The Mystery of Pungent Plankton and Malodorous Mats

Pond owners sometimes complain about a specific type of odor coming from their pond. Earthy, musty, swampy or even fishy are common descriptions of these odors. Most often these types of odors are a result of algae in your pond. Now let’s be very clear, algae are a very important and beneficial part of any aquatic system:

  • They provide oxygen to the system through photosynthesis
  • They are an important food source for many aquatic animals
  • Certain types of algae that grow as masses of filaments serve as a home to tiny organisms

But it’s easy to forget how great algae are when they decide to saturate the air with their special brand of perfume. The first thing you may be thinking is: “My pond doesn’t always smell like this. What’s causing it to stink now?“. Well, normally algae do not produce an odor strong enough to detect unless you hold it close to your nose. The smell usually only becomes intense under certain circumstances.

Common Algae Odor Issues

Cyanobacteria blooms

Cyanobacteria, also called blue-green algae are naturally found in just about every kind of water body, including your pond. Under certain environmental conditions, planktonic cyanobacteria start to multiply rapidly, with their population size becoming large enough to form a blue-green blanket on the water surface. This phenomenon is called a “bloom” and when it occurs it is pretty hard to miss. Cyanobacterial blooms can cause an intense odor and this is due to three main reasons:

  • Certain species of cyanobacteria produce chemicals with a pungent odor. So of course, the larger the number of smelly cyanobacteria you have, the stronger the scent.
  • As the cyanobacteria begin to die off and decompose, they produce a rotting plant smell.
  • Some cyanobacteria produce toxins that may kill fish and other aquatic animals. Also, as bacteria break down the dead cyanobacteria cells, oxygen in the water column is rapidly used up and this can cause fish kills. As the dead fish and other creatures decompose, they add to the already impressive stench.
Blue-green algae bloom

Blue-green algae bloom

Algal blooms

Although they are referred to as blue-green algae, cyanobacteria are not actually true algae. However, the same types of blooms created by cyanobacteria can also occur with true planktonic algae. Again these blooms can form a blanket on the surface of the water, the color of which can vary depending on the type of algae forming the bloom.While true algae do not usually produce odorous chemicals, an algal bloom can still produce an intense odor following the death and decomposition of algal cells. The depletion of oxygen from the water column that accompanies decomposition can again cause fish and other creatures to die and rot, adding to the smell.

Filamentous Mats

Planktonic algae become a problem when they form blooms. But there are other types of algae that can also be a problem.Filamentous algae can grow into masses commonly called algal mats. These mats are usually concentrated along the shallow edges of ponds, but can also be found floating throughout the entire pond. The odor produced from these mats is again most intense when the algae die and begin to decompose. But algal mats that cover the sediment along the edges of ponds can become thick enough to prevent the exchange of oxygen between the air and the sediments below the mat. When this happens, the lack of oxygen beneath the mat creates conditions in which noxious gases such as Hydrogen sulfide are produced. If the mats are moved, the gasses are released, creating a pungent “rotten egg” odor.

pond odors algae mat

Filamentous spirogyra mat

How best to handle algae related odor

As was said before, algae are a good thing. But too much of a good thing can turn bad real fast! In this case, it is the overgrowth of algae that creates conditions in which unpleasant odors can develop. Overgrowth of algae is caused by two main factors:


The No. 1 cause of algal blooms and filamentous mat formation is an abundance of nutrients in your pond. Algae require nutrients to grow, mainly Phosphorus and Nitrogen . If there is an excess amount of these nutrients available in the water, then algae will grow and grow and grow until they use it all up.


  • To prevent algal overgrowth, limit the use of fertilizers in areas around your pond. Remember, even if you only use fertilizer on the grass around your pond, it only takes one good rain shower to wash it all into the pond…then BOOM! Algae city!
  • You can also find ways to create buffer zones around your pond. A buffer zone is a strip of vegetation that surrounds your pond and helps to slow down the leaching of nutrients into your pond from the surrounding areas.

Water stagnation

Algal blooms only form in stagnant or extremely slowly moving water. The still waters are required for the algae to form their characteristic surface “blankets”. Filamentous mats also form more quickly in stagnant water than they do in flowing water.


  • Adding some form of flow or circulation to the system can help to break up the algal bloom and will help to prevent the formation of future blooms.
  • Aerators and fountains can help to circulate the water in your pond and prevent it from becoming stagnant. Just keep in mind that the size and number of aerators or fountains you need in your pond will depend on the size and depth of your pond (along with a couple other things.
  • Circulation can also help to deter filamentous algae growth in the open pond area, but it can still grow along the pond edges. Also, because of their cohesive structure, once formed, filamentous mats can withstand quite a bit of water movement without coming apart.

What to do if you already have a bad algae odor problem

Preventing algal blooms and mats is, of course, your best option when managing your pond. There are options to control existing problems.

Your lake management team should make the decision on whether to treat an overgrowth of algae with algaecides. When done properly, the method can be very effective, but it is important to have knowledge and experience when using any form of chemical control in aquatic systems. For example:

  • Using an algaecide to kill a toxic cyanobacteria bloom all at once may cause a mass of toxins to be released into the pond.
  • Killing a mass of planktonic or filamentous algae all at once can cause oxygen to be depleted from your pond. This can cause fish kills and make a bad odor problem even worse.
  • Some cyanobacteria can form jelly-like balls that are very difficult to kill with algaecides.

While labor intensive, it is sometimes necessary to physically remove filamentous mats from the pond, before doing any other form of treatment. If your pond has huge amounts of algae, this might be your best option.

A lake management professional would be well aware of all these issues and ensure that all factors are taken into consideration when coming up with a management plan. Hopefully, this article has helped to unravel the mystery of how algae can contribute to pond odors.

For questions about this or something specific to your pond, contact us today!

About Dr. Josette La Hée, Senior Environmental Research Consultant

Josette La HeeDr. La Hée conducts research for Aquatic Systems on urban aquatic ecology. The primary focus of her research is the use of sustainable management techniques for restoring impaired lakes and ponds. As Head of Limnological Research, Dr. La Hée spent three years working with biologists in our field offices. She designed and ran lake aeration studies on multiple lakes throughout Florida. Additionally, she developed the lake assessment and monitoring protocols used in our lake management programs. In addition to research, Dr. La Hée conducts teaching and training sessions for lake management groups.  She has presented research at conferences including the North America Lake Management Society and the Florida Aquatic Plant Management Society.