A Tale of Two Algae


The most frequent question that I’ve received in the past week and a half is about the green color of the water in ponds. Surpassing garden, lawn, and pasture questions, and managing one’s pond has always been a lead inquiry at my office.

The questions have been about the sudden appearance of “algae” and if it needs to be of any concern. There are two kinds of algae: planktonic and filamentous. The filamentous algae is the kind that starts growing on the bottom of a pond, then separates to float up and makes large clumps or rafts of fibrous algae on the surface. This algae is unsightly to many. Often called “pond scum” or “slime”, it always seems to get in the way when fishing. I often have to remove the slime off the hooks of the lure when fishing through this mess.

The other kind of algae is the planktonic type. If you remember any science classes, you will recall that plankton is typically a single-cell organism. Planktonic algae does not get hung up on a fishing lure but is more noticed by the green color that it gives water as it is suspended throughout the water. Where this type of algae is very abundant, your pond water will almost resemble pea soup.

Both are naturally occurring and should not cause any concern by their mere presence. Both provide oxygen and can indicate a healthy pond with nutrients. Yet the stringy, filamentous algae is not the preferred algae as it is often deemed unsightly and can hinder the ability to fish.

The planktonic algae may not give you the color of pond water that you wished for but it can be very beneficial. Think of these single-celled algae as the smallest and first link in the food chain. They convert sunlight and carbon dioxide into oxygen and organic compounds. This plant material is food for the next smallest group of organisms and continues the food chain in a pond that eventually leads to healthy, growing fish.

With their green pigment, we know algae conduct photosynthesis. Remember that photosynthesis is the mechanism for releasing oxygen, which is crucial for the growth and survival of fish and other aquatic life.

Read up on planktonic algae ‘blooms’ enough and you’ll find an abundance of literature explaining how to fertilize your pond to encourage its growth! Indeed, the recent uptick in calls may be from nutrients being washed into ponds from upstream, causing the green bloom without the landowner realizing it.

A very real downside of excessive planktonic algae blooms is that if it all dies suddenly, then your pond will have a crash of oxygen levels that your larger fish may not live through.

Accordingly, if you are looking to reduce the algae in your pond, many copper-based products are an excellent way to do this. When following the label and applying the correct amount, copper is completely safe for your fish, other vegetation, and animals.

You could treat for algae later in the summer months, but you will have more total vegetation to contend with and you could run into a low oxygen problem if you try and treat too much of your pond at once. 

When something is killed and begins to rot, the decomposition process consumes oxygen.  A dead squirrel on the side of the road will not cause any serious lack of oxygen, but the drop in oxygen in a pond full of decomposing algae is not easily replenished.

One may ask how to avoid any fish die-off in the summer months or during any time of the year. The solution is to treat no more than a third of the pond at a time so that fish have oxygen available. 

Let us say it again, herbicides approved for use in ponds will not kill fish. If they did kill fish, then I cannot imagine how they would have ever been approved. 

It is also important to state that there are products commonly used in ponds that have no business being used. If the label does not state that it can be used in a pond with specific instructions on how to apply it, then it is not legal to use in a pond. No, I don’t care what your neighbor said, if it is not labeled, then it is out of bounds.

If you wish to dive deep into farm pond management, go to the website fisheries.tamu.edu/pond-management/. This site has an abundance of pond and sport fish management resources and links. Also, feel free to call your local county Extension agent with the Texas A&M AgriLife Extension Service. To recap, do not fret about the planktonic algae; it is often considered beneficial to a pond’s health. Treat now with a copper-based product if you want to rid your pond of filamentous algae.

Cary Sims
Cary Sims is the County Extension Agent for agriculture and natural resources for Angelina County. His email address is cw-sims@tamu.edu Educational programs of the Texas AgriLife Extension Service are open to all people without regard to race, color, sex, disability, religion, age, or national origin.

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