Green water plants emerging from a green pond full of algae.

Types of algae in ponds and how to fight them

There are several types of algae we can find in our ponds, these include: planctonic (green) algae, filamentous algae, blue-green “algae” and euglena algae. They are diverse organisms that play a crucial role in pond ecosystems. While some algae are beneficial, excessive growth can lead to numerous problems. In this article, we will look at the various types of algae commonly found in ponds, problems they cause, and effective strategies for prevention and control.

What is Algae and Why Does it Thrive in Ponds?

Algae are simple, plant-like organisms that range from single-celled to multicellular organisms. Algae are found in various aquatic environments and their presence is essential for maintaining the balance of the ecosystem.

In pond ecosystems, algae serve as the foundation of the food chain. They are primary producers that convert sunlight and nutrients into organic matter through photosynthesis. This process not only provides energy for algae themselves but also creates oxygen, benefiting other organisms in the pond.

Several factors contribute to the growth of algae in ponds.

  • Nutrient levels, particularly phosphates and nitrates, play a significant role. These nutrients are essential for algae growth and can come from various sources lice decaying organic matter, fertilizers, and runoff from surrounding grounds. When nutrient levels exceed the optimal range, algae growth can become excessive and lead to undesirable consequences.
  • Sunlight exposure is another critical factor for algae growth. Algae require sunlight for photosynthesis, and ponds with high sun exposure can facilitate rapid algae growth. Shallow ponds or those lacking sufficient shading from vegetation are more prone to extensive algae blooms.
  • Water circulation also influences algae growth. Stagnant or poorly circulated water creates an ideal environment for algae to flourish. Adequate water movement and circulation help distribute nutrients, prevent stagnant areas, and inhibit the accumulation of algae.
  • The pH balance of the pond water can also impact algae growth. Most algae species thrive in neutral to slightly alkaline conditions. Fluctuations in pH levels, especially towards more alkaline conditions, can favor the growth of certain types of algae.

Understanding the factors that contribute to algae growth is crucial for effective pond management. Monitoring and managing nutrient levels, optimizing sunlight exposure through vegetation, promoting water circulation, and maintaining a suitable pH balance are key strategies to prevent excessive algae growth in ponds. By maintaining a balanced pond ecosystem, algae can contribute positively to the overall health and vitality of the pond and its inhabitants.

Types of algae

I didn’t want to bore you with the complex taxonomy of algae, so I figured, it would be best to simply list some common types of algae that one can find in a pond.

Planktonic Algae: The Foundation of Pond Food Chains

Planktonic algae, also known as phytoplankton, are single-celled algae that are widely distributed and can be found in various forms and colors. Common colors of planktonic algae blooms include green and even blood red (like euglena).

As microscopic organisms, planktonic algae are not visible to the naked eye. They float freely in the water column and are easily transported by currents and wind. Planktonic algae blooms can occur when environmental conditions are favorable, what means thre are high nutrient levels and sufficient sunlight. These blooms can give the water a distinct color and may create a cloudy or murky appearance.

Planktonic algae serve as the foundation of the pond food chain. Through photosynthesis, they convert sunlight and nutrients into organic matter, producing oxygen as a byproduct. This process is essential for maintaining the pond’s oxygen levels and providing a food source for other aquatic organisms.
Zooplankton, small invertebrates, and larval stages of various aquatic animals rely on planktonic algae as their primary food source. 


Excessive growth of planktonic algae can have detrimental effects on pond water quality and aesthetics. When nutrient levels, particularly phosphates and nitrates, exceed the optimal range, it can lead to algal blooms. These blooms can deplete dissolved oxygen levels, creating an unfavorable environment for fish and other aquatic organisms.

Moreover, dense planktonic algae blooms can reduce water clarity and visibility, impacting the aesthetic appeal of the pond. It can give the water a murky or muddy appearance, making it less visually appealing for recreational purposes.


In terms of fish safety, certain species of planktonic algae can produce toxins under certain conditions, leading to harmful algal blooms. These toxins can pose a threat to fish health and can even result in fish mortality in severe cases.

