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
Blue-green algae have many weapons in their arsenal to defeat good, edible algae, including the ability to adjust their buoyancy. Blue-green algae can rise to the surface during the day for sunlight, and then sink into nutrient-rich bottom waters to access nitrogen and phosphorus.
Blue-green algae also have other advantages: they contain toxins that can kill zooplankton and other organisms, they can store phosphorus for later use, and they can convert nitrogen gas near the surface into a nutrient. In fact, they have so many advantages that once blue-green algae take over, it is difficult for any good algae to displace them.
When HABs occur, excessive nutrients--i.e., phosphorus--make the bloom grow that much larger. Large algae blooms near the surface also block sunlight to the deeper algae cells. When the deeper cells become light-limited, they can die off at once. As a consequence, bacterial de-composition of the dead algae cells can consume all the available oxygen, resulting in aerobic odor events and fish kills.
Advantages Of Edible Green Algae
Green algae also have major advantages: they start growing earlier in the season and can reproduce many times faster than blue-green algae. When they’re suspended in the water column, green algae can consume the available nutrients very quickly and remain viable throughout the season, making it harder for blue-green algae to gain any traction.
Wind-mixing alone is not enough to help the good algae out-compete blue-green algae because blue-green algae have adapted over millions of years to predictable wind-mixing patterns. Additional circulation may be necessary, and has been shown to be effective in improving water quality.
Limitations Of Chemical And Mechanical Methods As A Solution
Several methods exist for controlling blue-green algae. The most common is aquatic herbicides, such as copper sulfate. Chemicals can provide short-term relief from harmful blue-green algae, at a typical cost of $200 to $1,000 per acre per year, but they have serious ecological impacts, and the long-term effects are unknown. Algaecides can actually create copper-resistant algae, requiring more algaecide applications each year and eventually creating a sterile pond environment.
Devices such as bubblers, aerators and fountains do some mixing, and attempt to infuse a pond with dissolved oxygen, but in shallow ponds they are generally not effective at controlling blue-green algae over a large surface area. Solar-powered and grid-powered long-distance circulation equipment impacts a larger surface area, so the use of this type of equipment is growing in ponds with HAB problems.
Circulation Helps Edible Green Algae Survive
With a pond circulator, water is drawn to the intake from all directions (usually just above the thermocline in a deep pond and closer to the bottom in a shallow pond), pushed upward, and then sent out across the surface in a thin layer. This circulation provides gentle mixing and surface renewal over a large area. Because of this circulation, green algae and diatoms continue to receive sunlight, remaining viable and available to zooplankton all summer long.
The goal of circulation is not to add oxygen to a pond because that’s the work of green algae. Rather, the purpose of circulation is to help the green algae survive. When the green algae survive the levels of zooplankton, fish, dissolved oxygen and pH will stay in balance. The blue-green algae never take over, and the water will remain in a healthy state. It’s a low-maintenance and economical way to ensure that the pond or lake remains the focal point of municipal parks.
Joel Bleth is the president and a co-founder of SolarBee, Inc. The company manufactures solar-powered long-distance circulation equipment for solving water-quality problems freshwater, potable water and wastewater reservoirs.