrations of nutrients, algal blooms can be sustained all throughout the year.
The presence of a large concentration of nutrients in the water ecosystem is known as eutrophication.
Agricultural runoff, stormwater, the discharge of sewage water, and other human activities contribute to eutrophication, which is the primary cause of algal blooms.
For instance, Lake Taihu (China) recorded phosphorus loads of ~1,400 metric tons/year in 2024, sustaining near-year‐round blooms.
Algal blooms are a vital component of the marine ecosystem.
They are oxygen producers, and almost half of the oxygen dissolved in the water ecosystem and atmosphere is available as a byproduct of photosynthesis by phytoplankton.
However, algal bloom due to eutrophication is the most problematic.
These are slimy, green mats of microscopic algae that are attached to one another.
Although such algae are not known to produce toxins, they pose other dangers to aquatic life, including oxygen depletion, blocking sunlight, affecting the photosynthesis of other phytoplankton, and impairing the aesthetics of the water body due to odor and an unpleasant sight.
An algal bloom that poses a potential hazard to the health of humans, marine life, and birds due to its toxin production or oxygen depletion, causing a threat to marine life due to high concentrations of algal bloom, can be classified as a harmful algal bloom.
Some algal species produce 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 through microbial action; however, during an algal bloom, the concentration of these toxins is extremely high.
Cyanotoxin is produced by a variety of cyanobacteria, while domoic acid is produced by diatoms, which is a known neurotoxin.
Cyanotoxins are further classified into two categories based on their biological effect.
Ingestion of such toxins through drinking water or swimming in such areas can pose a risk to a person’s health.
The toxins can also be passed indirectly to humans or birds through the food chain.
The concentration of toxins increases as the toxins are passed from smaller organisms to the next organism in the food chain.
The shellfish feed by filtering particles along with phytoplankton, and as a result, a concentrated level of toxin accumulates in them.
When human finally consumes the fish as food, the toxins are released into the body and impact human health.
In the case of a large algal bloom covering hundreds of kilometers, the water beneath the bloom becomes anoxic due to the decaying process of algal cells by microbial action, reducing the dissolved oxygen content.
Additionally, the high concentration of algal cells clogs the gills of fish, causing irritation.
Such conditions in an ecosystem lead to suffocation, causing the death of a large population of marine life.
Red tide is a natural phenomenon in marine ecosystems where algal species, such as diatoms or dinoflagellates, form an algal bloom under favorable environmental conditions, including warm water and high nutrient availability.
The water becomes discolored red as the concentration of diatoms or dinoflagellates reaches 1000 cells per milliliter, and the color intensity increases with the 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 humans and birds via the food chain, affecting human health.
Florida’s 2024 red tides spanned > 300 miles of coastline, causing widespread respiratory irritation and marine fish kills, resulting in $30 million in tourism losses.
To prevent the further expansion of algal blooms and the release of biotoxins into water bodies, it is essential to control the bloom before it damages the ecosystem and poses health risks to humans and marine life.
There are various methods that can help control algal blooms.
With this method, the filamentous algal blooms can be removed manually or with the help of machines.
The pumping of surface water, which contains most of the algal cells, can also help eliminate algal blooms to a great extent.
However, this method for controlling algal blooms is temporary, as the remaining cells in the water body can lead to algal blooms over time.
Filtration units are used to separate algal cells and purify drinking water.
The aeration systems help destratify the thermal and light layers in the water body, creating an unfavorable condition for the algal bloom of toxic cyanobacteria.
The greatest disadvantage of this technique is that it is not cost-effective, highly time-consuming, and cannot be used to curb algal blooms, which are larger in size.
In 2025, Toledo’s water utility installed rapid microfiltration systems, which reduced microcystin levels by 90% during Harmful Algal Bloom (HAB) events.
This method for controlling algal blooms involves adding chemical additives to water that can precipitate 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, typically derived from aquatic herbicides, are used to control algae, but they are costly and require frequent dosages to manage the algal population.
Apart from controlling the algal growth, chemical additives and algaecides can have ill effects on the water ecosystem.
The added chemicals can rupture the algal cells and release harmful toxins into the water body, which can cause the fatalities of fish and marine life.
These chemicals may also accumulate over time, leading to other unwanted problems in the aquatic ecosystem.
In 2024, Lake Rotorua’s phosphorus levels dropped from 0.12 mg/L to <0.01 mg/L after a lanthanum dosing trial, significantly reducing bloom intensity.
Various biological agents, like bacteria, viruses, and parasites, have been used to treat algal blooms.
Gymnodinium mikimotoi has been used as an algaecide to treat a dinoflagellate species.
Viruses are highly host-specific algaecides, which can effectively target a single species of algae.
However, such a process of biological control, where one organism controls another, can have deleterious effects on natural fauna and replace indigenous species with non-indigenous ones.
Most algal blooms that occur in freshwater or near the coastline are caused by eutrophication resulting from human activities.
To avoid the occurrence of algal blooms, it is preferable to take measures that do not encourage the creation of algal blooms.
The nuisance of HAB disrupting the water body over the years has led to the monitoring of water quality, which can help forecast possible algal bloom development and inform strategies to tackle them.
Such proactive strategies can help minimize health risks and economic impacts associated with algal blooms.
Detecting nutrient levels or identifying the presence of HABs from various locations within the water body can help in forecasting potential algal blooms.
NOAA’s Great Lakes HAB Prediction Tool (2025) achieves ~85% accuracy in forecasting bloom hotspots two weeks in advance, allowing managers to enact early warnings.
Local and state authorities can work together to design programs for monitoring and implementing containment measures for water management.
Policies should be strictly followed on the usage of fertilizers for agriculture and the discharge of sewage water into the water body.
The landscape modification of regions across freshwater and coastal oceans allows easy nutrient input due to agricultural runoff into the water body, causing eutrophication.
The creation of barriers around the water body can prevent the entry of nutrient-rich water sources into the water ecosystem.
Methods to contain the sources of nutrient runoff, like nitrogen and phosphorus from fertilizers, should be implemented.
Methods like drip irrigation can help in the focused application of fertilizers, avoiding any traces of nutrients from runoffs.
Implementing wastewater treatment strategies in industries for biological nutrient removal before discharge into water bodies can reduce nutrient concentrations in water bodies.
California vineyards adopting drip irrigation in 2024 reported a 40% reduction in nitrogen runoff; industrial pretreatment in 2025 led to 70% less phosphorus discharge from new permits.
Beneficial bacteria can be used as part of an eco-friendly and natural pond bioremediation strategy for effective lake cleaning products.
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 them 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 one of the best options to curb the menace of algal blooms.
