Guide to Different Types of Pond Algae (With Pictures)

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Algae (or alga, singularly), belonging to the kingdom Protista, are largely aquatic organisms that are typically fully photosynthetic but differ from plants in that they lack true roots, stems, leaves, and gametes (the male and female parts of plants). Algae can vary in size from less than two micrometers (in the case of micromonas, a species of green algae) to over 200 feet tall (in the case of some species of giant sea kelps)!

In total, there are eight main groups of algae: blue-green algae (also known as cyanobacteria), diatoms, chlorophyta (or green algae), euglenophyta, dinoflagellate, chrysophyta (commonly called golden algae), phaeophyta (often known as brown algae), and rhodophyta (or red algae). Among these, alga also comes in two forms: planktonic and filamentous (or string).

Why Is Identifying Algae Important? 

beneficial algae in a koi fish pond
Knowing which algae you have is important, as some types are beneficial to the ecos-system, whereas others can be harmful.

Knowing the type of alga that is present in your pond is exceptionally important. For example, the presence of cyanobacteria usually indicates stagnant waters with low dissolved oxygen levels and too many nutrients (most likely from fertilizer run-off), while green algae generally indicates good (or at least acceptable) water quality. Diatoms are naturally present in virtually every body of water, from the vast ocean to your little pond to moisture that has collected on a leaf, and are truly incredible microscopic organisms that are responsible for producing more oxygen than all of Earth’s rainforests combined. In fact, it is estimated that 25%-40% of our oxygen is manufactured by diatoms, or as much as every third breath that we breathe!

However, it is important to note that too much of any type of algae (with the exception of diatoms!) can be harmful, as they may lead to algal blooms and fish deaths. This is why identifying what algae is present in your pond, and taking appropriate steps to control it’s growth when necessary, makes up an important aspect of good pond and fish keeping.

Pond Algae Identification – Which Algae Is That?

Within the 8 main groups (phyla) mentioned above are dozens of smaller groups encompassing more than one million species of algae – here we will simply discuss the groups that are most common in garden ponds and lakes, with pictures to help with identification:

1) Green Water Algae

Some green algae is helpful even if it turns water green
Green water algae can turn ponds a “pea-soup” color, but they are also a natural and healthy food source to many different animals.

Green algae, belonging to the family chlorophyta, is the most diverse group of algae encompassing over 7,000 species. These algae are present in most healthy pond and lake ecosystems, as they are at the base of the food web. Their chloroplasts contain both chlorophyll A and B, accounting for their typical bright green coloration, though they may also be various hues of yellow. In addition to providing food for a variety of creatures from fish to insects to waterfowl, green algae are also primary producers, generating oxygen and energy/nutrients that are then utilized by organisms that are unable to produce their own. Conversely, as previously mentioned, too much green algae (often as a result of nutrient-rich water) can result in eutrophication, ultimately resulting in depleted oxygen levels and the death of your pond’s inhabitants, especially in warmer summer months months. It’s important to control the spread of green water algae before it gets to this point, with the most effective treatments being UV clarification, water dyes, and good filtration and maintenance.

2) Cyanobacteria (Blue-Green Algae)

cyanobacteria is harmful to water ecosystems
Cyanobacteria, commonly “blue-green algae”, is not actually an algae, but a micro-organism that grows similarly on the waters surface.

As indicated by its name, cyanobacteria, while commonly referred to as blue-green algae, is not a true algae but rather a type of bacteria that looks deceivingly similar to algae. They prefer shallow, warm, still water that is rich in nutrients…in other words, they thrive in unhealthy, low quality aquatic ecosystems! They typically form dense, scum-like floating mats on the water’s surface and can range in color from the characteristic blue-green to green, yellow, purple, or brown. If your pond or lake has a strong, unpleasant odor and algae-like mats that are viscous and slimy, you likely have a cyanobacteria bloom. Another way to determine whether you have an overabundance of cyanobacteria (the presence of some cyanobacteria is normal and not harmful) is to conduct a water quality test – poor water quality with low oxygen and high nitrogen levels are a decent indicator of cyanobacteria presence, particularly if accompanied by a foul smell and dead or dying/unhealthy fish. When testing your water, be sure to wear protective clothing such as rubber gloves and waders – cyanobacteria contain various toxins that are harmful if touched or ingested. Different types of cyanobacteria present different health hazards, so be sure to minimize your exposure and thoroughly clean yourself and your clothing if you come into contact with any.

blue-green algae cyanobacteria forms thick toxic sludge on pond
Left uncontrolled, blue-green algae forms a thick toxic sludge on ponds and lakes. Photo credit:

When their numbers aren’t out of control, cyanobacteria do have some ecological benefits: some species of fungi and lichen have formed a symbiotic relationship with cyanobacteria, allowing it to live in their roots where the bacteria help to fix nitrogen into a form that is usable by the plant or fungus. Cyanobacteria is also present in many soils, where they also aid in nitrogen fixation that is essential for proper ecosystem functioning. In addition, the chloroplasts of modern plants (the part a plant’s cells that conducts photosynthesis and produces food for the plant) actually developed from ancestral cyanobacteria! Plant chloroplasts evolved from cyanobacteria hundreds of millions of years ago via endosymbiosis, a process that entails one organism living within another in such a way that both organisms benefit while adapting and evolving together over time. With this in mind, plant life as we know it would not exist were it not for cyanobacteria  – so it’s certainly not all bad!

