Who’s Winning The Algae War?

The featured attraction at a municipal park is often a pond or lake, for fishing, swimming, boating, picnicking or walking on paths around the perimeter. But keeping recreational waters attractive and useable in these days of limited budgets can be challenging. Maintaining water quality can be expensive and unsuccessful. Algae blooms and bad odors can send park visitors elsewhere, despite the best efforts to provide an attractive open space.

It can cost hundreds of dollars to clean up the water quality, often using chemicals, yet the problem soon returns. A long-term solution requires an understanding of pond biology–why a pond goes “bad,” what makes it “clean,” and how to keep it that way all season without undue expense or maintenance.

Why Good Ponds Go “Bad”

Ponds are susceptible to what are commonly known as harmful algae blooms (HABs). These are caused by blue-green algae, more formally known as cyanobacteria. HABs generally occur in warm weather and in stagnant water conditions, when a pond has a high level of nutrients. HABs may present themselves as “pond scum,” and may also exist throughout the water column. They can produce potent toxins, so the problem is more than just unsightly. Failure to address these concerns can cause serious health issues for humans, pets and wildlife.

However, not all algae are harmful. In fact, a healthy pond is one where “good” green algae, or phytoplankton, dominate. When bad algae or HABs take over a pond, they have, in effect, won the algae war. To restore a pond’s water quality, the goal is to help green algae prevail.

Healthy Prey-Predator Relationships

A pond needs prey-predator relationships to remain healthy. Single-celled green algae and diatoms (cold-water brown algae) are beneficial primary producers. During photosynthesis, they capture energy from sunlight and convert essential nutrients–carbon, nitrogen and phosphorus–into useable energy for other living organisms. Green algae and diatoms are small enough to be eaten by zooplankton, which are eaten by fish. This ongoing prey-predator relationship keeps the levels of algae, zooplankton, fish, dissolved oxygen and pH all within a normal and healthy range.

A pond must have sufficient levels of dissolved oxygen to support the food chain. Dissolved oxygen is often measured in parts per million (ppm), with levels ranging from 0 ppm (complete lack of oxygen) to around 9 ppm (the saturation point in water during typical summer temperatures). The normal oxygen content in a healthy pond ranges from 5 to 9 ppm, day or night, based on normal photosynthesis and respiration (breathing) of a modest-sized algae crop controlled by zooplankton. If a HAB occurs–where the algae are inedible and not being controlled by zooplankton–then the dissolved oxygen may range as high as 12 to 25 ppm in the day, but drop to 0 to 4 ppm at night due to algal nighttime respiration.

When a pond’s prey-predator relationships are in balance and dissolved oxygen levels are normal, the pond will be clear with only a tint of green. The green tint is a sign that beneficial algae dominate in the pond. It means that green algae remain suspended in the upper warm water, where they can feed on nutrients, and be available for zooplankton. This scenario is most common in the spring and early summer, before warm temperatures give the “inedible” blue-green algae a chance to take over. If there is a period of little or no circulation when the weather warms up, the green algae may sink out of the sunlight, settle to the bottom, and die. This will create conditions that allow the blue-green algae to dominate.

Advantages Of Blue-Green Algae

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