Algae can be divided into three categories; Planktonic algae (phytoplankton), filamentous (or colonial) algae (pithophora and others) and macroalgae (chara and nitella). Phytoplankton populations are called blooms and they can make water appear green to yellowish-green in color. Depending on the dominant algal species, a bloom can also leave an oily or scummy appearance on the water surface. The phytoplankton bloom produces most of the dissolved oxygen in the water and are the base of the pond’s food chain. But a dense phytoplankton bloom can also create a fish kill by consuming all the oxygen during the night or by suddenly dying off, leading to an oxygen crash as the plankton decomposes. Dense phytoplankton blooms are caused by excessive nutrients, usually nitrogen or phosphorus. These excessive blooms can sometimes be prevented by limiting nutrient input. Phytoplankton blooms do not be controlled or “thinned” most of the time, but sometimes it can be beneficial to the pond by the judicious use of a registered algaecide.

Planktonic Algae Management Options

Physical Management Options

Floating, planktonic algae cannot be mechanically or physically controlled, except by replacing the pond water. Exchange of water from a well or other source that does not have an algae bloom will dilute the planktonic algae in the pond. This is not a practical option for most pond owners unless their ponds are very small and they have wells close by.

Non-toxic dyes or colorants prevent or reduce aquatic plant growth by limiting sunlight penetration. However, dyes do not limit plant growth in water less than 30-36”, so the plankton bloom will not be affected by aquatic dyes. Some examples of non-toxic dyes and other products include Aquashade, Blue Spring, and Crystal Blue, as well as other brands.

Biological Management Options

While many microscopic animals (zooplankton) eat planktonic algae, there are no practical ways to increase their populations, so no biological control is possible.

Herbicide Control Options

Always read the product label for directions and precautions, as the label is the law. Click on the name of the product to see the label. Read the label for specific water use restrictions.

The active ingredients that have been successful in treating planktonic include Copper ion (Copper sulfate, copper sulfate solutions & chelated complexes) and Sodium Carbonate Peroxyhydrate (SCP).

Copper ion

Copper Sulfate or “blue stone” is probably the most commonly used algal treatments because of its availability and comparative low cost. Copper sulfate comes in several forms depending on how finely it is ground. Smaller crystals will dissolve easier than larger crystals. The copper sulfate solutions are pre-dissolved, but essentially the same as crystals in use. In very hard water it is difficult to use copper sulfate because it binds with the calcium, precipitates out of solution, and renders the copper ineffective as an algaecide.

Unfortunately, copper is toxic to fish if used above labeled rates and can be toxic if the pond water is soft (low alkalinity below 50ppm CaCO3) or acidic, even at label rates. This describes most ponds that are fed by streams or rainwater. It is best to test the pond water’s alkalinity and adjust copper treatments for the alkalinity concentrations.

Copper Chelated Complexes

Free copper ions can be toxic to fish in soft water. In chelated copper complexes the copper ions are bound to inert ingredients that allow the slow release of the copper ions, which keeps copper ions active in the water for a longer period of time. This also permits the use of less copper and potentially are safer to use in soft (low alkalinity) water. Following label directions is essential.

Sodium Carbonate Peroxyhydrate

SCP is a contact herbicide that is only effective against algae. SCP is not effective on the macroalgaes, Chara or Nitella, or on any higher plants. Once the product is applied to water it converts to hydrogen peroxide, a powerful oxidizer, that causes damage to the algal cells. Hydrogen peroxide will eventually convert into water, leaving no chemical residue, making it safe for all applications. It is applied by itself, but can be followed up with other products, like copper sulfate, providing more effective algae control.

Always read and follow all label directions. Check label for specific water use restrictions.

One danger with trying to control a phytoplankton bloom is the potential of an oxygen depletion following an algaecide application. Dead algae will decay and consume oxygen, as well as reducing the production of dissolved oxygen by phytoplankton. Aeration, particularly at night, for several days after treatment may be necessary avoid an oxygen depletion.

Application

Algaecides can only kill algal cells that the chemical comes in contact with. The most effective is to dilute the algaecide in water according to label directions and then spray or inject it as widely as possible. Use of a boat will likely make this easier and more effective. While not as effective, using a hand-held pump sprayer to “spray” out a stream of algaecide over the water surface is one way to apply algaecide if better equipment is not available or practical.