3 Main Causes of Pond Algae

Water Gardeners everywhere fight the scourge of algae at some point in their gardening careers. Whether it is the sickly green pea soup look of a suspended algae outbreak or the fibrous mats of a string algae infestation, the result is the same: your dream of a beautiful oasis becomes a nightmare. Though at the time it may seem that this is a random occurrence, the explanation is commonplace and predictable.

Causes of Pond Algae

There are three main causes of pond algae:

Algae Cause #1: Excessive Nutrients

No living thing can thrive without abundant amounts of food. The algae in your pond is no exception. There are two main kinds of food or nutrients that algae need to flourish: phosphates and nitrates.


Phosphates, from phosphorus, are a naturally occurring plant nutrient that can enter your pond from many different sources. If you look at almost any plant or lawn fertilizer package, you will see that phosphates (phosphorous) are one of the three main ingredients (the other two are nitrogen and potassium, but unlike in other plants, potassium is not used as food by algae).

This is exactly why algae like phosphates so much – phosphates are plant food. Fortunately, phosphates are manageable as their sources are somewhat limited.

The most common sources of phosphates in your pond are the following:

  1. Source water

    Some municipal water systems allow low levels of phosphates to be passed through treatment. It is, however, much more likely for your phosphate levels to be significant if the water you are using to fill your pond is from a dugout that is fed with runoff from farmland. Well water also may occasionally contain low to moderate levels of phosphates due to leaching into groundwater.

  2. Runoff from your lawn

    If your pond is at a low spot, heavy spring rains can wash phosphates into your pond. Lawn fertilizer and fecal matter from pets all contain high levels of phosphorous and are common in most yards.

  3. Fish

    Those beautiful goldfish and koi can also be a source of phosphates in your water garden. Phosphorous can be found in some koi foods and also in fish waste, so feeding excessively contributes to phosphate build up.


Nitrates, also naturally occurring plant nutrients, are much more difficult to control than phosphates. Though rarely in source water, nitrates can be introduced into the system in the form of nitrogen from runoff (like phosphorous, nitrogen is also a main ingredient in plant fertilizer). However, runoff is unlikely the primary source of nitrates in your pond system. Nitrates happen to be a by-product of one of the most basic biological processes on the planet - decomposition. The diagram below illustrates this process: 

The Nitrogen Cycle:

Pond Nitrogen Cycle

The Nitrogen Cycle is the primary source of nitrates in your pond. To know the source of nitrates in your pond, you must look at what is in or being added to your pond that feeds this cycle.

Common elements that feed the nitrogen cycle are:

  1. Plant debris

    This can be rotting plants and algae left from the previous year that dies through the winter: fallen leaves, lawn clippings, etc.

  2. Fish waste

    Fecal matter from fish, excess food that remains uneaten, and even ammonia that is produced through fish respiration all serve to feed into the Nitrogen Cycle. This is a primary reason you should not overstock your pond with fish. (would be good to have a link to an article on appropriate fish stocking levels)

  3. Other organic material

    Anything that once was alive that gets into your pond and dies - bugs, small animals, fish or plant matter - will serve to elevate your nitrate levels, allowing algae to prosper.

It is impossible to eliminate all the sources of nitrates and phosphates in your pond. The truth is, you cannot and do not want to totally eliminate these nutrients from your backyard pond. Nitrates and phosphates are part of the overall ecosystem and essential for the health of your decorative plants. Your goal as a water gardener, however, is to create an environment where there are not excess levels of these nutrients. Achieving an optimal nutrient level creates an environment where your plants and fish thrive, but where algae is unable to overwhelm all that is good and beautiful.