3) String Algae (Filamentous)

string algae and green frog in a pond
String algae, or filamentous algae, forms hair-like strands under the water, often sticking to rocks, ornaments and pond liners.

String algae, also called filamentous algae, are single-celled organisms that link together to form – you guessed it! – long strings that in turn intertwine and form mats. Still water, plenty of sunlight, and the proper concoction of nutrients give rise to this algae, which starts off forming on rocks and substrate at the bottom of the water and then rises to the water’s surface as it links together, grows, and oxygen bubbles collect within the hair-like fibers, creating buoyancy. Belonging to the chlorophyta family and therefore a variety of green algae, string algae are most commonly green but can also be shades of yellow or brown. Some familiar filamentous algae species are blanketweed or watersilk (spirogyra), horsehair algae (pithophora), and cotton algae (cladophora).

Like most types of green algae, string algae are an essential food source for young fish, waterfowl, and aquatic insects, and also generate oxygen. Their propensity to colonize into mats can create issues such as clogging water filters and pumps, blocking sunlight, consuming dissolved oxygen, generating ammonia (and then converting that into potentially harmful nitrates and nitrites), and ultimately depleting water quality. You can control filamentous algae by utilizing naturally occurring microbes, vacuuming/raking out any mats that are present, and regularly monitoring your water quality to prevent algae overgrowth.

4) Euglena Algae

euglena algae red algae in a pond very toxic
Euglena blooms are quite distinctive, causing a deep green or crimson water color. Photo credit:

Euglena, belonging to the family euglenaceae and phyla euglenophyta, contains over 1,000 species and is incredibly diverse and resilient, able to exist in any water body around the world as well as most moist soil types. Typically green or red, this type of algae is often quite alarming – and for good reason. When euglena is present, you typically won’t know it until a bloom occurs, often bright crimson in color. These blooms are incredibly toxic, and will result in fish and vegetation die offs unless brought under control. Unfortunately, most euglena species do not respond to manual or biological controls, so you’ll have to either entirely drain your water body and replace it with fresh water, or utilize chemical products to kill off the bloom. The most effective chemical controls for euglena often contain copper or sodium carbonate. The downside of employing chemical controls, as discussed in previous articles, is the potential to harm the flora and fauna in your lake or pond. There are no known benefits of euglena, other than its presence indicating poor water quality and thus warning that something needs to change.

5) Chara Algae

chara algae muskgrass is good for ponds and lakes
Chara Algae – Photo by Mnolf available under a Creative Commons’s Attribution-Share Alike 3.0 Unported license.

Chara, or muskgrass, also belongs to the green algae family. This type of alga is often mistaken for a plant because they possess structures that look quite similar to leaves and stems. However, these are not true leaves or stems, nor does it possess reproductive structures (such as ovum or flowers). It’s not known to be overly detrimental to pond health, other than producing a pungent odor similar to that of garlic (giving rise to the name muskgrass!), and, like most algae, being prone to overgrowth. In fact, it’s known as the “filter algae,” as it naturally helps to filter out pollutants and add dissolved oxygen to the water. Muskgrass is commonly consumed by waterfowl and provides habitat for aquatic insects, which are in turn eaten by fish. Their root-like structures, called rhizoids, also help to stabilize the sediment at the water’s bottom, thus preventing murky water.

Is Algae Bad for Ponds, Lakes & Fish?

Overall, algae is natural and should not be immediately treated as a pest, as it’s often essential for proper and healthy ecosystem functioning. Aiding in water oxygenation and purification; providing food for fish, insects, and waterfowl; and offering spawning and shelter sites for fish, depending on the algae species, algae has a myriad of positive benefits. However, the point at which you should consider removing or at least controlling algae growth is when your water quality has diminished and/or you’ve noticed blooms or floating mats upon the water’s surface. To do this, you can try manual removal (rakes, filters, vacuums, etc.), algae-eating fish species like otocinclus catfish or plecostomus, biologic controls that will naturally break down the algae, chemical means (again, these may have adverse impacts on the rest of your pond’s residents as well), incorporating plants that will help to purify the water and soak up any excess nutrients that algae thrive on, or fully draining your pond and refilling it with fresh, clean water.

