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Dealing with Planktonic Algae: Strategies for Control and Maintenance

by Joe Cadieux on December 13, 2022

Good day all….

Planktonic Algae

Blue Green Algae (Cyanobacteria)                       Green Planktonic

They are widely diversified in form.

Planktonic algae are single celled organisms that live both solitary and colonially, and encompass a wide swath of the algal taxonomy…. They are very hard to control, and are most often best left alone. They are also a valuable part of the ecosystem. Otherwise known as phytoplankton, they inhabit the bottom of the food chain (along with zooplankton) for fish and other creatures of the depths. Most of the time the diversity of these wee folks makes for a balanced system, and allows for free competition for food and space, which is what I look for in healthy systems.

When conditions arise that lead to a monoculture (high nutrient load, heavy treating of weeds and algae, large runoff event, prolonged heat and sun exposure).  Too much of anything is bad.

The issue lies in which species is found in your particular waterbody.  Most genera (and species) are harmless and can controlled with the strategies described below. However, if you have a special few algal species (that are fairly common) in high concentrations, problems may ensue.

Some blue green algae (cyanobacteria) that are a common pest in small garden ponds and large ponds and lakes can be dangerous to higher lifeforms like fish and even people (microcystin toxin). While only harmful in large doses, the threat is real and must be taken seriously.  The big issue is that if you do not have a positive identification of these organisms before you treat, you risk the release of a toxin into the water as the algae die, potentially making the water poisonous.  If your see dead fish, or thick concentrations of Bluish Green soup, please get a sample tested to see if you have an issue that needs to be addressed.

Saying that…..If you have pea soup green water, there are ways to handle the problem.  My favorite for watergardens is installing an ultraviolet filter system . This reduces to near zero the potential for a toxic bloom to run through your system. The UV clarifier exposes the single celled invaders to UV light rendering them inert.  It will never allow the populations of planktonic algae to rise to the level of toxicity no matter which species invades your system.  For larger systems (0.2 acres and larger), UV systems aren’t as practical.  Historically Planktonic blooms are dealt with by adding dye to water to limit light penetration and starve out the algae as it cycles through the water column, it is definitely worth a try as it is the cheapest potential solution available…. If this doesn’t work (and I admit, most of the time it only helps a little) the following are good options for the control of persistent planktonic issues.

Sonic units:

Installing these units appropriately will control most algal species (including most filamentous as well).  They emit many amplitudes and frequencies of sonic waves. These waves disturb the interstitial fluids and organelles within individual algal cells, eventually killing them off.  These units are expensive but are a good option for the longterm control of planktonic algae.  Sonic units do not harm fish or people, they are silent and easy to install and maintain.

Silver ion filtration:

These units work like the UV systems listed in article above but you can link a couple of them together to lower the concentrations of planktonic algae (and other bacterial organisms like e coli). They require the install of and external pump as well, so they can get pricey. Here’s one sized for watergardens. 


If your planktonic algae is a temporary issue or if its brand new to your system, you may be able to drop it out of the system by glomming it together and sinking it to the bottom.  Flocculants also grab excess phosphorus and other particles in the water column to improve water clarity. The link is to a product sized for watergardens. Please contact me for large pond products that do the job.

We’ll be adding all of these (and many more) products (specifically large pond products) to Midwestponds.com in the near future. Please contact me through the comments section or email: joe@mwp.testpoint.dev for more product info and advice. Over the next few months I will be reviewing all of these products on this blog to completely flesh out how they have worked for my clients here in the midwest. So stay tuned, and if you would like a particular issue of product talked about, by all means shoot me a line.

Thanks So Much…


Senior Biologist



https://aquaplant.tamu.edu/plant-identification/alphabetical-index/planktonic-algae/ I https://thepondconnection.com/pond-and-lake-101/problem-ponds-lakes/  I https://midwestponds.com/planktonic-algae/