Ecosystem Imbalance

Excessive nutrients and algae in your pond can also be understood as an issue of ecosystem imbalance. Algae growth is an attempt by Mother Nature to fix a problem she perceives is occurring in your pond. This problem is that of imbalance. Nature likes balance and although you may not agree with her methods, she is very effective in restoring it. Algae efficiently removes the imbalance of excessive nitrates and phosphates from your water. It is fast growing, extremely hardy, and quickly will absorb all the excess nutrients available. This is why often when you test your water in the midst of an algae outbreak, the results do not show excessively high (or sometimes any sign of) nitrates and phosphates. This does not mean nitrates and phosphates are not a problem in your pond. It only means that you have an aggressive growth of algae that is consuming nitrates and phosphates as fast as they are being generated.

Ecosystem imbalance is caused by three things:

  1. Overabundance of nutrients due to inordinate amounts of organic waste (see Common Elements that Feed the Nitrogen Cycle above)
  2. Not enough live plants

    Desirable and attractive pond plants compete against algae for the nutrients in your pond, serving to balance the pond ecosystem. The key is having enough plants to out-compete the algae for these essential nutrients.

  3. Not enough naturally-occurring bacteria and enzymes

    An established pond ecosystem contains naturally-occurring bacteria and enzymes that serve to process a portion of the phosphates and nitrates present into a form that is unusable to algae. It can take a while for the necessary bacteria and enzymes to reach optimal levels, so supplementation with commercially available products, such as Laguna Bio Booster, helps.


Algae Cause #2: Excess Sunlight

Although there seems to be some debate whether algae actually are plants or are some other classification all to themselves, for our purposes it is helpful to think of them as a type of plant. As with other plants, another essential ingredient algae needs to survive is sunlight. The more light that your pond gets over the course of a day, the greater your chance of an algae outbreak.

The two main causes of excessive sunlight are:

  1. Physical position

    Your pond is positioned in your yard where it gets full sun for a majority of the day. You likely have limited structural shade sources: trees, buildings, etc.

  2. Too few plants

    You have no or a limited amount of shade producing plants: lilies, hyacinths, lotuses, etc. These plants provide direct cover for the surface of the pond, thus blocking the sunlight from reaching algae farther down. Bulrushes, cattails, irises, and other plants that reach a significant height also serve to shade the pond for at least part of the day.


Algae Cause #3: Stagnant Water

The third most common reason for an algae outbreak is stagnation of the water. What I mean by stagnation is that your water has excessive carbon dioxide levels and low oxygen levels. Carbon dioxide is another primary ingredient that algae requires and at high levels it serves as a growth accelerant. Overabundance of carbon dioxide in a stagnant pond creates algae outbreaks of the most unattractive kind (think: scummy, murky swamp). If your pond is stagnant, you can be assured that algae will overwhelm it and you will not regain control until you change the conditions.

Pond Algae & Sludge

The primary culprit for stagnation is the inadequate circulation of your pond water. Under-circulation has two obvious causes:

  1. Undersized pump

    You need to be circulating the entire volume of your pond water at least once every 2 hours - preferably more often. This is the minimum required to keep your pond at an acceptable level of CO2 and oxygen. This level of circulation is vital not only to limit the growth of algae but also to keep the overall ecosystem functioning well. Don’t underestimate how important water circulation is. You can address every other cause of algae, but if you are not providing adequate circulation, you will struggle. In my experience, this is one of the most common and yet often overlooked cause of algae issues for the backyard water.

  2. Poor surface agitation

    Even though you may have an adequate sized pump for your pond, it is still possible to be starving your pond of oxygen. This can be due to setting up your system so it circulates without significantly breaking the surface tension of the water. Running fountains, waterfalls, surface skimmers and other water features generally make this a non-issue since they are very effective in agitating the surface of the pond, allowing excess carbon dioxide to be released and oxygen to be absorbed. (need graphic of ponds with water features)

Once again, do not underestimate the importance of the circulatory aspect of your pond in controlling algae.


Although irritating, algae outbreaks in your pond are not mysterious; they can always be traced back to one or more of the causes listed above. Understanding is the key to success in any endeavour in life and caring for your garden pond is no exception. In learning the causes of algae growth in your pond, you have successfully taken the first step in transforming your pond from the murky green realm of nightmares to the crystal clear oases of your sweetest dreams.


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