E.g. diatom algae are safe

Filamentous Algae

Filamentous algae are quite common in freshwater ponds. These algae form long, thread-like strands or mats, giving them their characteristic appearance. They can have a slimy or cottony texture and are typically green in color, although they may also appear brown or black.

The growth of filamentous algae is influenced by various factors, with high levels of calcium and phosphorus being significant contributors. Additionally, stagnant water conditions and low water flow provide an ideal environment for their proliferation.


The overgrowth of filamentous algae can have detrimental effects on pond water quality and aesthetics. As these algae multiply and form dense mats, they can block sunlight from reaching submerged plants, impeding their growth. This reduction in submerged plant population can disrupt the balance of the pond ecosystem and impact the overall biodiversity.

Furthermore, the presence of excessive filamentous algae can lead to decreased dissolved oxygen levels in the water.

In terms of aesthetics, the presence of filamentous algae mats can detract from the visual appeal of the pond. The water may appear murky or covered in a green or brown “carpet”..

Fish safety

While filamentous algae themselves may not be directly toxic to fish, their excessive growth and mat formation can negatively impact fish populations. The dense mats can hinder oxygen exchange between the water and atmosphere, leading to decreased oxygen levels in the pond. Insufficient oxygen can be detrimental to fish health and may result in stress or even fish kills if prolonged

Blue-green “algae” – cyanobacteria

Despite their name, they are not true algae but belong to the Monera kingdom. Unlike other algae (but like bacteria), blue-green algae (or cyanobacteria) lack a nucleus and other membrane-bound organelles.


Cyanobacteria blooms can have great impacts on aquatic ecosystems and pose potential risks to fish and other organisms, including humans. One of the main concerns associated with them is the production of toxins called cyanotoxins. They are produced by some species of cyanobacteria and can be released into the water during a bloom. These toxins can negatively affect  the health of fish, mammals, birds, and humans. 

Fish safety

The toxins released by cyanobacteria can be harmful if ingested by fish. Cyanotoxins can affect their nervous system, liver, and other organs, leading to illness or even death. Fish mortality events have been observed during severe blooms, particularly in stagnant, nutrient-rich waters.

In addition to fish safety, blue-green algae blooms also pose health risks to humans and animals. Contact with or ingestion of water contaminated with cyanobacteria or their toxins can cause various health issues. Common symptoms of intoxication include vomiting, diarrhea, skin rashes, eye irritation, sore throat, cough, and headache. The severity of the symptoms can vary depending on the concentration and the exact type of toxins present in the water. If you suspect a cyanobacteria bloom in your pond, also don’t let your dog drink or bathe in the pond! Unfortunately many dogs have already died because of intoxication after swimming in cyanobacteria infested waters.

Euglena algae

Euglena algae, also belongs to the planktonic algae, that I’ve mentioned before, however, I found it important to list it separately.
It defied early classification attempts by scientists due to its unique characteristics, leading to the creation of a separate kingdom to classify it and its protist counterparts. Euglena is notorious for infesting ponds and can change its pigmentation from green to red in response to sunlight. This color change is caused by euglena forming a protective cyst to protect it from sunlight. This cyst, however, is not only a good shield against sunlight, but also makes euglena robust to many anti-algae treatments. 


Euglena poses a problem for pond owners as it consumes beneficial algae without contributing to a healthy aquatic environment. When deprived of sunlight, Euglena can shift from photosynthesis to a predatory mode, actively hunting for other organisms to sustain itself. This resilience makes Euglena resistant to common treatments like pond dyes or covers, as well as algaecides, which struggle to penetrate its robust “shield”.

To effectively manage Euglena, it is best to target it during its green and more vulnerable stage, typically in the early morning before it develops its radiation shell. However, eliminating Euglena alone is not sufficient, as it is likely to return if the pond’s conditions remain favorable. After using an algaecide to eradicate Euglena, it is crucial to promote a balanced pond ecosystem by reducing organic matter while supporting the growth of beneficial bacteria (there are many products available on the market that can do that!). By enhancing the overall health of the pond, recurring Euglena problems can be prevented.

