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:
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.
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:
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.
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.
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.
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.
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:
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.
Dr. 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.