Algae is the ever-present cloud that hangs over many pond operations, whether the pond is built for irrigation, recreational fishing, a backyard garden spot, a fire pond, or even just aesthetic enhancement. Algae can be unsightly and smelly, can clog pipes and pumps, and blooms can even create toxins that contaminate crops and are deadly to both animals and humans.
Fighting algae is a complex operation, partly because “algae” is not a single target. The term, as it is commonly used, refers to microscopic single-celled organisms that float freely within the water and give it a greenish tinge, as well as to single-celled varieties that form long threads, or filaments. These filaments can become large and dense, forming thick mats or pond scum. At the other end of the spectrum are multicellular forms such as seaweeds and giant kelp. The organisms commonly known as blue-green algae are actually a type of photosynthesizing bacteria which thrive in similar conditions to algae but are particularly dangerous thanks to the deadly toxins they produce.
All algae, whatever the type, prefers a certain set of conditions which are commonly found in artificial ponds and even in natural ponds where the ecosystem has been disturbed. Sun, carbon dioxide, and nutrients like phosphorus and nitrogen are what pond algae seek, and all three are typically abundant in your average irrigation pond. Algae is notoriously difficult to eliminate once it has gotten established, because the cycle of bloom - death - decay is relentless and self-sustaining.
Removing & Filtering Organics
Filtration, used to separate organic waste from incoming water, is an important step in discouraging algae. Decaying waste at the bottom of the pond releases nitrogen and phosphate into the water, which feeds algal growth. Be sure to clear traps of leaves, sticks, and other debris, and make sure you mow or blow clippings away from the pond. If you notice significant amounts of organic debris collecting at the bottom of your pond, it’s a good idea to schedule a full cleanout. In the case of a farm pond, this will typically involve the use of a hydraulic dredge, which is essentially a pond vac on steroids.
Turning off the Hospitality
Algae need plenty of sunlight to thrive because they use photosynthesis to produce their food. Irrigation ponds are, by necessity, located in open areas with little cover from trees, where sunlight is abundant. Just like plants, algae use CO2 during the daytime while they’re photosynthesizing, then consume O2 at night, so both need to be in abundant supply. Also, like plants, algae need nutrients to fuel photosynthesis, grow, and reproduce. Algae also prefer warm weather and relatively still water. The key to preventing and eliminating algae, then, is to change the conditions that support algal growth.
It’s impractical to cover an entire irrigation pond, whether it’s 4 or 40 acres, so limiting the amount of sunlight isn’t an effective strategy and, likewise, for the supply of oxygen and carbon dioxide. Wintertime is a natural break for the growth of algae, but it’s not possible to keep the surface layers of a pond cold all year around. That leaves two possible avenues for making a pond less welcoming to algae: reducing the nutrient supply and keeping the water moving.
Nutrients
Let’s face it: irrigation tailwater is going to be loaded with nutrients, as is stormwater that has passed over cultivated fields, livestock lots, etc. The same nutrients that are added to help your crops thrive, in turn, support the abundant growth of algae. But those nutrients are valuable, and you have probably paid good money to make it available to your crops to begin with. It’s possible to use aquatic and marginal plants to absorb most of the available nutrients and keep algae at bay, but in a perfect world, we’d keep the levels high and send them around to your crops again.
When you’re faced with managing a pond with a consistently high level of nutrients, the best strategy is to move it along, keeping detention times minimal. Whenever practical, plan to recycle your operational water back to the crops as quickly as possible, while keeping stored stormwater available to supplement as needed.
Of course, another way to manage the issue of nutrient-rich tailwater is to use an irrigation system that applies water with a high level of precision. This ensures that the soil receives the right amount of water and can absorb it, reducing the amount that needs to be applied. In turn, this leaves significantly less water to flow off the field. Smaller amounts of nutrient-rich water can be combined with stormwater storage without presenting such rich conditions for algae.
Aeration
Providing aeration is probably practical only for smaller irrigation ponds, and it doesn’t kill algae directly, but it can be helpful to make the pond environment less ideal. Aerating fountains, underwater diffusers, and similar tools work by increasing the amount of oxygen available through all levels of the pond. This supports and protects the colonies of beneficial bacteria that compete directly with algae for the available nutrients. A healthy bacteria colony can easily out-compete algae and keep it at bay.
Aerators and fountains also mix water vertically, moving warmer surface waters down and bringing up cool bottom water. Since warmer water holds more oxygen, this keeps the bacteria at lower depths more plentiful and active, helping them out-compete algae. This is particularly effective in deep ponds and reservoirs.
When it’s powerful enough, aeration can move the surface water around and eliminate areas of stagnation, where filamentous algae and blue-green algae (the deadly cyanobacteria) prefer to grow.