Fish safety

Regarding fish safety, Euglena algae does not pose a direct threat to fish. However, the use of algaecides and other chemical treatments should be carefully managed to prevent toxicity to fish populations. Additionally, the decomposition of dead Euglena cells can lead to oxygen depletion in the water, potentially endangering fish. It is advisable to treat the pond in sections and monitor oxygen levels closely, considering aeration to mitigate the risk of oxygen depletion.

How to prevent and fight algae overgrowth?

A well-designed pond ecosystem is essential in preventing the overgrowth of algae, ensuring a balanced and visually appealing environment. Several factors contribute to the overall health and equilibrium of a pond, including the pond substrate, aquatic plants such as water lilies, oxygen plants, and marsh plants, as well as proper nutrient management and water circulation.

  • The pond substrate provides a habitat for beneficial bacteria (fastest and less smelly with aerobic bacteria) and microorganisms that help break down organic matter and prevent nutrient buildup, which are often the primary cause of algae proliferation.
  • Aquatic plants play a dual role in algae control. Firstly, they provide shade, which limits the amount of sunlight reaching the pond water and reduces photosynthesis, a process algae rely on for growth. Especially good for that are plants covering the water surface e.g. water lilies.
    Secondly, their root systems absorb excess nutrients, depriving algae of their main food source. Submerged pleans release oxygen into the water during the day and help maintain proper oxygen levels, creating an unfavorable environment for algae growth. Here you can read more about oxygenating water plants.
    Moreover, marsh plants at the pond’s edges act as natural filters, trapping sediments and absorbing excess nutrients before they enter the water.
  • Proper nutrient management is crucial in controlling algae overgrowth. Excessive nutrients, such as nitrogen and phosphorus, often originate from organic waste, decaying plant matter, and runoff from surrounding areas.
    Regular removal of leaves and organic debris from the pond prevents their decomposition and subsequent nutrient release. Avoiding the use of chemical fertilizers near the pond and redirecting rainwater runoff away from the pond can also help prevent nutrient influx.
    If you keep fish, try to feed them less during algae treatment.
  • Water circulation is another critical factor in algae control. Stagnant water creates an ideal environment for algae growth, as it allows nutrients to accumulate and reduces the effectiveness of oxygenation. Installing aeration systems, such as fountains, waterfalls, or air pumps, promotes water movement and oxygenation, inhibiting algae growth. If you already have a pump/ fountain installed, increasing circulation also helps distribute beneficial bacteria throughout the pond, enhancing their ability to break down organic matter and nutrients.
  • If an appropriate pond substrate isn’t an option for you, or it didn’t work, other, boiological controls can offer a natural and environmentally friendly approach to algae management. Beneficial bacteria, commonly available in the form of commercial products, can be added to the pond to enhance the decomposition of organic waste and limit nutrient availability for algae.
  • Regular water changes are recommended to maintain optimal pond conditions. Partial water changes help dilute accumulated nutrients and prevent the buildup of waste products that can fuel algae growth.
  • Removing excess organic waste, such as fallen leaves and debris, from the pond surface and bottom can significantly reduce nutrient levels and the risk of algae blooms. Syphon out excessive debris and net out whatever free floating plant fragments you can find in the pond.

If you are not sure if you are dealing with the toxic cyanobacteria or a harmless algae in your pond, it’s best to seek professional assistance. Chemical treatments targeted at blue-green algae, such as copper-based algaecides, may be employed under the guidance of experts to control and manage the bloom effectively.

In conclusion, maintaining a well-designed pond ecosystem is vital for effective algae control and prevention in our ponds. Through proper substrate management, the presence of aquatic plants, nutrient control, water circulation, biological controls, regular water changes, and the removal of organic waste, a balanced and visually appealing pond environment can be achieved. However, in the case of blue-green algae, its best to look professional help to ensure the safety.

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