“Toxic Algae” and Your Pond

Servicing Aeration Systems
August 8, 2019
Fall Phragmites and Cattail Control
September 3, 2019

Photo: Dr. Jennifer Graham, USGS

Late Summer and a lot of media attention have brought a number of questions from our customers about “toxic algae blooms” on ponds and what to do about them.

First, it is important to distinguish between plain old green algae, which forms the base of the food web in your pond and is an integral part of any healthy ecosystem, and Cyanobacteria, more commonly called blue-green algae, which can produce toxins that can be deadly to animals and even people.

Generally, when we think of algae it is the more common green algae, which can describe a large number of different species of microscopic colonial and free-floating plants that form the necessary base of your pond’s food web.  In reasonable quantities, these are beneficial and necessary in bodies of water as the food source for small planktonic organisms that graze on them that then provide the food source for larger species on up to the fish that we are fond of having in ponds.  If there is too much nutrient in a pond or lake, green algae species can reach nuisance levels, however they are not toxic.

Blue-green algae, more correctly referred to as Cyanobacteria, are a different group altogether.  These are free-floating or colonial bacteria that produce energy through photosynthesis like plants, but they are very different than green algae, although blooms of Cyanobacteria can appear similar.  The most significant difference for pond-owners is that some (not all) Cyanobacteria can produce powerful toxins that can be dangerous to animals and humans.  It is this aspect of Cyanobacteria that creates the most public interest, especially related to dogs and other pets being exposed.

Not all Cyanobacteria are “blue-green” in color and can range from shades of blue and green all the way to yellows, browns, reds and black.  Unfortunately, identification of a Cyanobacteria bloom is not always a simple thing.

It is worth noting that there is nothing “new” about the issue of Cyanobacteria in ponds and lakes, only the amount of attention that it is receiving in the media.  These organisms have always been around and are not part of a sudden invasion.  They are mostly a summer-time issue and grow best under the same conditions that produce green algae, specifically high nutrient levels, intense sunlight and warm water.  This means that they are also susceptible to the same types of control methods that are used for green algae species, with prevention steps topping the list.

What can you do to prevent the growth of algae in general and specifically cyanobacteria?  The first step is always to limit the introduction of nutrients to the pond.  Nutrients come from numerous sources such as lawn fertilizers, grass clippings, decaying leaves, goose droppings, leaking and failed septic systems, rain run-off from high nutrient areas, and others.  Controlling the sources of nutrients is a key starting point.

The next step is to help reduce the nutrients that have already made it into the water.  Generally, this is accomplished by the addition of adequate aeration that is correctly sized for the pond and the use of beneficial pond bacteria treatments.  This combination helps the pond “digest” the nutrients that are present.  The movement and oxygenation of the water that come with aeration also discourage algal growth, including cyanobacteria.

Utilization of a pond dye can also “shade” your water and limit the amount of sunlight available to algae and cyanobacteria, thus limiting their growth potential.  Use of algaecides to kill algae and cyanobacteria can be useful if growth becomes excessive, but remember that as the algae break down, the nutrients return to the water and become available to grow another “crop” of algae, so this is an approach that only gives temporary relief.

One thing that is frequently overlooked is the role that other plants play in limiting algae and cyanobacteria growth in a body of water.  I am fond of pointing out that the combination of “water, sunlight and nutrients will cause something green to happen.”  The question is what type of green it is.  In other words, there is a practical trade-off at work between algae and other water plants.  By allowing macrophyte (rooted) plants to grow in your pond, you will limit the amount of nutrients that are available to algae and will therefore limit algae and cyanobacteria growth.  If you have nutrients in your pond water, but consistently kill all of the other various pond-weeds, you increase the opportunity for algae to get out of hand and this includes the possibility of toxic blue-green algae.  Too many pond weeds can be annoying, but totally eliminating them can also create other problems like more algae growth.  Chara, a common branched alga, that resembles a macrophyte weed, can also play this same role, by binding up nutrients and preventing the growth of more obnoxious algae species.

The reason for this article about cyanobacteria is to be informative and to provide basic strategies to prevent toxic algae blooms from creating problems in ponds and to provide good information about what to do with a suspected bloom.  It is not my intention to overstate the likelihood of toxic algae blooms, which remain relatively infrequent.  Your pond is there to enjoy and for the most part it remains safe, but just like the potential hazards of thin ice or deep water, there are some basic prevention and safety steps that should be included in your pond management plan.

If you find yourself looking at a suspected cyanobacteria (blue-green algae) bloom in you pond, here are some considerations:

  • Be safe, stay out of it, and keep pets out of the water. If it is indeed a toxic bloom, ingestion of even small amounts of water can be fatal to pets.
  • Get a positive identification. Private laboratory services are available to identify the cyanobacteria as well as to detect and identify potential toxins.
  • Consult with a professional to determine appropriate controls. Killing all of the cyanobacteria with an algaecide may create a sudden release of concentrated toxin.  The toxin will linger for some time in the water before breaking down and dissipating, but the timing is variable and unpredictable.
  • Consult with a professional to create a workable prevention and control strategy for the future. Just because you have a bloom this summer does not mean that it can’t be prevented in the future.

As always, we are available to work with you to take the steps necessary to prevent toxic algae from becoming a problem in your pond.  Give us a call for advice, products or to set up an appointment for an on-site consultation.