12 thoughts on “Guide to Different Types of Pond Algae (With Pictures)”

  1. In my new pond I have noticed small amounts of algae growing and was not sure if I should remove it or if it was harming my fish. After reading this article is was very helpful for me to identify my algae and know that my koi will be fine.

  2. My pond in central Texas develops floating mats of brownish/grey color. The fish and waterfowl apparently don’t mind and seem to be doing very well. The water has a green tint with about 2 feet of visibility. The mats are nevertheless fairly disgusting. I have considered fabricating a floating rake to pull the reachable mats to shore and onto the bank for sun drying and natural decomposition. I am concerned about toxic algae but believe this algae to be of the non-toxic variety. An aerator is in continuous operation in the deepest area (20’ deep). I have continuous water inflow from a dedicated well. Any thoughts or suggestions would be appreciated. And thanks!

    • Hi W. Robbins,

      This doesn’t sound like harmful or toxic algae, like you said. However, too much of any type of algae can be potentially harmful, especially when they form thick mats that block out sunlight. I think your idea of raking some of the mats out is a great one! A pond vacuum or net would work well, too. Don’t worry too much if you can’t get all of it – algae does help to provide oxygen to the water, so just raking/vacuuming some of it out as needed should be good.

      Let us know what you decide to do, and how it works for you!

  3. We came home from a short vacation and found orangey brown globs and some larger grey white blobs that resemble poop or mold on the pond edge(13 acres that has two cow pastures that utilizee it).

    • Hi Denise,

      Those both sound like different varieties of algae beginning to bloom, and both are often caused by nutrient imbalance. This makes sense, given that you mentioned two different cow pastures utilize the pond! Likely there is runoff from the pastures. I would recommend scooping out any algae that you can easily get to with a net, aquatic rake, or shovel, and then add in a beneficial bacteria supplement that will help feed on the algae and keep things more balanced. If you don’t already have them, I would also recommend installing a pond filter and an aerator to keep things filtered, circulated, and oxygenated to help prevent algae overgrowth in the future.

  4. I’ve lived in my home for 17yrs & it has a pond approx. 30′ long x 3′-7′ wide. It has 3 rocks w/holes that the water circulates through and helps aerate the water. Over the last 6mths or so there is a gelatinous brown algae (?) that has developed on the rock surfaces on the bottom and sides of the pond. I haven’t been able to identify it this is an algae or some other type of plant. Any idea what the heck it is?

    • Hi Barbara,

      Does the algae smell terribly? If not, it sounds like brown algae, also known as silica algae. Basically, it’s a buildup of diatoms (diatoms are the microscopic, helpful form of algae, and generate a ton of oxygen!) as they work to break down certain forms of organic matter present in the water, and they attach to surfaces like rocks that are easy to hold onto. You can remove the brown algae if you’d like, but it’s not harmful and in fact some fish enjoy eating it.

  5. We. Have a small pond, no fish (because my dog would go crazy). We are getting some green algae that looks like snot (sorry). I have parrots feather and water lillies in it to keep the algae out, but it’s not working. We also have a waterfall to keep the water moving and a pump with filter. What else can we do to keep this out?

    • Hi Cathy,

      It sounds like a type of slime algae, and those can indeed be a pain! The plants are a great addition, but those are more of a long-term solution to algae as they take time to develop enough to control large amounts of algae like that. I would suggest manually removing the algae from the surface with a rake, net, or pond vacuum, and also using some barley straw. As the barley straw breaks down, it releases compounds that kill off the algae and prevent regrowth. It takes some time to work, but it will work long term. As an added bonus, it usually only needs to be used every 6 months or so, as there’s a continual slow release of the compounds. You may also consider adding in some beneficial bacteria, as there’s likely an imbalance of nutrients if slime algae is forming. The beneficial bacteria will work to eat the algae and any harmful bacteria contributing to its growth.

      If you’d like some more info on general algae removal, we have an article here:
      Here is more info on barley straw:
      We have two articles exploring beneficial bacteria: and

      I hope that this helps! Best of luck!

  6. We have a small pond with 6 gold fish and 1 ghost koi we’ve only had the pond for about 3 to 4 months we have a water fall also which has a small amount of green algae on it but the pond itself has a black stuff round the sides it looks like dirt but when you wipe it it looks like sludge I don’t know what it is or if it’s harmful.

    • Hi Julie,

      Usually, sludge is a warning sign of a developing water quality imbalance. Test your water quality to see what the parameters are, and scrub off any of the sludge that you find before it has a chance to spread and contribute to any water quality issues. You can do this by hand or with an aquatic vacuum. Then I would add some beneficial bacteria to help break down organic matter and harmful bacteria, as a buildup of these is usually what causes black sludge to form.

      This guide focuses on black water, but also covers black sludge in various areas of the pond, why it’s there, etc.:
      If you’re interested in products to help remove it, we have a guide here that reviews different methods:


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